Menelaos Apostolou. Review of General Psychology. Volume 17, Issue 3. September 2013.
Homosexual men are less likely to have children and become fathers than heterosexual men (Iemmola & Camperio-Ciani, 2009; King et al., 2005). This indicates that male homosexuality has considerable fitness costs (i.e., the contribution to the gene pool of the future generation of an individual with this trait will tend to zero) and evolutionary forces should have eliminated it. The fact that it remains present in the population, however, testifies that it confers, or had conferred, to our ancestors’ sufficient fitness benefits that balance, or had balanced, its costs.
Another possibility is that male homosexuality does not have a genetic basis, so it is invisible to evolutionary forces. For instance, Symons (1979) argued that homosexuality results from a meme that takes advantage of the inborn male propensity to find sexual variety interesting. Nevertheless, this line of argumentation is not plausible, as the considerable fitness costs of this trait would create strong evolutionary pressures that would give rise to mechanisms that protect men from any environmental effects that are likely to influence their sexual orientation (Miller, 2000).
The human mind has been shaped predominantly by evolutionary pressures operating in ancestral times (Barkow, Cosmides, & Tooby, 1992). Accordingly, the male homosexuality trait should have also been the product of evolutionary forces working in ancestral human societies. Thus, a theory that aspires to account for the evolution of male homosexuality should integrate anthropological and historical evidence in order to reconstruct ancestral human conditions and identify the selection pressures that were likely to have favored it.
This is not the road taken, however, by existing evolutionary theories. Therefore, in order to address this limitation, the present article employs anthropological and historical evidence on mating patterns in order to identify the evolutionary pressures that were likely to have shaped male sexual orientation. Based on this evidence, it is proposed that homosexuality has evolved to increase a man’s inclusive fitness (i.e., the sum of personal reproductive success and any extra reproduction by relatives that results from the individual’s behavior) by reducing interfamily conflict and by promoting the reproductive success of older male siblings. Before this argument is developed further, current evolutionary theories on male homosexuality will be reviewed to demonstrate that they do not account for many of the observed aspects of male homosexuality.
Evolutionary Theories on Homosexuality
Hutchinson (1959) proposed that homosexual behavior is retained in the population because it co-occurs with another trait that is under positive selection. Although Hutchinson did not identify this trait, such a possibility was offered by McKnight (1997), who argued that women may prefer heterosexual men who tend to make good lovers, partners, and fathers by offering more “sensitivity, creativity and better communication skills than do boorish, unfaithful homozygous heterosexuals” (p. 106). Thus, what is selected is being more feminine rather than homosexuality per se, but being more feminine co-occurs with homosexuality.
In this argument, there are numerous alleles that make for femininity, and the typical male inherits only a few, which increase his reproductive success. But if he inherits a large number of such alleles, his development may be pushed so far toward being feminine that he becomes a homosexual (Miller, 2000; see also Zietsch et al., 2008). Still, the hypothesis that homosexuality is the outcome of the presence of many feminine alleles in a single individual remains to be proven. Furthermore, the desire for sexual variety and sex without emotional commitment are male traits that are yet present in homosexual men. In addition, as is the case with other evolutionary hypotheses, this argument does not account for many of the observed patterns of male homosexuality, such as the fraternal older-brother effect (i.e., male homosexual having more older brothers than male heterosexuals).
Another hypothesis is that a gene for male homosexuality is selected because it increases the fecundity of female relatives of male homosexuals (Hamer & Copeland, 1994). There are numerous mechanisms through which this can be achieved; for instance, a gene may enhance the sexual interest in men in both sexes (Hamer & Copeland, 1994). Empirical evidence supports the prediction that the female relatives of male homosexuals have more children (Camperio-Ciani, Corna, & Capiluppi, 2004; King et al., 2005). However, given the high reproductive cost of male homosexuality, it can be argued that it would be more optimal, and perhaps easier, for evolution to increase the fecundity of females without affecting the sexual orientation of their male offspring (e.g., by increasing their level of libido). Moreover, the fecundity argument does not account for a number of patterns of male homosexuality, such as the fraternal older-brother effect and its cross-cultural variability (see the next section).
A further possibility is that groups with male homosexuals may have advantages in their competition with groups who do not have any male homosexuals, so male homosexuality could have been the product of group selection forces. One argument is that selection takes place between families, with the ones who are fitter to withstand the competition replacing those who are less fit (Wade, 1979, 1982). Families with male homosexuals may enjoy less conflict over resources that are required for reproduction, and thus they may be fitter than families with no male homosexuals in their ranks.
This remains a theoretical possibility, but in order to assess the contribution of group selection forces, formal models are needed but are currently lacking (although there have been some attempts; see Kirby, 2003). In general, though, because, in most instances, group selection forces are weaker than lower-level selection forces (Williams, 1966), it is unlikely that the group selection has been the primary force driving the evolution of male homosexuality.
Moving on, Kirkpatrick (2000) proposed that male homosexuality enables the maintenance of same-sex alliances that aid resource competition or cooperative defense. The biggest weakness of this model is, nevertheless, that such alliances can be maintained, and are maintained, by psychological mechanisms that do not need to bear the considerable cost of opting out from the reproductive process (e.g., the feeling of friendship). Also, this model does not predict that male homosexuals have more older brothers, and there is no solid evidence that homosexuality actually facilitates same-sex alliances.
Another theory sees homosexuality as the product of parental manipulation. In this scenario, parents maximize their inclusive fitness by manipulating some of their children to become homosexual so as to provide support and assistance to other relatives (Ruse, 1988; Trivers, 1974). Nevertheless, there is no empirical support in favor of this hypothesis (Kirkpatrick, 2000).
Behavioral patterns such as altruism, which have obvious direct costs for the individual and no apparent direct benefits, usually arise from kin selection (Hamilton, 1964). More specifically, genes that predispose an individual to be altruistic toward close kin are likely to be selected, as they benefit copies of themselves found in the individual’s relatives (Hamilton, 1964). Based on this reasoning, Wilson (1975) proposed that male homosexuality has evolved through kin selection to increase the fitness of a male homosexual’s close kin.
In particular, Wilson (1975, 1978) argued that homosexual men, freed from the need to allocate energy in pursuing direct reproduction, may have helped their siblings and other relatives in ancestral environments to survive and reproduce by allocating to them material resources, childcare, and protection. Similarly, Ruse (1981) and Weinrich (1976) argued that benefits to kin may come through sharing of income, bequeathing of wealth after death, and helping with child rearing.
A number of studies attempted to examine the predictions that follow from this kin-selection hypothesis, namely, that homosexual individuals will act as helpers and will divert resources to assist their relatives. The empirical evidence produced did not find support for this argument, as homosexual men were not more likely to contribute monetary resources or undertake child rearing or any other helping behavior toward their close kin than their straight siblings (Bobrow & Bailey, 2001; Rahman & Hull, 2005).
Nonetheless, for the homosexuality trait to increase inclusive fitness, the homosexual man does not need to actively contribute resources to his relatives. This article argues that during human evolution, gaining long-term sexual access to women had been dependent on the resources a man controlled, and, to a large extent, these resources would come from his family. Therefore, when a man opts out from the reproductive process, more resources become available to his other male siblings, increasing, in effect, their reproductive success. Furthermore, conflict over resources decreases, which promotes family coherence and improves the survival and reproductive chances of the family members. In this respect, homosexual orientation increases inclusive fitness; accordingly, kin selection constitutes an important selection force driving the evolution of male homosexuality.
Mating in Preindustrial and in Ancestral Human Societies
The human mind is predominantly the product of evolutionary forces working in past environments (Barkow et al., 1992; Lee & DeVore, 1968), which means that male homosexuality is not a modern phenomenon and that it evolved in ancestral societies. This leads to the question of what were the prevailing conditions in these environments that made male homosexuality beneficial. The current section employs ethnographic and historical evidence in an attempt to reconstruct ancestral conditions and identify these selection pressures.
To begin with, women are the scarce reproductive resources for which men compete to gain access (Trivers, 1972). The anthropological record of preindustrial societies indicates that women’s mate choices are controlled by their parents, who choose husbands for them (Minturn, Grosse, & Haider, 1969; Stephens, 1963). In particular, in a study of 190 hunting and gathering societies, arranged marriage was found in approximately 70% of the societies in the sample, whereas marriage based on free courtship was found in less than 5% of the societies in the sample (Apostolou, 2007).
Likewise, another study of 186 preindustrial societies that based their substance on agriculture and animal husbandry, as well as on hunting and gathering, produced similar results: The most common pattern of long-term mating was arranged marriage, with female offspring being controlled more than male offspring (Apostolou, 2010b). Finally, in the vast majority of preindustrial societies, polygyny is practiced, and men with desirable qualities, and who can afford it, acquire a disproportionate share of wives (Apostolou, 2010b; Stephens, 1963).
The anthropological record is also informative on the ancestral human condition. More specifically, the genus Homo appeared on earth approximately 2 million years ago, and until approximately 10,000 years ago, all humans were living as hunters and gatherers (Lee & DeVore, 1968). Although these ancestral foragers did not leave behind any written evidence, it is likely that their way of life was comparable with the way of life of modern ones (Lee & DeVore, 1968). On this basis, it can be argued that the patterns of mating found among modern foragers (i.e., arranged marriage, control of women) had been prevalent during most of human evolution (Apostolou, 2007). This is corroborated by research based on phylogenetic analysis that attempts to reconstruct the condition in ancestral societies (Walker, Hill, Flinn, & Ellsworth, 2011).
Similarly, the patterns of mating found in contemporary agropastoral societies are likely to have been prevalent in ancestral agropastoral ones (Apostolou, 2010b). This is corroborated by evidence from historical resources that indicates that the prevailing patterns had been strong parental control over the mate choices of children, particularly over the female ones (Apostolou, 2012). All in all, it seems that during most of human evolution, sexual access to women has been controlled by their parents. Accordingly, if the evolutionary pressures exercised on male mate-seekers are to be understood, one needs to ask what parents demand from a prospective son-in-law in return for granting mating access to their daughters.
In a preindustrial context, where there are no social protection systems and resources are scarce, a wealthy and capable son-in-law would increase dramatically parents’ fitness, as he would be able to provide them and their family with resources that are needed for survival and reproduction (Apostolou, 2010a). In effect, evolutionary forces should have shaped in-law preferences to be sensitive to a prospective son-in-law’s control of wealth and ability to generate wealth (Apostolou, 2010a). Thus, when they choose a husband for their daughter, parents are interested in finding an individual who is wealthy, industrious, enjoys high social status, and is a good provider (Apostolou, 2010a; Borgerhoff Mulder, 1988; Koster, 2011). Consequently, men with few resources at their disposal are unlikely to be selected by parents as husbands for their daughters.
Moreover, to screen for men who satisfy these criteria, parents require a considerable amount of wealth from a prospective son-in-law. In many preindustrial societies, this takes the form of the institution of the bridewealth, which involves the transfer of substantial resources from the groom and his family to the family of the bride (Murdock, 1981). The payment of the bridewealth is mandatory and the marriage does not proceed without it. Gaining long-term mating access to a woman then becomes a costly business for men and their families. As a result, men who can bear this cost enjoy considerable reproductive success by gaining access to multiple women (i.e., polygyny) of high mate quality, whereas men with limited resources are excluded from mating (Goody & Tambiah, 1973). Adding to this effect, hypergyny, that is, women marrying up the social status hierarchy, pulls women out of the lower classes, leaving many lower-status men without wives (Boone, 1986).
In sum, there are good reasons to believe that male homosexuality evolved in a context where the mate choices of women were controlled by their parents, who looked for sons-in-law who had access to resources. These parental preferences resulted in long-term access to women being contingent on the resources a man controlled, which meant that men needed to compete intensively with each other for control of wealth.
Interfamily Conflict over Resources and the Evolution of Male Homosexuality
The consequence of the aforementioned patterns of human mating is that in a family with one son and one or more daughters, wealth is usually transferred to the former so that he can convert it into reproductive benefits (Hartung, 1976, 1982). However, if there is more than one son in the family, resources are divided between them. Given that fewer resources translate into getting a wife of lower mating value or fewer wives, this division reduces each of the sons’ reproductive success. Furthermore, the possibility of conflict dramatically increases as each son, motivated by the reproductive benefits that his family’s wealth can give him, will be willing to take out of the competition the other(s) and monopolize the family resources.
Fights over inheritance are likely to result in violence and considerable deterioration of within-family relations, which may compromise a family’s coherence, with negative implications for the family members. Accordingly, interfamily conflict should increase exponentially with the number of sons.
In consequence, evolutionary pressures are likely to favor mechanisms that reduce interfamily conflict over resources required for reproduction. It is proposed that such a mechanism is male homosexuality, which, by removing a son from the reproductive competition, decreases interfamily conflict, while it augments the reproductive success of the older brother(s), who have more resources available. More specifically, when a younger brother opts out from the competition over resources, more wealth becomes immediately available to his older brother(s), who can use it to gain access to high mate value women or marry polygynously. Thus, in the presence of the homosexuality trait, the mating success of the older brother(s) becomes contingent on the sexual orientation of the younger one(s). In this respect, male homosexuality constitutes a form of reproductive altruism toward kin.
Despite its considerable direct reproductive costs, this trait can be favored by selection because of the reproductive benefits it confers to other male relatives who remain active in the mating game, and to the rest of the kin that benefit from less conflict and higher family coherence. Note that, in this case, the homosexual man benefits his relatives without actually transferring resources to them or actively helping them.
Homosexuality has the additional benefit of removing a man from the reproductive process without removing him from the production process, and this is perhaps one of the reasons why evolution has favored male homosexuality instead of a mechanism that produces more female offspring. In particular, one possible solution for reducing interfamily conflict and increasing the reproductive success of male siblings would be a mechanism whereby once a mother has given birth to one or two sons, then she would switch to giving birth to daughters (instead of homosexual sons).
Nevertheless, this would deprive the family unit of the much needed contribution of young males, who are needed in subsistence activities (e.g., plowing the land) and in defending the family’s resources. Consequently, a mechanism that biases the sex ratio in favor of daughters when a son has already been born is less beneficial than a mechanism that biases sexual orientation in favor of homosexual sons. Male homosexuality, then, solves the problem of how to reduce interfamily conflict without compromising the subsistence and survival capacity of the family unit.
The Costs and Benefits of Male Homosexuality
It has been argued that one problem with kin selection models on male homosexuality is that the altruism toward kin, required to offset the loss of direct reproduction, should be considerable (McKnight, 1997). Nevertheless, this argument needs to be refined in the light of mating patterns prevailing in preindustrial societies. More specifically, in a preindustrial context, marriages are arranged and it is not up to the individual’s will to seek and find a mate. In effect, getting married does not depend on sexual orientation, so a male homosexual will most likely find himself married to a woman his parents have selected for him and he will have children. In fact, a childless homosexual man (and a woman) is probably a phenomenon confined only to postindustrial societies (Peters, 2006).
Thus, in a preindustrial setting where mating is regulated, the direct reproductive cost of male homosexuality is not as high as it first seems, and, consequently, it does not require a considerable benefit to balance it. This needs not to mean, however, that this trait is not costly. To begin with, a man who is not interested in attracting a high-quality female will not actively compete for resources and he will end up with less wealth and most likely with a wife of lower mate value. His marriage is also likely to be less stable as his wife may be less satisfied. This can lead her to seek sexual satisfaction outside marriage, decreasing marital stability and increasing the chances of cuckoldry. He is also unlikely to increase his reproductive success by engaging in casual sexual relationships with women before and after marriage. Thus, the point to be made is not that homosexuality in a preindustrial context is not reproductively costly, but it is not so costly as to require a huge inclusive fitness benefit to balance it.
Furthermore, when a male offspring is born in a family where older male siblings are present, he is likely to be in a disadvantageous position to lay claim on his family’s resources. This is because his brother(s), by virtue of being older (or having arrived first), may be positioned higher in the family hierarchy, which gives them an advantage over claiming resources. There is also the possibility that older siblings have already claimed resources, even before the younger brother was born. Accordingly, the younger brother is more likely to end up with fewer resources that can be used for reproduction, which compromises his mating success.
This disadvantage is demonstrated in the practice of primogeniture, that is, the right of the firstborn to inherit his family’s estate. For example, in medieval Portugal, primogeniture among the upper strata resulted in younger sons remaining landless, something that had been catastrophic for their prospects of marriage (Boone, 1986). These landless sons sought their fortune outside Portugal, taking part in colonization efforts. As a result, by the mid-16th century, almost 25% of noble males who reached adulthood subsequently died in expansionist or internecine warfare (Boone, 1986). Overall, the opportunity cost of homosexuality is smaller in the case of a man who is born after the firstborn and decreases further as he descends the birth-order hierarchy.
On the other hand, the benefits accrued can be considerable: A homosexual man, who will not be interested in getting an attractive woman or many women, most likely will not make a claim on his family’s resources required for this purpose. This will result in all his genetic relatives (close and distant) benefiting from reduced interfamily conflict, increasing in effect his inclusive fitness. Second, the fact that men are not constrained by their biology in the number of children they can father, along with the fact that polygyny is an option in most preindustrial societies for men who can afford it (Murdock, 1981), indicates that the resources that become available to an older brother(s) can be translated into many more children. This has a considerable inclusive fitness benefit for the homosexual younger brother.
Last but not least, a male homosexual is likely to be more willing than a heterosexual man to accept a woman of low mate value as a wife. For instance, he will be more willing to accept an older or an unattractive woman, simply because he is not interested in having sexual intercourse with her. In turn, his parents can use this to establish a beneficial alliance, something that they would not be able to do if their son was heterosexual (Weinrich, 1987). For example, they can arrange a marriage alliance with an influential family that has an unattractive daughter. If their son was a heterosexual, they would not be able to do so, because this would meet with strong resistance from him. In effect, male homosexuality may also bring additional inclusive fitness benefits in terms of beneficial marriage alliances.
Even in the minority of societies where women have some freedom to exercise mate choice, access to them is also contingent on wealth, as women place considerable value on the resources a man controls (Buss, 2003). For instance, among Serbian peasants in the last century, “Our girls like to marry only sons, because in that case there was no dividing the property and, aside from that, where several brothers lived together there was always some quarreling” (Halpern, 1967, p. 220).
Nonetheless, it is important to stress that bringing parents into the picture is critical for understanding the evolutionary forces that have shaped male homosexuality. One reason is that the good looks and the exciting personality of a prospective male suitor are considered to be much more important for women in a spouse than for their parents in a son-in-law (Apostolou, 2008; Buunk, Park, & Dubbs, 2008; Perilloux, Fleischman, & Buss, 2011). Consequently, in a scenario where women exercise choice there would be much less pressure on men to acquire resources that would enable them to gain mating access. This is because a good-looking man with a charming personality can gain long-term mating access to the opposite sex, even if he has few resources at his disposal. But this is not going to work in a scenario where parents choose spouses for their children, as they are not going to fall for his charms. Thus, parental control over mating augments the importance of resources and reduces the importance of looks and charms, increasing the inclusive fitness returns of male homosexuality in men with older brothers.
Predictions of the Competition over Resources Hypothesis
The proposed kin selection model of male homosexuality, which is based on the competition over resources hypothesis, makes several predictions that fit well into the observed patterns of male homosexuality. To begin with, in the proposed framework, male homosexuality is a trait that has evolved to increase the inclusive fitness of younger brothers. This predicts that homosexuality has a genetic basis, meaning that this trait should not be exclusively the consequence of environmental factors but that there should also be genes that predispose for it. Empirical evidence, which is based predominantly on twin studies, provides a strong support for this prediction (Alanko et al., 2010; Kendler, Thornton, Gilman, & Kessler, 2000; Långström, Rahman, Carlström, & Lichtenstein, 2010).
The key prediction of the model is that the prevalence of male homosexuality is associated with the presence of an older brother or brothers. In particular, division of resources and conflict over the family’s wealth required for gaining access to mates occurs when there is more than one brother in the family. Thus, male homosexuality is beneficial only in the presence of an older brother(s), which indicates that this trait should primarily be found among males who have older male siblings. The fraternal birth-order effect is the strongest known predictor of sexual orientation, with numerous studies establishing that having an older brother considerably increases the chance of male homosexuality (Blanchard, 2004; Blanchard & Bogaert, 1996; Blanchard & Lippa, 2007; Blanchard, Zucker, Bradley, & Hume, 1995; Blanchard, Zucker, Siegelman, Dickey, & Klassen 1998; Bogaert, 2003; Camperio-Ciani et al., 2004; Ellis & Blanchard, 2001; King et al., 2005; Purcell, Blanchard, & Zucker, 2000; Schwartz, Kim, Kolundzija, Rieger, & Sanders, 2010; VanderLaan & Vasey, 2011).
Moreover, each additional brother increases the competition over family resources and thus the interfamily conflict. Consequently, there is a positive relationship between the inclusive fitness benefit of male homosexuality and the number of older brothers, which predicts that the probability of male homosexuality should be positively correlated with the number of older male siblings, that is, the higher the number of older brothers, the higher the inclusive fitness benefit from male homosexual orientation. Accordingly, male homosexuality should not only be associated with the presence of an older brother, but also each additional brother would increase the chances of a man becoming homosexual.
This prediction is supported by research that indicates that with each older brother, a man’s odds of being homosexual increase by approximately 33% (Blanchard, 2004; VanderLaan & Vasey, 2011). Similarly, another study found that each additional brother increases the odds of male homosexuality by 20%, but the effect is nonlinear (Schwartz et al., 2010). In particular, the first and second older brothers have a small effect on the probability of male homosexuality, but the effect grows rapidly stronger with three or more older brothers.
The presence of the fraternal older-brother effect means that the mother’s body should somehow “remember” previous births. MacCulloch and Waddington (1981) proposed a mechanism that makes this possible. In particular, they argued that in some pregnant women, the male fetus provokes a maternal immune response, which increases with each successive male pregnancy. The maternal antibodies that are produced to male fetal antigenic molecules or tissues cross the placenta and disrupt the neurohormonal development that is crucial for sexual differentiation of the fetal brain. In consequence, the male offspring displays female-typical sexual preferences (see also Blanchard & Klassen, 1997). There are other possible mechanisms, but there has not been any consensus yet on which mechanism is responsible for the female birth-order memory that mothers exhibit.
A further prediction is that a woman’s sexual orientation should not be affected by the presence of older male siblings, as women do not compete with their older brothers for resources because they do not need resources to gain access to the opposite sex. Consistent with this, Blanchard (2004) reviewed data from previews studies on sexual orientation and concluded that the sexual orientation of women is invisible to the birth-order effect.
Similarly, homosexuality is more beneficial in a man than in a woman, as, in the latter case, it does not have a significant impact in reducing interfamily conflict and increasing the reproductive success of older brothers. Therefore, this trait should be more prevalent among men than among women. In accordance with this prediction, studies that attempt to estimate the prevalence of homosexuality find that this trait is almost twice as common in men as in women (Dickson, Paul, & Herbison, 2003; Laumann, Gagnon, Michael, & Michaels, 1994; Sell, Wells, & Wypij, 1995).
Furthermore, if male homosexuality has been forged by evolutionary forces, it should be part of human nature and, consequently, it should be present in most human cultures. Also, if this trait has emerged in ancestral environments, it should have been present in ancestral societies. On this basis, it is predicted that male homosexuality is found across different cultures and across different times. Historical and anthropological resources confirm this prediction (Barber, 1998; Murray, 2000).
As well as cross-cultural consistency, there should also be cross-cultural variability in the prevalence of male homosexuality. In more detail, in foraging societies, little wealth is produced, so there are few resources available to the family unit that can be employed for the purpose of reproduction. This is not the case, however, in agropastoral societies, where considerably more wealth, such as land and animals, is available. As there is more wealth around, parents place more emphasis on the resources a prospective son-in-law controls. Accordingly, one study found that parents are more interested in the wealth of a prospective son-in-law in agropastoral than in foraging societies (Apostolou, 2010a). In this case, then, they demand more resources for granting access to the reproductive capacity of their daughters.
For instance, bridewealth is more common (and its level is higher) in agropastoral societies than in foraging ones (Goody & Tambiah, 1973; Murdock, 1981), which makes the struggle over resources even more intense and strengthens the correlation between male reproductive success and resources. In turn, this indicates that there should be more conflict between male brothers over family resources in agropastoral societies, making male homosexuality more beneficial in this context. On this basis, it can be predicted that male homosexuality should be more prevalent in agropastoral societies than in foraging ones. A study of homosexuality in preindustrial societies provides support for this prediction (Barber, 1998).
Overall, the competition-over-resources hypothesis gives a good account of the known patterns of male homosexuality. In addition, it generates several new predictions that future research needs to explore. To begin with, less conflict over resources is expected in families where there are one or more homosexual younger brothers than in families with the same number of brothers who are all heterosexual. A further prediction is that, ceteris paribus, in a preindustrial context, men with one or more homosexual male siblings should enjoy a higher reproductive success (more wives and younger, more attractive wives, etc.) than men with the same number of heterosexual brothers.
In addition, when parents exercise strong control over mating, the inclusive fitness benefit of male homosexuality increases and its reproductive cost decreases. This is because parents are less likely to put an emphasis on looks, so they will be less likely to compromise on resources, meaning that, in societies where mate choice is regulated and arranged marriage is prevalent, men will need more resources to gain long-term access to women than men in societies where mating is not regulated. Furthermore, in societies where marriages are arranged, male homosexuals are more likely to get married and have children than in societies where mating is free and individuals have to seek and find their own mates. Accordingly, it is predicted that there is an inverse relationship between the prevalence of male homosexuality and the degree of parental control over mate choice.
Outside family factors, such as the availability of women and how competitive access is to them, may also have an effect. For instance, in an environment where the gender ratio favors men—that is, there are more women than men—competition over gaining access to women is likely to be lower and, consequently, the inclusive fitness benefit of male homosexual orientation should also be lower. Male homosexuality, then, is likely to be less prevalent in a context where the gender ratio favors men than where it is balanced or where it favors women.
General Discussion
In the competition-over-resources hypothesis put forward here, where mate choice is dominated by parents, male homosexuality has been positively selected because it increases the reproductive success of older brothers and decreases interfamily conflict. This model accounts for the fraternal older-brother effect, the biological basis of male homosexuality, homosexuality being more prevalent in men, male homosexuality being present across different cultures and across different times, and male homosexuality being more prevalent in agropastoral than in foraging societies.
The key factor in this model is the fraternal older-brother effect: For homosexuality to provide inclusive fitness benefits, a male homosexual should have older brothers. Thus, this model does not account for all the cases of male homosexuality because there are homosexual men without older male siblings. More specifically, it is estimated that around 20% of male homosexuals owe their sexual orientation to the fraternal birth-order effect (Blanchard, 2004). This is most likely an underestimation, given that male homosexuality may be triggered by the presence of an older brother, even if he is not present at the time of study.
In particular, a male fetus may not have been born due to abortion; still, the mother’s body may “remember” the pregnancy of a male child, triggering male homosexuality in later births (Bogaert, 2006). In addition, in certain cases, an older brother may have been absent at the time of the study due to death, a possibility that has not been taken into consideration in many studies. Even if this is so, there still remains a considerable share of male homosexuality that is not accounted for by the older-brother effect and, consequently, by the proposed model.
Moreover, in hunting and gathering societies where parental control over mating is less restrictive and few resources are produced, gaining access to women is much less strongly correlated with the resources that a man controls, and families possess little wealth. In this case, male homosexuality will have very small inclusive fitness benefits in reducing family conflict and in increasing the mating success of older brothers. Consequently, male homosexuality attributed to the older-brother effect is most likely a product of recent evolutionary pressures operating during the period of evolution humans spent as agropastoralists. If this hypothesis is correct, male homosexuality should be more prevalent in agropastoral than in foraging societies (which appears to be the case), and the older-brother effect should not be present in foraging societies (that is, in societies that have not made the transition to agropastoralism), a hypothesis that remains to be tested.
Overall, the consequence of the transition from hunting and gathering to agropastoralism is likely to have been an increase in the prevalence of male homosexuality. The next question to address is the impact on the prevalence of male homosexuality of the transition from preindustrialism to postindustrialism. In postindustrial societies, considerably more resources are produced, which increases interfamily conflict over wealth. Also, women—although they are no longer controlled by their parents—similar to their parents, require resources or evidence of a resource-generating ability to grant access (Buss, 2003). In consequence, men who possess more resources remain more successful in the mating market, which should have a positive impact on the prevalence of male homosexuality.
Nevertheless, there are several evolutionary forces that work against male homosexuality attributed to the older-brother effect. To begin with, polygyny is not allowed and the use of contraception prevents sexual relationships with different mates being converted into reproductive benefits. In addition, women are not controlled by their parents, and men with low resource acquisition capacity can be successful in attracting mates as long as they possess other qualities such as beauty and charm. Therefore, opting out from the mating competition is costlier in a postindustrial setting as the opportunity cost is higher. More importantly, in such a context, a homosexual man is unlikely to have his marriage arranged by his parents and, consequently, less likely to have children, which, in turn, makes male homosexuality costly.
In sum, it seems that, in postindustrial societies, the inclusive fitness benefits of homosexuality are lower and its costs are higher than in agropastoral preindustrial ones. Because postindustrial societies have recently made the transition from agropastoralism, it is likely that the prevalence of male homosexuality due to the older-brother effect lies above the optimum level justified by modern conditions and it is likely to be selected against.
One further question that needs to be addressed is why evolution has favored homosexuality instead of asexuality (Kirkpatrick, 2000). The evolutionary framework put forward here gives a plausible answer: In a preindustrial context, parents are likely to arrange the marriage of their male homosexual sons with women of their choice. These marriages are likely to result in children, as a homosexual man does not lose his capacity to reproduce and he is likely to divert his sexual urges to his wife, even if she is not the desirable target. Nevertheless, an asexual man would not be able to have children from such a marriage. This translates into asexuality being much more reproductively costly than homosexuality, at least in a context where marriages are arranged. It follows that evolutionary forces are much less likely to favor asexuality than homosexuality.
Research indicates that parents react negatively and with distress, at least at first, when they find out that their children have a homosexual orientation (Wisniewski, Robinson, & Deluty, 2010), particularly their sons (Conley, 2011; although parents, in many instances, respond in a supportive fashion; see Rothman, Sullivan, Keyes, & Boehmer, 2012). On this basis, it can be argued that although male homosexuality may, on the one hand, reduce interfamily conflict, on the other, it may increase it. Still, one primary reason for parents’ negative reaction that has been identified by literature is that they worry that their children’s homosexuality will compromise their chances of having grandchildren (Conley, 2011). This means that in a preindustrial context, where families are larger (and thus, parents have alternative solutions) and marriages are arranged (thus, they can marry their homosexual son[s]), distress from homosexual orientation should be small. In addition, the fraternal older-brother effect ensures that there are older brothers available to provide parents with grandchildren.
Although the present framework aims to account for the evolution of male homosexuality, it can also provide us with certain insights on the evolution of female homosexuality. In a preindustrial context where marriages are arranged, female homosexuality is going to have little impact on a woman’s reproductive success. That is, irrespectively of her sexual orientation, a woman in this context will have her marriage arranged by her parents. Moreover, women do not need to be sexually attracted to their partners for sexual intercourse to become possible, and they are physically weaker than men, who can force sex on them. This is not to say, of course, that female homosexuality is not costly; for instance, it may result in less stable marriages and a higher frequency of divorce, which may be distractive for children. But it says that this trait is not so costly in a woman and it may require, in effect, a small inclusive fitness benefit to balance it—a benefit that remains to be identified.
Overall, because finding a mate and having children with is not up to a woman, her sexual orientation is a weak predictor of her mating success. In effect, in a context where mate choice is regulated by parents, female homosexuality has small reproductive costs, meaning that a moderate, or even small, fitness gain can maintain it in the population. What this fitness gain is, however, needs to be identified.
It has been argued that the resources parents ask in return for allowing reproductive access to their daughter (i.e., the bridewealth) is one of the factors that leads to conflict between brothers over family resources, which, in turn, makes male homosexuality in younger brothers evolutionary optimal. In a small minority of preindustrial societies, the institution of dowry, where the parents of the bride give considerable wealth to the groom, is practiced (Murdock, 1981); because, in this case, a man does not need many resources to gain long-term access to a woman, the question that comes about is whether male homosexuality is beneficial in this context. Although the practice of dowry is rare, answering this question is important if one takes into consideration that this was practiced in Imperial Rome and in Classical Greece, from which many modern Europeans descended.
In the societies where dowry is practiced, parents still maintain a strong interest for the resources that a prospective son-in-law controls, and they actually use dowry to attract wealthy individuals as spouses for their daughters. In Classical Greece,
A girl’s kyrios was under strong moral obligation to dower her and to dower her with as much as possible, both in order to find her the best husband (preferably a wealthy husband), and as an expression of his own standing. (Schaps, 1979, pp. 74-75)
Thus, in this context, men still have to compete with each other for gaining control over resources, as this will get them long-term access to women. Competition can be even fiercer, because the man who will monopolize resources will not only gain long-term access to woman but also get resources from her parents in the form of dowry. Consequently, male homosexuality in dowry-practicing societies has a positive effect in reducing interfamily conflict over resources, and increasing the fitness of older brothers who can gain access to higher quality women and also receive additional resources (i.e., a dowry). In effect, in dowry-practicing societies, the frequency of male homosexuality may be higher, a hypothesis that waits future testing.
To summarize, in a context where mating is regulated and parents require evidence of controlling resources in order to grant mating access to their daughters, homosexuality is likely to have been favored by evolution in men with older brothers by providing inclusive fitness benefits in terms of lower interfamily conflict and higher reproductive success of older male siblings. The predictions of the model fit well the known patterns of male homosexuality, indicating that it may provide a partial solution to the mystery of its evolutionary origins.