How can homosexuality be explained from an evolutionary perspective?
Homosexuality from an evolutionary perspective
Maastricht University
April, 2019
Faculty of Psychology & Neuroscience
Bachelor thesis Word count: 5439
1
, How can homosexuality be explained from an evolutionary perspective?
Abstract
One of the remaining mysteries in socio-biology is how homosexuality has been maintained
in the population, being so unbeneficial for reproductive success as it is. With significantly
less offspring than heterosexuals, homosexuals have slim chances of spreading their genes, as
the laws of natural selection dictate. However, the estimate percentage of homosexuals in
Western societies fluctuates around a sufficient 10% so there must be an explanation.
Scientists have found significant differences between DNA, hormones, brain structures and
brain activity of homosexuals and heterosexuals. All explain parts of homosexuality, but
neither is able to provide a full explanation. Theorists have also suggested paradigms such as
kin selection, helpers at the nest and the strengthening of social bonds as phenotypic traits
which could explain homosexuality, but these are not robust against each other. A definitive
explanation is yet to be discovered. Further research could focus on the functioning of
relevant pleiotropic genes, and researchers can merge their powers to combine theories that
together could make up for a full explanation of homosexuality.
2
, How can homosexuality be explained from an evolutionary perspective?
1. Introduction
Homosexuality is a popular debate, not only within a scientific framework, but also in society.
Marriage between same-sex individuals is becoming morally accepted and legally possible in
more countries every year and hash tags such as ‘#ComingOut’ are trending on social media
(Asal, Sommer, & G. Harwoord, 2012). Companies and governments make clear where they
stand in the debate by renouncing the ‘Nashville Statement’ (2018) : a document in which
Christianity rebukes homosexuality and gender identification. Even commercial television
programs are dedicated to show their audience that there is no shame in identifying with the
‘LGBT+’-community.
Homosexuality is a widely researched topic in both a biological and psychological
context. A large body of empirical research contributes towards knowledge concerning
different components of homosexuality, but no ultimate explanation has yet been formulated.
Some studies on homosexuality have only addressed sexual behaviour between same-
sex individuals, whereas other studies mainly focussed on sexual attraction towards same-sex
individuals. Although these are distinct categories, these are not disjoint; there is a substantial
overlap between these two components of homosexuality. It is estimated that about 10% of
the world population is homosexual, and even more people engage in same-sex sexual
behaviour alone (without attraction) (Sell, Wells, & Wypij, 1995).
The observation that about 10% of the population in one way or another self-identifies
as homosexual contradicts the theory of homosexuality as being the result from a random
gene mutation. It should be mentioned that no gene has yet been discovered that can reliably
predict sexual orientation (Eaves & Hatemi, 2008). According to Goldstein (2009), a variation
in genetics can be categorized as a trait when the according phenotype is commonly observed
in the population. As homosexuality is commonly observed in populations around the world,
this is sufficient enough to call homosexuality a phenotypic trait. According to Darwin’s laws
(which will be elaborated further in section two of this literature review),a trait is expected to
be maintained in the population, only if it has a beneficial effect for the species’ survival
within the current environment. Through a process Darwin called ‘adaptation’, species are
expected to develop phenotypes that are in any way beneficial for their survival, and to drop
phenotypes that are unbeneficial. Darwin explains how beneficial traits that are embedded
within the genes are likely to be inherited by the next generation, thereby maintaining the trait
in the population (Darwin, 1859). Therefore it can be expected that any commonly observed
3
