Encyclopedia of Evolutionary Psychological Science

Living Edition
| Editors: Todd K. Shackelford, Viviana A. Weekes-Shackelford

Alliance Formation Theory

  • Frank MuscarellaEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-3-319-16999-6_44-1


Dominance Hierarchy Alliance Formation Human Male Homosexual Behavior Bisexual Behavior 
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A theory holding that homosexual behavior in human males derives from selection for behavior that reinforces male alliances.


The incidence of exclusive same-sex sexual preference (exclusive homosexuality) in humans is greater in men than in women and varies across cultures and historical periods (LeVay 2011). The incidence in contemporary Western cultures is estimated to range between 2 % and 6 % (Barthes et al. 2013). Exclusive homosexuality is an evolutionary paradox, because it does not contribute to reproduction; consequently, it is unclear how genes that may contribute to it are inherited and maintained in the population. The trend in theory and research has been to identify a single explanation. However, recently, some authors have made compelling arguments for more complex and multicomponent explanations that involve both ultimate (evolutionary) and proximate (environmental, developmental) factors (e.g., LeVay 2011; Poiani 2010).

The alliance theory of male-male sexual behavior in humans posits the development of one of the components that may contribute to the existence of exclusive homosexuality in humans: the evolutionary component. It takes into account theory of human evolution and unique species-specific aspects of human evolution. Nonexclusive homosexual behavior occurs in many animals (Bagemihl 1999), and its causes and functions may be similar in some species and quite different in others (Poiani 2010). Consequently, human homosexual behavior should be studied in the context of the evolution of humans and closely related species. The expression of homosexual behavior among primates varies across species and tends to be confined to Old World monkeys, apes, and humans. This suggests both shared phylogenic origins and current adaptive value (Poiani 2010).

The Alliance Theory

The alliance theory holds that homosexual behavior reinforced alliances between hominin (human ancestor) males that contributed to their ability to survive and ultimately to find female mates and reproduce (Kirkpatrick 2000; Muscarella 2000, 2006). This may have been particularly adaptive in the case of juvenile males who are likely to have been peripheralized or pushed to the outer edge of the group for some period of time as is seen with juveniles in other species. The juveniles remain peripheralized until they are able to work their way up the male dominance hierarchy. At this time they are vulnerable to predation, intraspecies aggression, and lack of resources. The use of homosexual behavior as sociosexual behavior that reinforces alliances is seen in other primates. Dyadic relationships increase the status of both individuals, and their alliance allows both to more effectively defend against aggressors and prevent the taking of resources such as food. The expression of homosexual behavior in human males across history and across cultures can often be interpreted as supporting alliances between the individuals (Kirkpatrick 2000). There may have been even greater selection pressure for the expression of homosexual behavior as a mechanism of alliance formation in humans because of a factor unique to human evolution.

The Role of Dominance Hierarchies

As described in the alliance theory, one of the most important aspects of human evolution was the development of complex social organization and the characteristics that allowed for this including the abilities to cooperate, form alliances, and live in a dominance hierarchy. Dominance hierarchies would have been in flux for hominins, and one’s positions of subordination and dominance would have changed across the lifespan due to developmental changes and changes in the membership of the hierarchy itself as a function of births, deaths, illness, and injury. Human males in particular appear to have evolved numerous behavioral characteristics that facilitate the development and maintenance of within-coalition dominance hierarchies (Geary et al. 2003). In many social animals homosexual behavior serves a sociosexual role for the expression and maintenance of positions of dominance or subordination in a hierarchy and for the establishment of pair-bonds that lead to alliance formations (Poiani 2010). This is particularly evident in humans’ closest primate relatives, chimpanzees and bonobos (Muscarella 2000). Homosexual behavior in humans is speculated to be an exaptation, that is, it may have evolved originally as a sociosexual behavior related to the expression of dominance and subordination and selection acted upon it further, because it also contributed to the development and reinforcement of alliances that contributed directly to survival and indirectly to reproduction (Muscarella 2006).


The alliance theory actually describes selection for bisexual behavior, that is, nonexclusive homosexual behavior under certain circumstances and heterosexual behavior, which is the sine qua non of reproduction and the passing on of genes that may be related to homosexual behavior. The theory does not account for exclusive homosexual behavior which is posited to be due to an interaction between the species-specific and genetically influenced expression of homosexual behavior characteristic of humans and developmental, ecological, cultural, and psychological factors (Kirkpatrick 2000; LeVay 2011; Muscarella 2006; Poiani 2010). The alliance theory does not address female homosexual behavior although it may also be related to alliance formation (Muscarella 2000; Poiani 2010). However, the evolutionary nature of human female dominance hierarchies and coalitional strategies is very different from that of human males (Geary et al. 2003), which may affect the expression of homosexual behavior in human females (Muscarella 2015). The alliance theory has heuristic value, but to date, there is no empirical evidence that supports it.



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© Springer International Publishing Switzerland 2016

Authors and Affiliations

  1. 1.Barry UniversityMiami ShoresUSA