Emerging Trends in The Social and Behavioral Sciences

Sexual Behavior

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Sexual Behavior

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Sexual Behavior
MELISSA EMERY THOMPSON

Abstract
Research on human sexual behavior is a multidisciplinary pursuit that seeks to
understand one of the most vital and complex aspects of our biology. Foundations
for this study include the basic principles of sexual selection, including differential
reproductive roles of male and female, and the processes of sexual differentiation.
Study of human sexual behavior is made vastly more complex by emotional
involvements and the diversity of sexual behaviors exhibited by our species.
Historically, sex researchers have struggled to overcome the methodological
challenges involved with objective study of behavior in our own species and with
the ethical and political implications of their work. Early research focused on
merely quantifying the spectrum of human sexual behaviors and on understanding
the physiological and psychological processes of sexual response. Subsequent
work developed the concept of gender and began to address how behavioral and
corporeal manifestations of sex can diverge from one another. Modern studies
incorporate a variety of advanced scientific techniques to investigate mechanistic
and functional hypotheses for specific behaviors. This review highlights four
prominent research topics, highlighting current understanding, cutting edge work,
and key issues for future research: mate preferences, concealed ovulation, sexual
coercion, and homosexuality. In each of these areas of research, there is strong
evidence for biological influences on behavior. It is also clear that known biological
mechanisms only partly explain actual behavioral patterns, suggesting strong
mediation by cultural, environmental, and developmental processes.

INTRODUCTION
Sexual behavior is central to the biology of a species, being vital to how
the species persists and changes over time. The study of sexual behavior in
humans has produced particularly intense research effort, perhaps because
of the unusual diversity of sexual behavior in our species and the central role
sex plays in health, wellbeing, and popular culture. These factors magnify
the complexity of gaining an objective understanding of sexual behavior in
humans. Thus, at every stage in its history, scientific research on sex has been
fraught with controversy for reasons ranging from inadequacy of sampling
methods to ethical concerns to wider social implications. Sex research has
Emerging Trends in the Social and Behavioral Sciences. Edited by Robert Scott and Stephen Kosslyn.
© 2015 John Wiley & Sons, Inc. ISBN 978-1-118-90077-2.

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both been shaped by and has steered social and political attitudes. As in any
science, the field has built incrementally on its successes and failures, gradually incorporating new techniques and theoretical approaches. A field that
began by simply attempting to catalog and describe the nature of sex in our
species has moved forward into a vastly interdisciplinary research area incorporating advanced scientific methodologies such as genetics, neuroscience,
and endocrinology, while acknowledging the complex interactions between
culture, environment, experience, and biology in shaping behavior.
The study of human sexual behavior is prominently represented in several international research organizations, including the Society for the Scientific Study of Sexuality, the International Academy of Sex Research, the
Society for Sex Therapy and Research, the Human Behavior and Evolution
Society, and the American Sociological Association. In addition to targeted
journals, such as the Journal of Sex Research, Archives of Sexual Behavior, and Sex
Roles, research on sexual behavior figures prominently in broader journals in
anthropology, psychology, sociology, and biology.
The following review provides a background to research in the field of
sexual behavior, including historical perspectives, prominent approaches to
modern studies of sex, and a highlight of four particularly active research
areas. It should be noted that this is by no means an exhaustive treatment
of sex research. Per the author’s expertise, this review highlights research
driven by or intersecting with a biological approach. For further discussion
of related topics, refer to entries on gender in the Sociology series and on sex
differences in behavior in the Anthropology series.
FOUNDATIONS OF SEXUAL BEHAVIOR
SEXUAL SELECTION
Foundations of sexual behavior in humans, as in other animals, stem from the
core biological definition of male and female sex. Females are defined as the
high-investment sex, demonstrated initially by a relatively large gamete (the
ovum, or egg) containing the nourishment for early growth of the zygote.
In mammals, where females bear the burden of pregnancy and lactation, the
inequality in investment is particularly great. Thus, females and the investment they provide become a limiting resource for males who must compete for opportunities to fertilize them (Bateman, 1948). This has far-reaching
implications for sexual behavior, as outlined in Darwin’s presentation of sexual selection (Darwin, 1871) and its many subsequent elaborations. Males are
generally expected to compete amongst themselves for access to females, a
process that can shape behavior (e.g., aggression), morphology (e.g., body
size, sexual ornaments), and physiology (e.g., sperm production). Males also

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have the potential to produce offspring at a faster rate than females, often
leading to relatively promiscuous mating behavior and a reluctance to invest
too heavily in one mate when other opportunities are available. Females, on
the other hand, are expected to be more conservative in their selection of
mates, choosing males that are willing to provide an investment of resources
or who have genetic traits that benefit offspring viability (Trivers, 1985). The
difference in reproductive interests between the two sexes can create sexual
conflict in which the strategies used by one sex cause negative effects for the
other (Parker, 1979). Sexual coercion is a prominent outcome of sexual conflict in which males (typically) use force or the threat of force to circumvent
female mate choice.
The above patterns are generalizations, and different species manifest them
in different ways and to differing degrees. The human species presents a
number of complications. Chiefly, the high cost of rearing human infants
is thought to necessitate significant investment by fathers, often leading to
long-term partnerships at the expense of mate-seeking behavior (Lancaster,
J. B., & Lancaster, C. S., 1983). Whereas ostentatious displays and ornaments
such as colorful feathers are more typical of males in other species, human
females in most cultures use adornments to advertise beauty, youth, or fecundity to potential mates, suggesting that males’ investment may compel them
to be more selective. Humans display greater variability in sexual practices
and more frequent nonconceptive sex than is typical of animals in nature.
In addition, variation in cultural practices and the involvement of complex
emotional and cognitive processes shape norms of sexual behavior.
SEXUAL DIFFERENTIATION
Biological research into human sexuality often references the developmental
origins of male and female and persisting differences in the hormonal physiology of the two sexes. Initial sex determination is dependent on whether an
individual receives an X (female) or a Y (male) chromosome from the father.
The presence of a functional Y chromosome leads to maturation of the testes
from an undifferentiated fetal gonad. Once the testes develop, they produce
the hormone testosterone which, directly or indirectly, guides the maturation of the remainder of the male physical phenotype. Females’ gonads and
external genitalia derive from the same embryonic structures as males’ in
the absence of the Y chromosome and testosterone. This is an oversimplification of a process that, in fact, can be altered at various stages. The influence
of these developmental processes on sexual behavior is far less clear-cut.
Testosterone impacts male sexual behavior, as can be seen from the effects
of castration in many species. However, it can have both “organizational”

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effects, priming the neuroanatomical substrates at an early age of development, and “activational” effects, interacting with these substrates to produce
behavioral responses in the mature organism. Testosterone may have important influences on the sexual behavior of females, as well, and it is known to
have complex interactions with other hormones in both sexes. Females experience a cyclical fluctuation in the hormones progesterone and estradiol that
accompanies the process of ovulation and menstruation. It is clear that sexual behavior in the human female is less strictly governed by these hormonal
fluctuations than is the case for most other species.
BRIEF HISTORY OF SEX RESEARCH
The modern study of human sexual behavior can be traced to Alfred Kinsey,
founder of what is now the Kinsey Institute for Research in Sex, Gender, and
Reproduction, and lead author of the Kinsey Reports on Sexual Behavior in
the Human Male (Kinsey, Pomeroy, & Martin, 1948) and Sexual Behavior in the
Human Female (Kinsey, Pomeroy, Martin, & Gebhard, 1953), among the first
attempts to understand human sexual behavior from a scientific perspective.
Kinsey’s work was accomplished primarily through extensive interviews
about sexual experiences and had the overarching goal of documenting the
scope of human sexual behavior, as well as the frequency and sociocultural
predictors of particular behaviors. Because his work emphasized the diversity of sexual experience, it was an important attempt to remove stigmas
not only from the examination of sexual behavior but from the behaviors
themselves. In particular, Kinsey documented the frequency of homosexual
behavior, arguing that many individuals are not easily classified as either
exclusively homosexual or exclusively heterosexual. While documenting
differences in sexual drive between males and females, Kinsey reported a
greater breadth and investment in sexual experience among women than was
publicly recognized. These features made his research highly controversial
at the time of publication. Though Kinsey’s works are still highly regarded,
they are criticized for methodological issues, such as reliance on self-reports,
self-selection of individuals willing to discuss their experiences openly, and
overrepresentation of gay subjects, sex workers, and sex offenders.
Beginning in the late 1950s, William Masters and Virginia Johnson extended
the study of sexual behavior by documenting patterns of human sexual
arousal and response (Masters & Johnson, 1966). In contrast to Kinsey’s
interview approach, Masters and Johnson conducted their research in a
laboratory setting, monitoring physiological responses during masturbation
or intercourse. Their work led to the recognition and description of the four
phases of sexual response (excitement, plateau, orgasm, and resolution)
and of multiple orgasmic potential in women. In addition, Masters and

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Johnson targeted the assessment and treatment of sexual dysfunction,
linking psychological and sexual health. However, by attempting to define
“normal” versus “dysfunctional” behavior, their research moved away from
Kinsey’s emphasis on the diversity and individuality of sexual experience.
This included their controversial treatment of homosexuality as a sexual
dysfunction, one which they attempted to cure through conversion therapy.
Such claims contributed to the maintenance of homosexuality in the American Psychiatric Association’s Diagnostic and Statistical Manual of Mental
Disorders until 1973.
After such intense scrutiny on the mechanics of sex, later attention focused
on sexual identity. John Money redefined the term gender, and associated
terms such as gender role, to highlight the distinction of biological sex from
sex-typical behavior (Money & Ehrhardt, 1996). At the time, Money’s view
that gender was learned rather than innate was widely accepted, meeting
agreement with a growing field of feminist scholarship that sought to reject
ideas about the biological determination of gender roles. While Money’s
work was ground-breaking in highlighting the flexibility in behavioral
manifestations of sex, he later earned fierce criticism because his recommendations led to a widespread and problematic medical practice of sexual
reassignment for infants with ambiguous genitalia. Later gender scholars, to
varying degrees, have acknowledged a more complex construction of gender, with influences of genes, hormones, environment, and developmental
history. The development of gender identity and gender roles continues to
be a major focal point of sex research.
MODERN APPROACHES IN SEX RESEARCH
While early work in sex research relied primarily on narratives and on
description and classification, the modern study of sexual behavior is truly
interdisciplinary, cross-cutting many traditional fields and methodologies.
Those with a biological leaning focus on understanding the diverse factors
that predict variation in sexual behavior and reproductive patterns. This
includes integration of data on genetics, hormone physiology, neurobiology,
ecology, and demography. Those from a pure social science background
explore the cultural contexts of sexual behavior and how sex shapes and
is shaped by identity. These approaches meet in studies of sex from a
public health perspective, where investigators study the risk factors and
consequences for sexually-transmitted diseases, early/unwanted pregnancy,
sexual violence, and other socially-relevant problems.
I highlight here several specific disciplines at the forefront of modern sex
research:

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•

•

•

•

Evolutionary psychology seeks to understand how evolution has shaped
the brain and behavior. Substantial constituents of this field focus on
mate choice and the dynamics between mated partners. This field relies
on the principle that, even in modern contexts, deep-rooted behavioral
adaptations influence neuroanatomical structure and responses to
everyday situations in ways that, accumulated over time, can positively
impact fitness. These responses can be investigated by correlative
studies or elicited in response to stimuli in experimental paradigms.
Biological anthropologists also study sex within an adaptive framework,
asking what characteristics of sexual behavior typify our species, how
these characteristics differ from closely related species, through what
evolutionary processes these features came about, how ecological context impacts the expression of sexual behavior, and how behavioral patterns affect reproductive success.
Developmental psychologists study the impacts of biological mechanisms
and rearing environments on the development of sexual behavior, particularly sexual orientation, gender identity, and sex differences. Much
research focuses on transgendered individuals and those with intersex
conditions.
Gender studies scholars, variably represented in programs such as
women’s studies, sociocultural anthropology, gay/lesbian/bisexual/
transgender studies, sociology, and visual and literary arts, focus on the
cultural and biological contexts of gender, often with an eye towards
the ethical and political landscape. While some gender scholars decry
scientific approaches to gender, others interact closely with scientific
literature to negotiate the cultural construction of biological processes.
KEY QUESTIONS AND EMERGING TRENDS

MATE CHOICE
A primary topic of study of sexual behavior in any species is partner choice.
Following the predictions of sexual selection theory, females are predicted
to be relatively choosy, given their limited reproductive opportunities,
and to select features that reflect male genetic quality and/or willingness
to invest resources in offspring. Males are predicted to be less choosy for
sexual encounters, though perhaps more choosy in selection of a mate that
will be invested in over a longer term. In choice of a long-term partner,
males are expected to select signs of reproductive potential. Studies have
generally borne out these predictions. Studies of mate choice have primarily
been conducted in laboratory settings in which individuals are asked
to rate attractiveness of subjects or make a forced choice. These studies

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require careful design to assure that the variable of interest is isolated from
competing stimuli. This may involve, for example, computer manipulation
of images to alter specific facial features. Another common design assesses
the attractiveness of odors by asking raters to smell t-shirts worn by another
individual for several days under controlled conditions. Several consistent
features of mate choice have been reported:
•

•

•

Fluctuating Asymmetry (Gangestad, Thornhill, & Yeo, 1994). Asymmetries between the left and right halves of the body are generated by developmental instability, such as pathogen exposure. Both men and women
find photographs and odors of people with low asymmetry to be more
attractive.
Immunocompatibility (Havlicek & Roberts, 2009). Women demonstrate a
preference for the scents of partners with different profiles of the major
histocompatibility complex (MHC), a set of genes that regulate immune
response. By selecting partners with different MHC alleles from their
own (or rare alleles), females maximize the ability of their offspring to
respond to a variety of pathogens.
Sex-typical Features (Geary, Vigil, & Byrd-Craven, 2004). Men’s and
women’s faces show average differences in a number of key features
as a result of differential exposure to sex hormones. Males are reported
to find relatively feminized female faces (indicative of high estrogen
levels) more attractive. Females find relatively masculine male faces
most attractive, though this tendency varies with context. Women show
correlated tendencies to prefer other masculine, testosterone-dependent
features, such as deep voice, muscularity, and masculine displays. Men
are reported to find a combination of relatively small waist and large
hips most attractive in female partners.

Such studies have been criticized for focusing too closely on Western industrialized populations, and, in particular, on populations of college students
who may not be behaving with marriage, or reproduction, in mind (Henrich, Heine, & Norenzayan, 2010). Certainly, the effect sizes of such studies
are small, and the primacy of physical features over others (e.g., personality) has been exaggerated. However, it is important to note that the preferences outlined above have ample precedents in other species, thus their
appearance in humans should not be surprising. Nevertheless, it is important
to consider that humans experience substantial cultural differences in standards of beauty. Emerging research asks whether mate preferences exhibit
cross-cultural variation, and if so, whether this can be attributed to differences in ecology or reproductive practices. For example, a study of Tanzanian
hunter-gatherers found that the preference for symmetrical faces was even

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more pronounced than in European cultures, perhaps because pathogen vulnerability is more ecologically relevant (Little, Apicella, & Marlowe, 2007).
Moving forward, research on mate choice needs to address two longstanding issues. First, mate preferences in experimental studies may not
reflect actual choices given the availability of partners. Second, preferences
for particular characteristics have been identified in isolation from others.
Emerging research will assess how combinations of traits affect perceptions of attractiveness, the influence of individual rater characteristics on
preferences, the comparability between preferences and realized mate characteristics, the consistency of characteristics across successive mates, and
the effects of preferred versus nonpreferred characteristics on relationship
quality. Important questions might include whether a disparity between preferred and actual mate characteristics leads to a shorter relationship, greater
conflict, or a higher likelihood of infidelity. Longer term studies should
address whether such choices have demonstrable effects on fitness. For
example, recent evidence suggests that female body proportions preferred
by males are associated with higher fecundity (Jasienska, Ziomkiewicz,
Ellison, Lipson, & Thune, 2004).
CONCEALED OVULATION
Many primates display prominent advertisements of ovulation, often accompanied by a behavioral “estrus”, during which sexual behavior is enhanced.
By contrast, humans lack such an obvious estrous display, and sexual behavior can occur at any time, lacking a strict reliance on hormones. This contrast
generated a proliferation of arguments for the evolution of concealed ovulation in our species, many linking this phenomenon with the maintenance
of long-term pair bonds and paternal effort. Subsequent scholars criticized
that lack of visible estrus did not mean that ovulation was completely concealed and that “continuous” sexual receptivity did not preclude cyclic fluctuations in sexuality (Thornhill & Gangestad, 2008). Thus, emerging research
addresses these issues:
•

A large number of studies have examined frequency of sexual intercourse, masturbation, sexual fantasies, and sexual desire across the menstrual cycle. These studies provide frustratingly little consensus, perhaps
because non-hormonal factors, such as daily experiences and relationship fluctuations, can have a strong impact. Some recent studies indicate
that women may exhibit signs of enhanced receptivity, such as dressing more attractively or behaving more flirtatiously around the time of
ovulation (Haselton, Mortezaie, Pillsworth, Bleske-Rechek, & Frederick,
2007). Others, however, suggest that women may be more risk averse at
ovulation (Chavanne & Gallup, 1998).

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Several lines of evidence suggest men have some ability to distinguish
between women’s preovulatory and postovulatory cycle phases, either
through scent or in response to subtle changes in attractiveness (Haselton & Gildersleeve, 2011). Available data are not sufficient to suggest that
men can pinpoint ovulation, that they consciously attempt to do so, nor
that they can discern available cues outside of a controlled, experimental setting and in the absence of female behavioral cues. Newer research
directions will focus more directly on these questions.
A relatively new line of inquiry asks whether extended sexual receptivity coupled with subtle changes in attractiveness may allow women
to pursue a mixed mating strategy. The hypothesis is that women face
a tradeoff between two types of males, those that provide investment
(likely to be long-term partners) and those that provide high quality
genes (who may not be stable mates because of high appeal to other
females). Extended receptivity allows women to pursue relationships
with both types of males, while a subtle estrous period allows females
to attract the highest quality male at the time of conception (Thornhill &
Gangestad, 2008). Supporting evidence finds that women’s preferences
for particular male features (e.g., masculine features, symmetry) wax
and wane across the cycle. This line of research is controversial, yet consistent with models from other pair-bonded species and with a low but
non-negligible rate of cuckoldry in humans. This pattern sets up interesting lines of inquiry regarding sexual conflict. For example, men have
been found to display increased mate guarding towards their partners
near ovulation (Haselton & Gangestad, 2006).

The above research, dominated by scholars in evolutionary psychology,
is both compelling in its bulk and controversial in its implications. Like
studies of mate choice, far too many of these studies have relied on young,
well-nourished Western samples and warrant replication in cross-cultural
settings. Among the important considerations are that women in many
traditional cultures, closer in lifestyle to our evolutionary past, spend
the bulk of their lives pregnant and lactating. They experience very few
ovulatory menstrual cycles and frequently conceive while still nursing their
previous child, suggesting the existing literature could be unrepresentative
of hormonal conditions in our evolutionary past (Lancaster & Kaplan,
2009).
SEXUAL COERCION
Sexual coercion is a class of behaviors incorporating sexual harassment,
domestic violence, rape, and other behaviors which forcibly limit female

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behavior. These behaviors are of special importance and research difficulty
because of their public health impacts. One body of research related to sexual
coercion seeks to understand the extent of sexual violence and the personal
and contextual risk factors that predict it. The Centers for Disease Control
and World Health Organization have produced comprehensive reports
on sexual violence in the United States and globally. Other researchers
have produced survey results, primarily on college students (who are both
common victims and perpetrators), in addition to analysis of attributes
of convicted sex offenders. A second body of research acknowledges the
pervasiveness of sexual violence across cultures, and thus seeks to understand the biological roots of the behavior. This line of research focuses on
violence between intimate partners, comprising the majority of assaults.
This approach is rooted in sexual conflict theory and uses models of sexual
aggression in nonhuman species, including closely-related primates, in
generating and testing hypotheses about human sexual violence (Muller
& Wrangham, 2009). Both research directions must overcome considerable
difficulties with reporting bias, as victims are less likely to report sexual
violence than other types of crime, particularly in cases of acquaintance
and spousal violence. Many recent studies rely on paired surveys of men
and women and incorporate broader inquiries about sexual conflicts within
relationships.
A recent work entitled A Natural History of Rape (Thornhill & Palmer, 2000)
focused on the argument that human rape, like its analog in nonhuman
species, is a behavioral adaptation by which men with limited sexual access
attempt to gain fertilization opportunities. This text ignited considerable
controversy both for its ethical implications and its tenuous empirical
backing (Travis, 2003). However, it also generated renewed research interest
in the topic. A more recent hypothesis poses that rape and other forms
of sexual violence within relationships are tactics to protect the paternity
certainty of the male partner (Goetz, Shackelford, Romero, Kaighobadi, &
Miner, 2008). Similar punishment and intimidation tactics are utilized in
the context of mate guarding among our closest primate relatives (Emery
Thompson & Alvarado, 2012). A growing literature links intimate partner
violence with suspicions of infidelity and the use of other types of possessive
behaviors and suggest that predictors of rape are congruent with those for
other forms of sexual violence within relationships. An important outcome
of this work has been the push to recognize that biological underpinnings
and proximate motivations are independent yet potentially complementary
ways to explain such complex behaviors (Goetz et al., 2008). The hope is that
a better understanding of the evolutionary backdrop to sexual violence will
aid in identifying individual and contextual risk factors.

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HOMOSEXUALITY
While the early study of homosexual behavior fixated on questions of
prevalence and normality, it has since become clear that forms of homosexual behavior are common in non-human species and across human
cultures in widely varying environments. Emerging research has focused on
discerning the biological factors affecting predispositions to homosexuality.
Complementary work attempts to resolve how homosexual partnerships
differ from those of heterosexual partnerships. These lines of research merge
with a parallel literature on gender, attempting to identify correlations and
divergence of gender identity with sexual orientation.
Several lines of evidence support biological predispositions to homosexuality:
•

•

•

Genetics. Numerous studies report that close relatives of both male and
female homosexuals have a higher likelihood of also being homosexual
themselves (Bailey & Bell, 1993). Candidate genetic regions have been
identified, including one on the X chromosome (Xq28 region) which may
explain the high concordance of male homosexuality through the maternal line (Hu et al., 1995).
Intrauterine Effects. Birth order is one of the strongest known predictors
of male homosexuality, with the odds of homosexual identification
increasing by 33% for each older brother a man has (Blanchard, 2004).
It is hypothesized that this occurs because previous male fetuses prime
mothers to produce a specific immune response to an antigen found on
male cells (histocompatibility-Y, HY), leading to effects on brain development. M stress during gestation may have similar effects on neural
development, though this hypothesis is yet not strongly supported.
Specific effects that such fetal exposures may have on the brain are
also unclear. Early work had pointed to a region of the hypothalamus,
analogous to a part of the brain responsible for sexual behavior in
other species, which was larger in heterosexual men than in women
or homosexual men (LeVay & Hamer, 1994). This finding remains
controversial.
Hormonal Effects. A proliferation of studies have attempted to link
prenatal androgen or estrogen exposure to homosexual behavior
(Cohen-Bendahan, van de Beek, & Berenbaum, 2005). Many of these
use sexually-dimorphic characteristics, such as digit length ratios,
to estimate the uterine hormonal environment. These results do not
reach a uniform conclusion, perhaps because these measures are too
indirect. Other studies use individuals with known atypical hormonal
environments and provide support that prenatal androgen exposure
plays a contributing role in sexual orientation. Genetic males who are

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insensitive to androgens adopt the typical sexual behavior of females.
By contrast, genetic females with high uterine androgen exposure have
a higher than average likelihood of becoming homosexual or bisexual
(Meyer-Bahlburg, Dolezal, Baker, & New, 2008).
While there are compelling biological predictors of homosexuality, each single predictor explains only a fraction of the occurrence of homosexuality, suggesting that these factors each have only mild penetrance and/or that there
may be large environmental influences. Curiously, there is little research to
suggest which specific environmental features could play a role. Another
flaw to this work is that it has primarily relied on self-identified homosexual orientation (yes or no), when such identification may be stigmatizing and
when identification may not clearly be one or the other.
Some of the most intriguing new work focuses on the question of why, if
homosexual behavior does not lead offspring production, it has persisted
through evolutionary time. A prominent new hypothesis proposes that
certain genes or processes that promote homosexual behavior also influence
other traits that may, in fact, lead to high fitness outcomes. One replicated
finding is that homosexual men have female maternal relatives with high
fecundity (Camperio-Ciani, Corna, & Capiluppi, 2004). This suggests that
such a gene is maintained by a process known as sexually-antagonistic
coevolution, in which a gene that is evolutionary advantageous when
expressed in one sex is disadvantageous in the other. An alternative, but not
mutually-exclusive hypothesis, is that incomplete penetrance of a so-called
“gay” gene could lead to high reproductive success. For example, psychologically masculine females or feminine men are less likely to identify as
heterosexual than their peers, but when they do, have a higher than average
number of sexual partners (Zietsch et al., 2008).
CONCLUSION
The modern study of sexual behavior targets diverse aspects of sexuality,
including partner preference, the dynamics between mated pairs, and
the causes and consequences of nontraditional sexual behavior or gender
identity. Cutting edge research is not merely descriptive but investigates
the roles of biological and psychological mechanisms in shaping behavior,
in addition to asking broader questions about the functions of the sexual
behaviors observed in our species. Modern interdisciplinary perspectives
are now better equipped to balance the biological and cultural aspects of sexuality and address the confluence of multiple developmental influences on
behavior.

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Havlicek, J., & Roberts, S. C. (2009). MHC-correlated mate choice in humans: A
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Henrich, J., Heine, S., & Norenzayan, A. (2010). The weirdest people in the world?
The Behavioral and Brain Sciences, 33, 61–83. doi:10.1017/s0140525x0999152x
Hu, S., Pattatucci, A. M. L., Patterson, C., Li, L., Fulker, D. W., Cherny, S. S., … ,
Hamer, D. H. (1995). Linkage between sexual orientation and chromosome Xq28
in males but not in females. Nature Genetics, 11, 248–256.
Jasienska, G., Ziomkiewicz, A., Ellison, P. T., Lipson, S. F., & Thune, I. (2004). Large
breasts and narrow waists indicate high reproductive potential in women. Proceedings of the Royal Society of London B, 271, 1213–1217.
Kinsey, A. C., Pomeroy, W. B., & Martin, C. E. (1948). Sexual behavior in the human
male. Philadelphia, PA: W.B. Saunders.
Kinsey, A. C., Pomeroy, W. B., Martin, C. E., & Gebhard, P. H. (1953). Sexual behavior
in the human female. Philadelphia, PA: W.B. Saunders.
Lancaster, J. B., & Kaplan, H. S. (2009). The endocrinology of the human adaptive
complex. In P. T. Ellison & P. B. Gray (Eds.), Endocrinology of social relationships (pp.
95–118). Cambridge, MA: Harvard University Press.
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LeVay, S., & Hamer, D. H. (1994). Evidence for a biological influence in male homosexuality. Scientific American, 270, 44–49.
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in human faces in two cultures: data from the UK and the Hadza, an isolated
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3113–3117. doi:10.1098/rspb.2007.0895
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Meyer-Bahlburg, H. L., Dolezal, C., Baker, S., & New, M. (2008). Sexual orientation
in women with classical or non-classical congenital adrenal hyperplasia as a function of degree of prenatal androgen excess. Archives of Sexual Behavior, 37, 85–99.
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Muller, M. N., & Wrangham, R. W. (Eds.) (2009). Sexual coercion in primates: An evolutionary perspective on male aggression Against Females. Cambridge, MA: Harvard
University Press.
Parker, G. A. (1979). Sexual selection and sexual conflict. In M. S. Blum & N. A. Blum
(Eds.), Sexual selection and reproductive competition (pp. 123–166). New York, NY:
Academic Press.
Thornhill, R., & Gangestad, S. (2008). The evolutionary biology of human female sexuality.
New York, NY: Oxford University Press.
Thornhill, R., & Palmer, C. T. (2000). A natural history of rape: Biological bases of sexual
coercion. Cambridge, MA: The MIT Press.
Travis, C. B. (Ed.) (2003). Evolution, gender, and rape. Cambridge, MA: MIT Press.
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Zietsch, B. P., Morley, K. I., Shekar, S. N., Verweij, K. J. H., Keller, M. C., Macgregor, S., … Martin, N. G. (2008). Genetic factors predisposing to homosexuality
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424–433. doi:10.1016/j.evolhumbehav.2008.07.002

MELISSA EMERY THOMPSON SHORT BIOGRAPHY
Melissa Emery Thompson received her PhD in biological anthropology from
Harvard University in 2005 and is currently Assistant Professor of Anthropology at the University of New Mexico. Her work combines field and laboratory approaches to study various dimensions of the reproductive biology
and behavior of humans and nonhuman primates. She has published papers
on diverse topics relating to this subject, including reproductive ecology, sexual signaling, mate preferences, reproductive competition, sexual coercion,
sex differences in sociality, and life history evolution.
http://www.unm.edu/∼memery
http://kibalechimpanzees.wordpress.com
RELATED ESSAYS
The Sexual Division of Labor (Anthropology), Rebecca Bliege Bird and Brian
F. Codding
Cooperative Breeding and Human Evolution (Anthropology), Karen L.
Kramer
Patterns of Attachment across the Lifespan (Psychology), Robyn Fivush and
Theodore E. A. Waters
Controlling the Influence of Stereotypes on One’s Thoughts (Psychology),
Patrick S. Forscher and Patricia G. Devine
Family Formation in Times of Labor Market Insecurities (Sociology), Johannes
Huinink
The Neurobiology and Physiology of Emotions: A Developmental Perspective (Psychology), Sarah S. Kahle and Paul D. Hastings
The Future of Marriage (Sociology), Elizabeth Aura McClintock
Stereotype Threat (Psychology), Toni Schmader and William M. Hall
Social Neuroendocrine Approaches to Relationships (Anthropology), Sari M.
van Anders and Peter B. Gray



Sexual Behavior
MELISSA EMERY THOMPSON

Abstract
Research on human sexual behavior is a multidisciplinary pursuit that seeks to
understand one of the most vital and complex aspects of our biology. Foundations
for this study include the basic principles of sexual selection, including differential
reproductive roles of male and female, and the processes of sexual differentiation.
Study of human sexual behavior is made vastly more complex by emotional
involvements and the diversity of sexual behaviors exhibited by our species.
Historically, sex researchers have struggled to overcome the methodological
challenges involved with objective study of behavior in our own species and with
the ethical and political implications of their work. Early research focused on
merely quantifying the spectrum of human sexual behaviors and on understanding
the physiological and psychological processes of sexual response. Subsequent
work developed the concept of gender and began to address how behavioral and
corporeal manifestations of sex can diverge from one another. Modern studies
incorporate a variety of advanced scientific techniques to investigate mechanistic
and functional hypotheses for specific behaviors. This review highlights four
prominent research topics, highlighting current understanding, cutting edge work,
and key issues for future research: mate preferences, concealed ovulation, sexual
coercion, and homosexuality. In each of these areas of research, there is strong
evidence for biological influences on behavior. It is also clear that known biological
mechanisms only partly explain actual behavioral patterns, suggesting strong
mediation by cultural, environmental, and developmental processes.

INTRODUCTION
Sexual behavior is central to the biology of a species, being vital to how
the species persists and changes over time. The study of sexual behavior in
humans has produced particularly intense research effort, perhaps because
of the unusual diversity of sexual behavior in our species and the central role
sex plays in health, wellbeing, and popular culture. These factors magnify
the complexity of gaining an objective understanding of sexual behavior in
humans. Thus, at every stage in its history, scientific research on sex has been
fraught with controversy for reasons ranging from inadequacy of sampling
methods to ethical concerns to wider social implications. Sex research has
Emerging Trends in the Social and Behavioral Sciences. Edited by Robert Scott and Stephen Kosslyn.
© 2015 John Wiley & Sons, Inc. ISBN 978-1-118-90077-2.

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EMERGING TRENDS IN THE SOCIAL AND BEHAVIORAL SCIENCES

both been shaped by and has steered social and political attitudes. As in any
science, the field has built incrementally on its successes and failures, gradually incorporating new techniques and theoretical approaches. A field that
began by simply attempting to catalog and describe the nature of sex in our
species has moved forward into a vastly interdisciplinary research area incorporating advanced scientific methodologies such as genetics, neuroscience,
and endocrinology, while acknowledging the complex interactions between
culture, environment, experience, and biology in shaping behavior.
The study of human sexual behavior is prominently represented in several international research organizations, including the Society for the Scientific Study of Sexuality, the International Academy of Sex Research, the
Society for Sex Therapy and Research, the Human Behavior and Evolution
Society, and the American Sociological Association. In addition to targeted
journals, such as the Journal of Sex Research, Archives of Sexual Behavior, and Sex
Roles, research on sexual behavior figures prominently in broader journals in
anthropology, psychology, sociology, and biology.
The following review provides a background to research in the field of
sexual behavior, including historical perspectives, prominent approaches to
modern studies of sex, and a highlight of four particularly active research
areas. It should be noted that this is by no means an exhaustive treatment
of sex research. Per the author’s expertise, this review highlights research
driven by or intersecting with a biological approach. For further discussion
of related topics, refer to entries on gender in the Sociology series and on sex
differences in behavior in the Anthropology series.
FOUNDATIONS OF SEXUAL BEHAVIOR
SEXUAL SELECTION
Foundations of sexual behavior in humans, as in other animals, stem from the
core biological definition of male and female sex. Females are defined as the
high-investment sex, demonstrated initially by a relatively large gamete (the
ovum, or egg) containing the nourishment for early growth of the zygote.
In mammals, where females bear the burden of pregnancy and lactation, the
inequality in investment is particularly great. Thus, females and the investment they provide become a limiting resource for males who must compete for opportunities to fertilize them (Bateman, 1948). This has far-reaching
implications for sexual behavior, as outlined in Darwin’s presentation of sexual selection (Darwin, 1871) and its many subsequent elaborations. Males are
generally expected to compete amongst themselves for access to females, a
process that can shape behavior (e.g., aggression), morphology (e.g., body
size, sexual ornaments), and physiology (e.g., sperm production). Males also

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have the potential to produce offspring at a faster rate than females, often
leading to relatively promiscuous mating behavior and a reluctance to invest
too heavily in one mate when other opportunities are available. Females, on
the other hand, are expected to be more conservative in their selection of
mates, choosing males that are willing to provide an investment of resources
or who have genetic traits that benefit offspring viability (Trivers, 1985). The
difference in reproductive interests between the two sexes can create sexual
conflict in which the strategies used by one sex cause negative effects for the
other (Parker, 1979). Sexual coercion is a prominent outcome of sexual conflict in which males (typically) use force or the threat of force to circumvent
female mate choice.
The above patterns are generalizations, and different species manifest them
in different ways and to differing degrees. The human species presents a
number of complications. Chiefly, the high cost of rearing human infants
is thought to necessitate significant investment by fathers, often leading to
long-term partnerships at the expense of mate-seeking behavior (Lancaster,
J. B., & Lancaster, C. S., 1983). Whereas ostentatious displays and ornaments
such as colorful feathers are more typical of males in other species, human
females in most cultures use adornments to advertise beauty, youth, or fecundity to potential mates, suggesting that males’ investment may compel them
to be more selective. Humans display greater variability in sexual practices
and more frequent nonconceptive sex than is typical of animals in nature.
In addition, variation in cultural practices and the involvement of complex
emotional and cognitive processes shape norms of sexual behavior.
SEXUAL DIFFERENTIATION
Biological research into human sexuality often references the developmental
origins of male and female and persisting differences in the hormonal physiology of the two sexes. Initial sex determination is dependent on whether an
individual receives an X (female) or a Y (male) chromosome from the father.
The presence of a functional Y chromosome leads to maturation of the testes
from an undifferentiated fetal gonad. Once the testes develop, they produce
the hormone testosterone which, directly or indirectly, guides the maturation of the remainder of the male physical phenotype. Females’ gonads and
external genitalia derive from the same embryonic structures as males’ in
the absence of the Y chromosome and testosterone. This is an oversimplification of a process that, in fact, can be altered at various stages. The influence
of these developmental processes on sexual behavior is far less clear-cut.
Testosterone impacts male sexual behavior, as can be seen from the effects
of castration in many species. However, it can have both “organizational”

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EMERGING TRENDS IN THE SOCIAL AND BEHAVIORAL SCIENCES

effects, priming the neuroanatomical substrates at an early age of development, and “activational” effects, interacting with these substrates to produce
behavioral responses in the mature organism. Testosterone may have important influences on the sexual behavior of females, as well, and it is known to
have complex interactions with other hormones in both sexes. Females experience a cyclical fluctuation in the hormones progesterone and estradiol that
accompanies the process of ovulation and menstruation. It is clear that sexual behavior in the human female is less strictly governed by these hormonal
fluctuations than is the case for most other species.
BRIEF HISTORY OF SEX RESEARCH
The modern study of human sexual behavior can be traced to Alfred Kinsey,
founder of what is now the Kinsey Institute for Research in Sex, Gender, and
Reproduction, and lead author of the Kinsey Reports on Sexual Behavior in
the Human Male (Kinsey, Pomeroy, & Martin, 1948) and Sexual Behavior in the
Human Female (Kinsey, Pomeroy, Martin, & Gebhard, 1953), among the first
attempts to understand human sexual behavior from a scientific perspective.
Kinsey’s work was accomplished primarily through extensive interviews
about sexual experiences and had the overarching goal of documenting the
scope of human sexual behavior, as well as the frequency and sociocultural
predictors of particular behaviors. Because his work emphasized the diversity of sexual experience, it was an important attempt to remove stigmas
not only from the examination of sexual behavior but from the behaviors
themselves. In particular, Kinsey documented the frequency of homosexual
behavior, arguing that many individuals are not easily classified as either
exclusively homosexual or exclusively heterosexual. While documenting
differences in sexual drive between males and females, Kinsey reported a
greater breadth and investment in sexual experience among women than was
publicly recognized. These features made his research highly controversial
at the time of publication. Though Kinsey’s works are still highly regarded,
they are criticized for methodological issues, such as reliance on self-reports,
self-selection of individuals willing to discuss their experiences openly, and
overrepresentation of gay subjects, sex workers, and sex offenders.
Beginning in the late 1950s, William Masters and Virginia Johnson extended
the study of sexual behavior by documenting patterns of human sexual
arousal and response (Masters & Johnson, 1966). In contrast to Kinsey’s
interview approach, Masters and Johnson conducted their research in a
laboratory setting, monitoring physiological responses during masturbation
or intercourse. Their work led to the recognition and description of the four
phases of sexual response (excitement, plateau, orgasm, and resolution)
and of multiple orgasmic potential in women. In addition, Masters and

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Johnson targeted the assessment and treatment of sexual dysfunction,
linking psychological and sexual health. However, by attempting to define
“normal” versus “dysfunctional” behavior, their research moved away from
Kinsey’s emphasis on the diversity and individuality of sexual experience.
This included their controversial treatment of homosexuality as a sexual
dysfunction, one which they attempted to cure through conversion therapy.
Such claims contributed to the maintenance of homosexuality in the American Psychiatric Association’s Diagnostic and Statistical Manual of Mental
Disorders until 1973.
After such intense scrutiny on the mechanics of sex, later attention focused
on sexual identity. John Money redefined the term gender, and associated
terms such as gender role, to highlight the distinction of biological sex from
sex-typical behavior (Money & Ehrhardt, 1996). At the time, Money’s view
that gender was learned rather than innate was widely accepted, meeting
agreement with a growing field of feminist scholarship that sought to reject
ideas about the biological determination of gender roles. While Money’s
work was ground-breaking in highlighting the flexibility in behavioral
manifestations of sex, he later earned fierce criticism because his recommendations led to a widespread and problematic medical practice of sexual
reassignment for infants with ambiguous genitalia. Later gender scholars, to
varying degrees, have acknowledged a more complex construction of gender, with influences of genes, hormones, environment, and developmental
history. The development of gender identity and gender roles continues to
be a major focal point of sex research.
MODERN APPROACHES IN SEX RESEARCH
While early work in sex research relied primarily on narratives and on
description and classification, the modern study of sexual behavior is truly
interdisciplinary, cross-cutting many traditional fields and methodologies.
Those with a biological leaning focus on understanding the diverse factors
that predict variation in sexual behavior and reproductive patterns. This
includes integration of data on genetics, hormone physiology, neurobiology,
ecology, and demography. Those from a pure social science background
explore the cultural contexts of sexual behavior and how sex shapes and
is shaped by identity. These approaches meet in studies of sex from a
public health perspective, where investigators study the risk factors and
consequences for sexually-transmitted diseases, early/unwanted pregnancy,
sexual violence, and other socially-relevant problems.
I highlight here several specific disciplines at the forefront of modern sex
research:

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EMERGING TRENDS IN THE SOCIAL AND BEHAVIORAL SCIENCES

•

•

•

•

Evolutionary psychology seeks to understand how evolution has shaped
the brain and behavior. Substantial constituents of this field focus on
mate choice and the dynamics between mated partners. This field relies
on the principle that, even in modern contexts, deep-rooted behavioral
adaptations influence neuroanatomical structure and responses to
everyday situations in ways that, accumulated over time, can positively
impact fitness. These responses can be investigated by correlative
studies or elicited in response to stimuli in experimental paradigms.
Biological anthropologists also study sex within an adaptive framework,
asking what characteristics of sexual behavior typify our species, how
these characteristics differ from closely related species, through what
evolutionary processes these features came about, how ecological context impacts the expression of sexual behavior, and how behavioral patterns affect reproductive success.
Developmental psychologists study the impacts of biological mechanisms
and rearing environments on the development of sexual behavior, particularly sexual orientation, gender identity, and sex differences. Much
research focuses on transgendered individuals and those with intersex
conditions.
Gender studies scholars, variably represented in programs such as
women’s studies, sociocultural anthropology, gay/lesbian/bisexual/
transgender studies, sociology, and visual and literary arts, focus on the
cultural and biological contexts of gender, often with an eye towards
the ethical and political landscape. While some gender scholars decry
scientific approaches to gender, others interact closely with scientific
literature to negotiate the cultural construction of biological processes.
KEY QUESTIONS AND EMERGING TRENDS

MATE CHOICE
A primary topic of study of sexual behavior in any species is partner choice.
Following the predictions of sexual selection theory, females are predicted
to be relatively choosy, given their limited reproductive opportunities,
and to select features that reflect male genetic quality and/or willingness
to invest resources in offspring. Males are predicted to be less choosy for
sexual encounters, though perhaps more choosy in selection of a mate that
will be invested in over a longer term. In choice of a long-term partner,
males are expected to select signs of reproductive potential. Studies have
generally borne out these predictions. Studies of mate choice have primarily
been conducted in laboratory settings in which individuals are asked
to rate attractiveness of subjects or make a forced choice. These studies

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require careful design to assure that the variable of interest is isolated from
competing stimuli. This may involve, for example, computer manipulation
of images to alter specific facial features. Another common design assesses
the attractiveness of odors by asking raters to smell t-shirts worn by another
individual for several days under controlled conditions. Several consistent
features of mate choice have been reported:
•

•

•

Fluctuating Asymmetry (Gangestad, Thornhill, & Yeo, 1994). Asymmetries between the left and right halves of the body are generated by developmental instability, such as pathogen exposure. Both men and women
find photographs and odors of people with low asymmetry to be more
attractive.
Immunocompatibility (Havlicek & Roberts, 2009). Women demonstrate a
preference for the scents of partners with different profiles of the major
histocompatibility complex (MHC), a set of genes that regulate immune
response. By selecting partners with different MHC alleles from their
own (or rare alleles), females maximize the ability of their offspring to
respond to a variety of pathogens.
Sex-typical Features (Geary, Vigil, & Byrd-Craven, 2004). Men’s and
women’s faces show average differences in a number of key features
as a result of differential exposure to sex hormones. Males are reported
to find relatively feminized female faces (indicative of high estrogen
levels) more attractive. Females find relatively masculine male faces
most attractive, though this tendency varies with context. Women show
correlated tendencies to prefer other masculine, testosterone-dependent
features, such as deep voice, muscularity, and masculine displays. Men
are reported to find a combination of relatively small waist and large
hips most attractive in female partners.

Such studies have been criticized for focusing too closely on Western industrialized populations, and, in particular, on populations of college students
who may not be behaving with marriage, or reproduction, in mind (Henrich, Heine, & Norenzayan, 2010). Certainly, the effect sizes of such studies
are small, and the primacy of physical features over others (e.g., personality) has been exaggerated. However, it is important to note that the preferences outlined above have ample precedents in other species, thus their
appearance in humans should not be surprising. Nevertheless, it is important
to consider that humans experience substantial cultural differences in standards of beauty. Emerging research asks whether mate preferences exhibit
cross-cultural variation, and if so, whether this can be attributed to differences in ecology or reproductive practices. For example, a study of Tanzanian
hunter-gatherers found that the preference for symmetrical faces was even

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EMERGING TRENDS IN THE SOCIAL AND BEHAVIORAL SCIENCES

more pronounced than in European cultures, perhaps because pathogen vulnerability is more ecologically relevant (Little, Apicella, & Marlowe, 2007).
Moving forward, research on mate choice needs to address two longstanding issues. First, mate preferences in experimental studies may not
reflect actual choices given the availability of partners. Second, preferences
for particular characteristics have been identified in isolation from others.
Emerging research will assess how combinations of traits affect perceptions of attractiveness, the influence of individual rater characteristics on
preferences, the comparability between preferences and realized mate characteristics, the consistency of characteristics across successive mates, and
the effects of preferred versus nonpreferred characteristics on relationship
quality. Important questions might include whether a disparity between preferred and actual mate characteristics leads to a shorter relationship, greater
conflict, or a higher likelihood of infidelity. Longer term studies should
address whether such choices have demonstrable effects on fitness. For
example, recent evidence suggests that female body proportions preferred
by males are associated with higher fecundity (Jasienska, Ziomkiewicz,
Ellison, Lipson, & Thune, 2004).
CONCEALED OVULATION
Many primates display prominent advertisements of ovulation, often accompanied by a behavioral “estrus”, during which sexual behavior is enhanced.
By contrast, humans lack such an obvious estrous display, and sexual behavior can occur at any time, lacking a strict reliance on hormones. This contrast
generated a proliferation of arguments for the evolution of concealed ovulation in our species, many linking this phenomenon with the maintenance
of long-term pair bonds and paternal effort. Subsequent scholars criticized
that lack of visible estrus did not mean that ovulation was completely concealed and that “continuous” sexual receptivity did not preclude cyclic fluctuations in sexuality (Thornhill & Gangestad, 2008). Thus, emerging research
addresses these issues:
•

A large number of studies have examined frequency of sexual intercourse, masturbation, sexual fantasies, and sexual desire across the menstrual cycle. These studies provide frustratingly little consensus, perhaps
because non-hormonal factors, such as daily experiences and relationship fluctuations, can have a strong impact. Some recent studies indicate
that women may exhibit signs of enhanced receptivity, such as dressing more attractively or behaving more flirtatiously around the time of
ovulation (Haselton, Mortezaie, Pillsworth, Bleske-Rechek, & Frederick,
2007). Others, however, suggest that women may be more risk averse at
ovulation (Chavanne & Gallup, 1998).

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•

•

9

Several lines of evidence suggest men have some ability to distinguish
between women’s preovulatory and postovulatory cycle phases, either
through scent or in response to subtle changes in attractiveness (Haselton & Gildersleeve, 2011). Available data are not sufficient to suggest that
men can pinpoint ovulation, that they consciously attempt to do so, nor
that they can discern available cues outside of a controlled, experimental setting and in the absence of female behavioral cues. Newer research
directions will focus more directly on these questions.
A relatively new line of inquiry asks whether extended sexual receptivity coupled with subtle changes in attractiveness may allow women
to pursue a mixed mating strategy. The hypothesis is that women face
a tradeoff between two types of males, those that provide investment
(likely to be long-term partners) and those that provide high quality
genes (who may not be stable mates because of high appeal to other
females). Extended receptivity allows women to pursue relationships
with both types of males, while a subtle estrous period allows females
to attract the highest quality male at the time of conception (Thornhill &
Gangestad, 2008). Supporting evidence finds that women’s preferences
for particular male features (e.g., masculine features, symmetry) wax
and wane across the cycle. This line of research is controversial, yet consistent with models from other pair-bonded species and with a low but
non-negligible rate of cuckoldry in humans. This pattern sets up interesting lines of inquiry regarding sexual conflict. For example, men have
been found to display increased mate guarding towards their partners
near ovulation (Haselton & Gangestad, 2006).

The above research, dominated by scholars in evolutionary psychology,
is both compelling in its bulk and controversial in its implications. Like
studies of mate choice, far too many of these studies have relied on young,
well-nourished Western samples and warrant replication in cross-cultural
settings. Among the important considerations are that women in many
traditional cultures, closer in lifestyle to our evolutionary past, spend
the bulk of their lives pregnant and lactating. They experience very few
ovulatory menstrual cycles and frequently conceive while still nursing their
previous child, suggesting the existing literature could be unrepresentative
of hormonal conditions in our evolutionary past (Lancaster & Kaplan,
2009).
SEXUAL COERCION
Sexual coercion is a class of behaviors incorporating sexual harassment,
domestic violence, rape, and other behaviors which forcibly limit female

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EMERGING TRENDS IN THE SOCIAL AND BEHAVIORAL SCIENCES

behavior. These behaviors are of special importance and research difficulty
because of their public health impacts. One body of research related to sexual
coercion seeks to understand the extent of sexual violence and the personal
and contextual risk factors that predict it. The Centers for Disease Control
and World Health Organization have produced comprehensive reports
on sexual violence in the United States and globally. Other researchers
have produced survey results, primarily on college students (who are both
common victims and perpetrators), in addition to analysis of attributes
of convicted sex offenders. A second body of research acknowledges the
pervasiveness of sexual violence across cultures, and thus seeks to understand the biological roots of the behavior. This line of research focuses on
violence between intimate partners, comprising the majority of assaults.
This approach is rooted in sexual conflict theory and uses models of sexual
aggression in nonhuman species, including closely-related primates, in
generating and testing hypotheses about human sexual violence (Muller
& Wrangham, 2009). Both research directions must overcome considerable
difficulties with reporting bias, as victims are less likely to report sexual
violence than other types of crime, particularly in cases of acquaintance
and spousal violence. Many recent studies rely on paired surveys of men
and women and incorporate broader inquiries about sexual conflicts within
relationships.
A recent work entitled A Natural History of Rape (Thornhill & Palmer, 2000)
focused on the argument that human rape, like its analog in nonhuman
species, is a behavioral adaptation by which men with limited sexual access
attempt to gain fertilization opportunities. This text ignited considerable
controversy both for its ethical implications and its tenuous empirical
backing (Travis, 2003). However, it also generated renewed research interest
in the topic. A more recent hypothesis poses that rape and other forms
of sexual violence within relationships are tactics to protect the paternity
certainty of the male partner (Goetz, Shackelford, Romero, Kaighobadi, &
Miner, 2008). Similar punishment and intimidation tactics are utilized in
the context of mate guarding among our closest primate relatives (Emery
Thompson & Alvarado, 2012). A growing literature links intimate partner
violence with suspicions of infidelity and the use of other types of possessive
behaviors and suggest that predictors of rape are congruent with those for
other forms of sexual violence within relationships. An important outcome
of this work has been the push to recognize that biological underpinnings
and proximate motivations are independent yet potentially complementary
ways to explain such complex behaviors (Goetz et al., 2008). The hope is that
a better understanding of the evolutionary backdrop to sexual violence will
aid in identifying individual and contextual risk factors.

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HOMOSEXUALITY
While the early study of homosexual behavior fixated on questions of
prevalence and normality, it has since become clear that forms of homosexual behavior are common in non-human species and across human
cultures in widely varying environments. Emerging research has focused on
discerning the biological factors affecting predispositions to homosexuality.
Complementary work attempts to resolve how homosexual partnerships
differ from those of heterosexual partnerships. These lines of research merge
with a parallel literature on gender, attempting to identify correlations and
divergence of gender identity with sexual orientation.
Several lines of evidence support biological predispositions to homosexuality:
•

•

•

Genetics. Numerous studies report that close relatives of both male and
female homosexuals have a higher likelihood of also being homosexual
themselves (Bailey & Bell, 1993). Candidate genetic regions have been
identified, including one on the X chromosome (Xq28 region) which may
explain the high concordance of male homosexuality through the maternal line (Hu et al., 1995).
Intrauterine Effects. Birth order is one of the strongest known predictors
of male homosexuality, with the odds of homosexual identification
increasing by 33% for each older brother a man has (Blanchard, 2004).
It is hypothesized that this occurs because previous male fetuses prime
mothers to produce a specific immune response to an antigen found on
male cells (histocompatibility-Y, HY), leading to effects on brain development. M stress during gestation may have similar effects on neural
development, though this hypothesis is yet not strongly supported.
Specific effects that such fetal exposures may have on the brain are
also unclear. Early work had pointed to a region of the hypothalamus,
analogous to a part of the brain responsible for sexual behavior in
other species, which was larger in heterosexual men than in women
or homosexual men (LeVay & Hamer, 1994). This finding remains
controversial.
Hormonal Effects. A proliferation of studies have attempted to link
prenatal androgen or estrogen exposure to homosexual behavior
(Cohen-Bendahan, van de Beek, & Berenbaum, 2005). Many of these
use sexually-dimorphic characteristics, such as digit length ratios,
to estimate the uterine hormonal environment. These results do not
reach a uniform conclusion, perhaps because these measures are too
indirect. Other studies use individuals with known atypical hormonal
environments and provide support that prenatal androgen exposure
plays a contributing role in sexual orientation. Genetic males who are

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EMERGING TRENDS IN THE SOCIAL AND BEHAVIORAL SCIENCES

insensitive to androgens adopt the typical sexual behavior of females.
By contrast, genetic females with high uterine androgen exposure have
a higher than average likelihood of becoming homosexual or bisexual
(Meyer-Bahlburg, Dolezal, Baker, & New, 2008).
While there are compelling biological predictors of homosexuality, each single predictor explains only a fraction of the occurrence of homosexuality, suggesting that these factors each have only mild penetrance and/or that there
may be large environmental influences. Curiously, there is little research to
suggest which specific environmental features could play a role. Another
flaw to this work is that it has primarily relied on self-identified homosexual orientation (yes or no), when such identification may be stigmatizing and
when identification may not clearly be one or the other.
Some of the most intriguing new work focuses on the question of why, if
homosexual behavior does not lead offspring production, it has persisted
through evolutionary time. A prominent new hypothesis proposes that
certain genes or processes that promote homosexual behavior also influence
other traits that may, in fact, lead to high fitness outcomes. One replicated
finding is that homosexual men have female maternal relatives with high
fecundity (Camperio-Ciani, Corna, & Capiluppi, 2004). This suggests that
such a gene is maintained by a process known as sexually-antagonistic
coevolution, in which a gene that is evolutionary advantageous when
expressed in one sex is disadvantageous in the other. An alternative, but not
mutually-exclusive hypothesis, is that incomplete penetrance of a so-called
“gay” gene could lead to high reproductive success. For example, psychologically masculine females or feminine men are less likely to identify as
heterosexual than their peers, but when they do, have a higher than average
number of sexual partners (Zietsch et al., 2008).
CONCLUSION
The modern study of sexual behavior targets diverse aspects of sexuality,
including partner preference, the dynamics between mated pairs, and
the causes and consequences of nontraditional sexual behavior or gender
identity. Cutting edge research is not merely descriptive but investigates
the roles of biological and psychological mechanisms in shaping behavior,
in addition to asking broader questions about the functions of the sexual
behaviors observed in our species. Modern interdisciplinary perspectives
are now better equipped to balance the biological and cultural aspects of sexuality and address the confluence of multiple developmental influences on
behavior.

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13

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Zietsch, B. P., Morley, K. I., Shekar, S. N., Verweij, K. J. H., Keller, M. C., Macgregor, S., … Martin, N. G. (2008). Genetic factors predisposing to homosexuality
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MELISSA EMERY THOMPSON SHORT BIOGRAPHY
Melissa Emery Thompson received her PhD in biological anthropology from
Harvard University in 2005 and is currently Assistant Professor of Anthropology at the University of New Mexico. Her work combines field and laboratory approaches to study various dimensions of the reproductive biology
and behavior of humans and nonhuman primates. She has published papers
on diverse topics relating to this subject, including reproductive ecology, sexual signaling, mate preferences, reproductive competition, sexual coercion,
sex differences in sociality, and life history evolution.
http://www.unm.edu/∼memery
http://kibalechimpanzees.wordpress.com
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