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S
Sexual Orientation and Human
Sexuality
Jaroslava Varella Valentova and Marco Antonio
Correa Varella
Department of Experimental Psychology, Institute
of Psychology, University of São Paulo, Cidade
Universitária São Paulo, SP, Brazil
Synonyms
Bisexuality;Heterosexuality;Homosexuality;
Same-sex sexuality;Sexual attraction;Sexual
minorities;Sexual preference
Definition
Sexual orientation refers to a psychological mech-
anism directing one’s sexuality towards individ-
uals based on their apparent sex. Individuals form
a continuum between extreme point of exclusive
heterosexuals, i.e., oriented exclusively towards
individuals of the opposite sex, and exclusive
homosexuals, i.e., oriented exclusively towards
individuals of the same sex, with bisexuals being
similarly attracted to both sexes and other individ-
uals being attracted to both sexes in various
degrees (e.g., predominantly heterosexual with
some potential to same-sex attraction). Sexual
orientation refers more to attraction or desire
than behavior or identity, because orientation is
not necessarily manifested by overt sexual behav-
ior, associated with self-identification of sexual
orientation label, or aligned with affectional bond-
ing (love).
Introduction
The majority of males and females predominantly
prefer opposite-sex sexual and/or romantic part-
ners. Such sexuality has been accepted as a stan-
dard default, possibly because it is easier to see its
biological/evolutionary relevance. Indeed, in sex-
ually reproducing species, preference for and sex-
ual activities with opposite-sex partners evolved
as a mechanism for combining genomes through
complementary gametes gaining genetic variabil-
ity and direct fitness. Thus, scholars have argued
that homosexuality is an evolutionary puzzle
because it impedes the reproductive success of
their owners. However, on the phylogenetic
level, evolutionary fitness is more than direct fit-
ness, and on the ontogenetic level, sexuality is
more than fertile heterosexual penile-vaginal
intercourse. Although direct reproduction is the
ultimate evolutionary force behind most sexual
activities, sexuality in general gained many other
proximate functions during its evolution, e.g., as a
social and pair bonding mechanism or means of
resource acquisition. Such functions are interme-
diate goals to achieve direct reproduction, e.g., by
means of survival and alliance formation and/or
indirect reproduction by helping kin, also known
#Springer International Publishing AG 2016
T.K. Shackelford, V.A. Weekes-Shackelford (eds.), Encyclopedia of Evolutionary Psychological Science,
DOI 10.1007/978-3-319-16999-6_3622-1
as inclusive fitness. Therefore, many forms of
sexuality, such as oral or anal sex, masturbation,
sexual preferences for different species, prefer-
ences of same-sex individuals, or individuals out-
side of reproductive age, cannot per se offer direct
reproductive success of the individual, but still
these nonfertile forms of sexuality can offer
other adaptive sociosexual functions (e.g., pair
bonding, alliance formation, resource acquisition,
well-being, etc.) that might indirectly foster future
reproduction, regardless of sexual orientation. It is
important to note that personal motivations for
sexual activities, homosexual or heterosexual
ones, are quite different from their possible socio-
sexual functions or possible evolutionary adaptive
values. Sexual relations are mostly motivated by
pleasure, love, self-esteem/revenge, and/or
obtaining resources. Such personal motivations
triggering homo/heterosexual behavior should
not be confused with the socioecological or evo-
lutionary reasons that maintain sexual activity in
the population.
Further, sexual orientation is a psychological
mechanism that generates a continuous array of
individual variation and not a dichotomous psy-
chological trait (e.g., Kinsey et al. 1948; Petterson
et al. 2015). In this sense, exclusive homosexual-
ity and exclusive heterosexuality present the
extreme opposite poles of the continuum, while
the majority of individuals fall in between,
although closer to the heterosexual extreme. Fol-
lowing this line, homosexuality or nonhetero-
sexuality (including bisexuals) is more than
exclusive homosexuality, and sexual orientation
does not equal homosexuality. One possibility
would be to focus on the adaptive value of the
mechanism itself (i.e., sexual orientation); the
other would be to explore the possible adaptive
value of one of its manifestations (e.g., exclusive
homosexuality). The adaptiveness of the mecha-
nism itself is usually not the focus of the debate.
The debate is mostly centered on whether and
how the individual differences generated by this
mechanism are adaptive. A mistake would be to
extrapolate a possible nonadaptiveness of an
extreme point within the continuum (exclusive
homosexuality) to the other variations along the
whole continuum of sexual orientation (e.g.,
predominantly heterosexual, bisexual, predomi-
nantly homosexual, etc.). Similarly, a severe clin-
ical depression or severe forms of autism are most
probably not adaptive; they are not only neutral
but rather maladaptive. However, they both are
extremes on a continuum of adaptive variation,
such as the capacity of sadness and systemizing
abilities, respectively (for an overview about
adaptive individual differences see, Buss and
Greiling 1999). In the same line, bisexuality or
some degree of homosexual tendencies among
predominantly heterosexual individuals as well
as some heterosexual tendencies among predom-
inantly homosexual individuals could be viewed
as adaptive variation of sexual orientation. From
this perspective, even if exclusive homosexuality
does not have any possible adaptive value, it
would be a mistake to assume that the same is
true for the whole continuum of sexual
orientation.
Evolutionary Theories of Sexual
Orientation
Sexual orientation has been described as a mech-
anism navigating or directing sexuality towards
individuals of the opposite sex, same sex, or in
varying degrees to both sexes (Bailey 2009; Bai-
ley et al. 2016; Savin-Williams 2016; Vrangalova
and Savin-Williams 2012). Similar to some het-
erosexual practices (e.g., extra-pair affairs), atti-
tudes toward same-sex sexuality can vary greatly
between cultures. In some cultures, same-sex sex-
uality constitutes an integral part of the sociocul-
tural system; in others such sexuality is
criminalized and persecuted (for review see, Bai-
ley et al. 2016). For these reasons, individuals can
vary in their overt manifestation of sexual prefer-
ence, from a tendency hidden from public or even
self to open preferences and behaviors. There are
also other motives why sexual preferences, behav-
iors, and identities do not need to be aligned.
Nonpreferred sexual behavior can be manifested,
for example, under conditions without access to
preferred sexual partners; it can be done for finan-
cial motives or simply to satisfy one’s partner.
Similarly, romantic affection can be largely
2 Sexual Orientation and Human Sexuality
disconnected from sexual desires (Diamond
2003). For example, a person can repeatedly fall
in love only with individuals of the same sex, but
prefer and actively seek sexual partners of both
sexes. Thus, sexual orientation refers to sexual
desires, attraction, and preferences rather than to
overt sexual behavior, identity, or affective
feelings.
Sexual orientation has a genetic component
that explains approximately one third of the vari-
ation between individuals (e.g., Zietsch
et al. 2012; for review, see Bailey et al. 2016).
This suggests that sexual orientation is a complex
trait that is influenced by genetic and environmen-
tal factors. Sex hormones are one of the environ-
mental factors that influence the development of
sexual orientation. According to the organiza-
tional hypothesis, the neural system of homosex-
ual individuals is influenced by sex-atypical
hormone levels during early developmental
phases. Systematic differences in neuroanatomy
between homosexual and heterosexual individ-
uals suggest that compared to their heterosexual
counterparts homosexual men were, on average,
less androgenized and homosexual women more
androgenized during early, probably prenatal,
development (for review, see Bailey et al. 2016;
LeVay 2010; Wilson and Rahman 2005). Neither
genetic nor hormonal influences suggest that sex-
ual orientation is necessarily fixed and rigid over
the lifespan. Sexual orientation develops during
ontogeny whereas sexual identity can change over
time, particularly among women (Diamond
2008). Such a fluidity of sexual orientation does
not equal to a conscious choice of different sexual
desires and attractions (Diamond 2008). Thus,
sexual orientation is not a choice nor a cultural
invention; it has genetic and neurohormonal
underpinnings and is not necessarily immutable.
Furthermore, occasional same-sex behavior and
stable same-sex preferences have been
documented along a number of nonhuman animal
species (Sommer and Vasey 2006). Thus, a varia-
tion on the sexual orientation continuum is not a
human specific and has its phylogenetic roots.
Moreover, it is commonly accepted that homo-
sexuals do not have any reproductive success.
Although it has been shown that reproductive
success of transgendered male homosexuals
(males who adopt female look and behavior) is
close to zero (Vasey et al. 2014), this does not
need to apply to other forms of nonheterosexual
individuals. In one study, 32.8 % of men and
65.4 % of women reported a potential for homo-
sexual response, suggesting that a huge percent-
age of predominantly heterosexual people do have
a propensity to experience same-sex sexual attrac-
tion and/or interaction (Santtila et al. 2008). In this
study, the potential for homosexual response was
based on responses (quite impossible, very
unlikely, quite unlikely, not likely, not unlikely,
relatively likely, very likely) on the following
question: “If a, in your opinion, handsome man
[to male participants]/beautiful woman [to female
participants], whom you like, suggested sexual
interaction with you, how likely would you be
able to do it (if you could define the nature of the
interaction and nobody else would know about
it)?”. In line with this finding, a large-scale study
demonstrated that almost 10 % of heterosexually
identified men reported that they have engaged in
same-sex sexual activities (Pathela et al. 2006).
This applies even more to non-Western
populations, such as Samoa, where the majority
of predominantly heterosexual men engage in sex
with both women and transgender males called
Fa’afafine (Petterson et al. 2015). Further, 20 % of
homosexually identified white American men
reported having been married to a woman at
some point in their life, and 50 % of homosexual
men reported having produced at least one child
(Bell and Weinberg 1978). According to the more
recent demographic data (2013) of the USA, 37 %
of LGBT-identified individuals report having at
least one child. Keeping in mind these assump-
tions, we can outline some of the empirically
supported evolutionary theories explaining the
adaptiveness of human homosexuality and
nonheterosexuality.
1. Homosexuality as an adaptation: Kin selection
theory
One of the oldest and partly supported theories of
homosexuality as an adaptation was postulated by
E. O. Wilson (1978), inspired by Hamilton’s kin
Sexual Orientation and Human Sexuality 3
selection theory. Homosexual individuals, who do
not invest into their own exclusive reproduction,
may propagate their genes indirectly by
supporting reproduction of close relatives, with
whom he/she shares own genes. Studies
conducted in the USA, Great Britain, or Japan
did not support the theory (for review, see Bailey
et al. 2016), which can be explained by persisting
relatively negative attitudes toward homosexual
individuals in these countries. On the other hand,
systematic investigations of Fa’afafine, who are
officially recognized as “third gender”or a trans-
gender type of male homosexuals (individuals
who are biologically males but have feminine
characteristics and adopt feminine gender roles)
among the population of Samoa, supported the
theory of kin selection. In particular, Fa’afafine
show higher avuncular tendencies (support of
nieces and nephews) compared to men who prefer
women as sexual partners, which supposedly
increases their fitness (e.g., Vasey and
VanderLaan 2010).
Samoan Fa’afafine fit a specific subgroup of
homosexual men, the transgender type of male
homosexuals that is a specific although still het-
erogeneous group. This subgroup of transgender
individuals whose sexuality is aimed at individ-
uals of the same biological sex is frequent in many
other societies, such as India (Hijra), Thailand
(Kathoey), or native North American tribes
(Berdache). The transgendered homosexuals
mostly prefer same-sex relationships, although
usually with cisgender (i.e., biological sex aligned
with gender role) men. Sexual and/or romantic
relations between partners of different gender
roles are documented in numerous populations
around the world (Crapo 1995). Furthermore,
age stratified type of same-sex relationships (i.e.,
one of the partners is much younger than the
other) has been documented across populations,
such as ancient Greece, Middle Ages Japan, or
contemporary New Guinea (Crapo 1995). Such
relationships, as opposed to gender-stratified
ones, do not need to imply one transgendered
and one cisgender partner, rather both of them
are cisgender but of different age categories,
although such interactions can be temporary and
do not need to imply predominant same-sex
preferences of none of the partners. Both
age-stratified and gender-stratified styles of
same-sex relationships exist in the modern West-
ern culture, although the most frequent style of
relationships is egalitarian between two individ-
uals who identify as homosexuals and are of sim-
ilar gender role, status, education, and age. This
latter style of same-sex relationships seems to be
specific for contemporary Western populations.
On the other hand, the other two styles of same-
sex relationships appear in populations that share
more common sociocultural characteristics with
human ancestral populations (e.g., geographical
connectedness of the kin and strong familiar
bonds), and the evolutionary strategy can thus
still be adaptive in the present as it could have
been advantageous in the ancestral environment
(VanderLaan et al. 2014). Importantly, evolution-
ary strategy of transgender homosexuals can dif-
fer from the one adopted by cisgender
homosexuals or nonheterosexuals. In other
words, some evolutionary hypotheses (e.g., kin
selection) can be supported in one specific sub-
group of homosexual individuals (e.g., in trans-
gender homosexual men) in specific society
contexts (e.g., large families, close kin bonds,
geographical proximity) but not in others (e.g.,
in cisgender homosexual men in big anonymous
cities with prevalent homophobia).
2. Homosexuality as a by-product of
sex-atypicality
Miller (2000) suggested that male homosexuality
per se is not adaptive, but rather it is a by-product
of another adaptation. He proposed that some
characteristics that are, on average, more typical
for women than men, such as lower aggressive-
ness, higher cooperation, and social skills in gen-
eral, became adaptive also in men during the
changes in social structure of relatively recent
human evolution. Futher, more feminine charac-
teristics in men can be typical for slow life history
of men who invest more in relationship commit-
ment and parental care than in mating success
(Jeffery 2015). Thus, men who have more femi-
nine and fewer masculine characteristics can
thrive better in a more complex and
4 Sexual Orientation and Human Sexuality
predominantly monogamous society, including
having relatively higher reproductive success.In
this scheme, male homosexuality would thus be
an extreme nonadaptive by-product of a general
adaptive male shift to feminization. Gender non-
conformity (lower masculinity in men) has a
strong genetic component (for review, see Bailey
et al. 2016), and women find femininity in men
attractive (Perrett et al. 1998). Furthermore,
numerous studies indicate that nonheterosexual
men are, in comparison to heterosexuals, more
feminine (for review, see Bailey et al. 2016).
Homosexual men would thus form an extreme
point on a continuum between male masculinity
and male femininity, while relatively feminine
heterosexual men would have some adaptive
advantages, at least in urban, middle class Western
groups.In line with the kin selection theory
outlined above, this theory also aims to explain
the existence and maintenance of feminine male
homosexuals in the population. This theory, none-
theless, does not apply to masculine male homo-
sexuals and other nonheterosexuals nor to female
homosexuals. Rice et al. (2013) suggested that
homosexual orientation is a by-product of
sex-atypical epigenetic marks that feminize
males and masculinize females during embryonic
development. Thus, the opposite logic of the the-
ory could also be applied to explain masculine
female homosexuals who would represent an
extreme pole of a general female masculinization.
This might have been adaptive for some women
during the evolutionary past, for example, by con-
centrating more social and physical power, lead-
ing to higher independence, better self-defense
and defense of their kin, and higher resource
acquisition. Such characteristics might have been
advantageous in the ancestral and also recent soci-
ety, where mortality in men has been relatively
higher than in women.
3. Predominant homosexuality as an adaptation:
Sneaking and conditional sexual strategy
Following the previous line of reasoning, predom-
inant homosexuality in feminine males can be also
understood as a sneaking sexual strategy (Jeffery
2015). In humans, it has been suggested that the
most frequent strategy would be exclusive or at
least predominant heterosexuality with males of
rather masculine characteristics, while bisexuality
and predominant homosexuality linked to
increased femininity in men would represent a
sneaking sexual strategy. From this point of
view, feminine men who have some potential for
opposite-sex activities can be successful in oppor-
tunistic heterosexual activities and secure direct
reproductive success. This can happen in particu-
larly because feminine characteristics were shown
to be attractive to women (e.g., Perrett et al. 1998),
at least under certain conditions. At the same time,
more feminine bisexual or predominantly homo-
sexual men can be perceived as weak rivals to
more masculine exclusively or predominantly het-
erosexual men and can thus reduce intrasexual
male-male competition. Consequently, feminine
predominantly homosexual men can be accepted
more as friends of heterosexual men’s female
partners, thus offering opportunities to sneak cop-
ulations.High numbers of predominantly homo-
sexual individuals reporting some opposite-sex
attraction, behavior, and also biological offspring
(see above) would tend to support this theory. The
theory thus does not attempt to explain exclusive
homosexuality or transgender homosexual men
who do not have any interest in sexual encounters
with opposite-sex partners. It rather shows that
various degrees of bisexuality and predominant
homosexuality can serve as an alternative and
successful reproductive strategy.This theory need
not apply only to men because bisexual and pre-
dominantly homosexual women can also show
some opposite-sex attraction and behavior, as
well as some reproductive success. On average,
lesbian women are more masculine than hetero-
sexual women (Bailey and Zucker 1995), and
masculine heterosexual women report higher
sociosexuality (tendency for uncommitted sexual
variety) than feminine women (Mikach and Bai-
ley 1999). We can thus speculate that masculine
predominantly homosexual women can secure
direct reproductive success without the necessity
of creating and maintaining long-term bonds with
men but rather with other women.Compared to
predominant female homosexuality or bisexual-
ity, a fluid sexual orientation might have helped
Sexual Orientation and Human Sexuality 5
ancestral women secure resources and care for
their offspring under conditions without a primary
male partner by receiving parental investment
from other women (Kuhle and Radtke 2013).
According to this theory, fluidity of female sexual
orientation can be understood as a conditional
sexual strategy, meaning that variations between
opposite-sex and same-sex sexuality can be adap-
tive in different conditions, such as different age
or life phases.
4. Nonheterosexuality as an adaptation: Same-
sex alliance formation
This theory holds that various degrees of sexual
feelings and behaviors aimed toward both males
and females offer an individual advantage over an
exclusive orientation toward only one sex
(Kirkpatrick 2000). The main reason towards
nonheterosexual interactions would be formation
and maintenance of same-sex alliances that are
crucial in many other primates and in particular
in the human evolutionary past. The author shows
anthropological and primatological evidence
supporting the view that strong and emotional
same-sex alliances can offer a reproductive
advantage to the individual. More specifically,
same-sex alliances reduce competition and
increase cooperation between members of the
same sex, while maintained access to partners of
the opposite sex ensures potential reproductive
success. In this perspective, heterosexual sex
among stable partners outside the fertile period
(i.e., outside the fertile window within the female
menstrual cycle and after menopause) or any sex-
ual activities that do not lead to fertilization (e.g.,
oral or anal sex) serve the same function, i.e.,
reducing disputes, increasing cooperation, and
thus maintaining the stable dyad (Bártová and
Valentová 2012).The question is whether
nonheterosexual behavior was originally selected
for alliance formation or whether alliance forma-
tion is rather a newly acquired sociosexual func-
tion of same-sex sexual behavior. In this sense,
nonheterosexuality would serve proximate socio-
sexual functions for which nonprocreative sexu-
ality in general was co-opted, such as alliance
formation, dyad maintenance, lowering stress,
peacemaking and reconciliation, or resource
gains.
5. Homosexuality as a by-product: Overdomi-
nance and sexually antagonistic gene
hypothesis
Both behavioral and molecular genetic studies
have shown that the male homosexuality is linked,
among others, to the X chromosome and is thus
inherited from the maternal lineage (Sanders
et al. 2014). Further, female relatives of male
homosexuals have more children and
grandchildren than control group of women with
no homosexual individuals among relatives (e.g.,
Ciani and Pellizzari 2012; for review, see Bailey
et al. 2016). It is thus suggested that greater fecun-
dity and fertility of female relatives of homosex-
ual men can explain the persistence of a
nonadaptive form of male sexuality through sex-
ually antagonistic selection (e.g., Ciani and
Pellizzari 2012; Zietsch et al. 2008). In other
words, sexually antagonistic genes can bring
advantages for one sex (e.g., higher fecundity in
women) but can be principally disadvantageous
for the other sex (e.g., can cause exclusive homo-
sexuality in men).However, as shown by recent
models, the frequency of only female carriers
cannot explain a stable proportion of exclusive
male homosexuals in the population but a large
proportion of both nonhomosexual female and
male carriers can (Chaladze 2016). Based on
behavioral genetic studies, a considerable propor-
tion of men carrying the homosexual genes are not
exclusively or predominantly homosexual. It was
suggested that homosexual phenotype can only be
expressed in a homozygous form of the underly-
ing alleles, while individuals with a heterozygous
form (only one allele of the gene) carry the phe-
notype but the phenotype is not expressed (for
review, see McKnight 1997). Such heterozygous
carriers are supposed to possess some advanta-
geous characteristics that can increase reproduc-
tive success of their owners.So far, this theory was
supported by a study showing that heterosexual
twins of homosexual men, who most probably
share the genetic component of homosexuality
although not homosexual orientation, report
6 Sexual Orientation and Human Sexuality
higher number of female sexual partners than
heterosexual men with a heterosexual twin
(Zietsch et al. 2008). Thus, heterosexual men
who carry the homosexuality-linked genetic com-
ponent, potentially in a heterozygous form, do
have higher mating success (considered a proxy
of a reproductive success) than the control group
of heterosexual men, which can explain its rela-
tively stable prevalence in the population.
The theories of by-product thus focus more on
explanation of the extreme exclusive form of male
homosexuality, rather than the rest of the
nonheterosexual continuum. The theories can
apply to other forms of nonheterosexuals, though,
because they can also be carriers of the homosex-
ual genotype. It has been shown that a potential
for homosexual response has a stronger genetic
component than homosexual behavior or self-
labeled identification (Santtila et al. 2008). Thus,
any degree of nonheterosexual tendencies might
be more suitable for genetic analyses than exclu-
sive homosexuality with no heterosexual
potential.
Furthermore, the theories of homosexuality as
a by-product originally did not apply to women,
although in principle they can work similarly,
since genetic component has been documented
also in female sexual orientation (for review, see
Bailey et al. 2016), and the potential to homosex-
ual response is even more frequent in women than
men (Santtila et al. 2008). Thus, exclusive male
and female homosexuality indeed can be a
by-product of otherwise adaptive variation of sex-
ual orientation.
Conclusion
During the last century, human and other animal
(vertebrates and invertebrates) sexual orientation
has been extensively studied within many scien-
tific areas. Despite important findings in the areas
of psychology, anthropology, medicine, neurosci-
ence, genetics, endocrinology, and development,
evolutionary reasoning has suffered numerous
conceptual and terminology issues. These mis-
conceptions have led to the view that homosexu-
ality can only be exclusive and thus an
evolutionary paradox. However, when essentialist
categorical thinking is discarded, it might become
clearer that the majority of variation on the con-
tinuum of sexual orientation can offer adaptive
advantages for their carriers. This view can be
supported by changing the perspective of sexual-
ity equaling only with reproduction, and by
acknowledging the fact that at least some homo-
sexual and nonheterosexual individuals within the
whole spectrum of sexual orientation do repro-
duce and raise their offspring.
Most outlined theories present adaptive rea-
sons for the evolution of nonheterosexual orienta-
tions, either by stressing indirect reproduction via
kin selection or direct reproduction via sneak cop-
ulations or same-sex alliance formation that can
increase survival and future direct reproductive
capacity. Even by-product theories offer plausible
evolutionary reasoning for the origins and main-
tenance of nonheterosexual orientations. In this
sense, nonheterosexual orientations would have
been passed on throughout generations together
with the adaptive trait of sex-atypicality, advan-
tages of an increased fertility in the other-sex kin,
or in carriers who do not express the homosexual
phenotype. These theories are not mutually exclu-
sive, and together they can explain a bigger pro-
portion of the sexual orientation continuum.
Moreover, these theories offer a stronger case
against the argument that it is impossible for any
form of homosexuality to have evolved.
Finally, it is important to realize that the
outlined theories are not just-so-stories nor naïve
panadaptationism as critics often portrait it. The
theories offer specific hypotheses with predictions
that can be empirically tested. We attempted to
provide the existing empirical support for each
theory, although we realize that more research
using a variety of sources of evidence and
methods is still needed. Only convergent evidence
of numerous empirical tests can give decisive
support and scientific credibility to a particular
adaptive hypothesis.
Sexual orientation is a complex psychological
mechanism that can profit from analysis within an
evolutionary framework, where otherwise hetero-
sexuality might be taken for granted. Future stud-
ies should pay more attention to female
Sexual Orientation and Human Sexuality 7
nonheterosexuality, to sex-typical nonheter-
osexuals, bisexuals, integration and empirical
confrontation of the existing theories, and to
empirical testing of the theories in Western as
well as non-Western populations with varying
concepts of same-sex sexuality.
Cross-References
▶Homosexuality
▶Kin Selection
▶Mate Preferences
▶Sexual Strategies
References
Bailey, J. M. (2009). What is sexual orientation and do
women have one? In D. A. Hope (Ed.), Nebraska
symposium on motivation: Vol. 54. Contemporary per-
spectives on lesbian, gay, and bisexual identities
(pp. 43–63). New York: Springer.
Bailey, J. M., Vasey, P. L., Diamond, L. M., Breedlove,
S. M., Vilain, E., & Epprecht, M. (2016). Sexual ori-
entation, controversy, and science. Psychological sci-
ence in the public interest: A journal of the American
Psychological Society, 17(2), 45.
Bailey, J. M., & Zucker, K. J. (1995). Childhood sex-typed
behavior and sexual orientation: A conceptual analysis
and quantitative review. Developmental Psychology,
31(1), 43.
Bártová, K., & Valentová, J. (2012). Evolutionary perspec-
tive of same-sex sexuality: Homosexuality and homo-
sociality revisited. Anthropologie, 50(1), 61.
Bell, A. P., & Weinberg, M. S. (1978). Homosexualities:
A study of diversity among men and women. New York:
Simon and Schuster.
Buss, D. M., & Greiling, H. (1999). Adaptive individual
differences. Journal of Personality, 67(2), 209–243.
Chaladze, G. (2016). Heterosexual male carriers could
explain persistence of homosexuality in men:
Individual-based simulations of an X-linked inheri-
tance model. Archives of Sexual Behavior.
doi:10.1007/s10508-016-0742-2.
Ciani, A. C., & Pellizzari, E. (2012). Fecundity of paternal
and maternal non-parental female relatives of homo-
sexual and heterosexual men. PloS one, 7(12), e51088.
Crapo, R. H. (1995). Factors in the cross-cultural pattern-
ing of male homosexuality: A reappraisal of the litera-
ture. Cross-Cultural Research, 29, 178–202.
Diamond, L. M. (2003). What does sexual orientation ori-
ent? A biobehavioral model distinguishing romantic love
and sexual desire. Psychological review, 110(1), 173.
Diamond, L. M. (2008). Sexual fluidity: Understanding
women’s love and desire. Cambridge, MA: Harvard
University Press.
Jeffery, A. J. (2015). Two behavioral hypotheses for the
evolution of male homosexuality in humans. In The
evolution of sexuality (pp. 207–219). Cham, CH:
Springer International Publishing.
Kinsey, A. C., Pomeroy, W. B., & Martin, C. E. (1948).
Sexual behavior in the human male. Philadelphia:
W. B. Saunders.
Kirkpatrick, R. C. (2000). The evolution of human
homosexual behavior. Current anthropology, 41(3),
385–413.
Kuhle, B. X., & Radtke, S. (2013). Born both ways:
The alloparenting hypothesis for sexual fluidity in
women. Evolutionary Psychology, 11(2). doi:1474704
91301100202.
LeVay, S. (2010). Gay, straight, and the reason why: The
science of sexual orientation. New York, NY: Oxford
University Press.
McKnight, J. (1997). Straight science?: Homosexuality,
evolution and adaptation. Routledge, London.
Mikach, S. M., & Bailey, J. M. (1999). What distinguishes
women with unusually high numbers of sex partners?
Evolution and Human Behavior, 20(3), 141–150.
Miller, E. M. (2000). Homosexuality, birth order, and evo-
lution: Toward an equilibrium reproductive economics
of homosexuality. Archives of Sexual Behavior, 29(1),
1–34.
Pathela, P., Hajat, A., Schillinger, J., Blank, S., Sell, R., &
Mostashari, F. (2006). Discordance between sexual
behavior and self-reported sexual identity: A -
population-based survey of New York City men.
Annals of Internal Medicine, 145(6), 416–425.
Perrett, D. I., Lee, K. J., Penton-Voak, I., Rowland, D.,
Yoshikawa, S., Burt, D. M., et al. (1998). Effects of
sexual dimorphism on facial attractiveness. Nature,
394(6696), 884–887.
Petterson, L. J., Dixson, B. J., Little, A. C., & Vasey, P. L.
(2015). Viewing time measures of sexual orientation in
Samoan cisgender men who engage in sexual interac-
tions with Fa’afafine. PloS one, 10(2), e0116529.
Rice, W. R., Friberg, U., & Gavrilets, S. (2013). Homosex-
uality via canalized sexual development: A testing pro-
tocol for a new epigenetic model. Bioessays, 35(9),
764–770.
Sanders, A. R., Martin, E. R., Beecham, G. W., Guo, S.,
Dawood, K., Rieger, G., et al. (2014). Genome-wide
scan demonstrates significant linkage for male sexual
orientation. Psychological Medicine, 45, 1379–1388.
Santtila, P., Sandnabba, N. K., Harlaar, N., Varjonen, M.,
Alanko, K., & von der Pahlen, B. (2008). Potential for
homosexual response is prevalent and
genetic. Biological Psychology, 77(1), 102–105.
Savin-Williams, R. C. (2016). Sexual orientation: Catego-
ries or continuum? commentary In Bailey et al. (Eds),
Psychological science in the public interest: A Journal
of the American Psychological Society,17(2), 37.
8 Sexual Orientation and Human Sexuality
Sommer, V., & Vasey, P. L. (2006). Homosexual behaviour
in animals: An evolutionary perspective. New York,
NY: Cambridge University Press.
VanderLaan, D. P., Ren, Z., & Vasey, P. L. (2014). Male
androphilia in the ancestral environment: An ethnolog-
ical analysis. Human Nature, 24, 375–401.
Vasey, P. L., Parker, J. L., & VanderLaan, D. P. (2014).
Comparative reproductive output of androphilic and
gynephilic males in Samoa. Archives of Sexual Behav-
ior, 43(2), 363–367.
Vasey, P. L., & VanderLaan, D. P. (2010). Avuncular ten-
dencies and the evolution of male androphilia in
Samoan Fa’afafine. Archives of Sexual Behavior,
39(4), 821–830.
Vrangalova, Z., & Savin-Williams, R. C. (2012). Mostly
heterosexual and mostly gay/lesbian: Evidence for new
sexual orientation identities. Archives of sexual behav-
ior, 41(1), 85–101.
Wilson, G. D., & Rahman, Q. (2005). Born gay? The
psychobiology of sex orientation. London: Peter Owen.
Wilson, E. O. (1978). On human nature. Chicago: Harvard
University Press.
Zietsch, B. P., Morley, K. I., Shekar, S. N., Verweij, K. J.,
Keller, M. C., Macgregor, S., et al. (2008). Genetic
factors predisposing to homosexuality may increase
mating success in heterosexuals. Evolution and
Human Behavior, 29(6), 424–433.
Zietsch, B. P., Verweij, K. J., Heath, A. C., Madden, P. A.,
Martin, N. G., Nelson, E. C., & Lynskey, M. T. (2012).
Do shared etiological factors contribute to the relation-
ship between sexual orientation and depression? Psy-
chological Medicine, 42, 521–532.
Sexual Orientation and Human Sexuality 9
