Heterosexual Male Carriers Could Explain Persistence of Homosexuality in Men: Individual-Based Simulations of an X-Linked Inheritance Model

Abstract

Homosexuality has been documented throughout history and is found in almost all human cultures. Twin studies suggest that homosexuality is to some extent heritable. However, from an evolutionary perspective, this poses a problem: Male homosexuals tend to have on average five times fewer children than heterosexual males, so how can a phenomenon associated with low reproductive success be maintained at relatively stable frequencies? Recent findings of increased maternal fecundity of male homosexuals suggest that the genes responsible for homosexuality in males increase fecundity in the females who carry them. Can an increase in maternal fecundity compensate for the fecundity reduction in homosexual men and produce a stable polymorphism? In the current study, this problem was addressed with an individual-based modeling (IBM) approach. IBM suggests that male homosexuality can be maintained in a population at low and stable frequencies if roughly more than half of the females and half of the males are carriers of genes that predispose the male to homosexuality.

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ORIGINAL PAPER
Heterosexual Male Carriers Could Explain Persistence of
Homosexuality in Men: Individual-Based Simulations
of an X-Linked Inheritance Model
Giorgi Chaladze
1
Received: 23 March 2015 / Revised: 30 September 2015 / Accepted: 15 March 2016/ Published online: 11 April 2016
Springer Science+Business Media New York 2016
Abstract Homosexuality has been documented throughout
history and isfound in almost all human cultures. Twin studies
suggest that homosexuality is to some extent heritable. However,
from an evolutionary perspective, this poses a problem: Male
homosexu als tend to have on ave rage five times fewe r children
than heterosexual males, so how can a phenomenon associated
with low reproductive success be maintained at relatively stable
frequencies? Recent findings of increased maternal fecundity of
male homosexuals suggest that the genes responsible for homo-
sexuality in males increase fecundity in the females who carry
them. Can an increase in maternal fecundity compensate for the
fecundity reduction in homosexual men and produce a stable
polymorphism? In the current study, this problem w as addressed
with an individual-based modeling (IBM) approach. IBM suggests
that male homosexuality can be maintained in a population at low
and stable frequencies if roughly more than half of the females and
half of the males are carriers of genes that predispose the male to
homosexuality.
Keywords Homosexuality Simulation 
Population Genetics X-linked Sexual orientation
Introduction
Homosexuality has been documented throughout history and is
found in almost all human cultures (Kirkpatrick, 2000). The
oldest depictions of homosexuality come from Mesolithic rock
art (Nash, 2001),althoughgenerallyitisacceptedthatthepreva-
lence of homosexuality in human societies is low (Diamond,
1993; Grulich, Visser, Smith, Rissel, & Richters, 2003; Sell,
Wells, & Wypij, 1995). Homosexual males on average have less
reproductive success than heterosexuals (Iemmola & Ciani,
2009; Rieger, Blanchard, Schwartz, Bailey, & Sanders, 2012;
Van de Ven, Rodden, Crawford, & Kippax, 1997). Consequently,
from an evolutionary perspective,homosexuality is a paradox:
How can a phenomenon associated with low reproductive suc-
cess be maintained at relatively stable frequencies (Hutchinson,
1959; Wilson, 1975)?
Research suggests that sexual orientation is influenced by
genetic factors. Specifically, monozygotic (MZ) twins, who share
all of their genes, are more likely to be concordant in their sexual
orientation than are dizygotic (DZ) twins or simple siblings, who
only share, on average, half of their genes (Kendler, Thornton,
Gilman, & Kessler, 2000; Kirk, Bailey, Dunne, & Martin, 2000).
In an effort to identify the gene or genes that influence sexual
orientation, Hamer, Hu, Magnuson, Hu, and Pattatucci (1993)
conducted a genetic study in a group of 40 families in which there
were two gay brothers. Their data indicated a linkage between the
Xq28 markers and sexual orientation. More recently, a genome-
wide study replicated results showing linkage of homosexuality
not only to Xq28, but also to elements on chromosome 8 (Sanders
et al., 2014).
It is, however, also demonstrable that homosexual men, com-
pared to heterosexual men, tend to come from larger families
(Ciani, Corna, & Capiluppi, 2004; Ciani & Pellizzari, 2012;
Iemmola & Ciani, 2009; King et al., 2005). Greater fecundity in
relatives of homosexual men could potentially explain the coun-
terintuitive persistence of homosexuality through sexually antag-
onistic selection—genes predisposing homosexuality in men
increase fecundity in females who carry them (Ciani & Pel-
lizzari, 2012; Iemmola & Ciani, 2009; Zietsch et al., 2008).
However, the positive fecundity increment accruing to moth-
ers of homosexual men is not large: 1.16 times higher per Ciani
et al. (2004), 1.31 times higher per Iemmola and Ciani (2009),
&Giorgi Chaladze
giorgi.chaladze.1@iliauni.edu.ge
1
Institute of Ecology, Ilia State University, 3/5 Cholokashvili
Ave, 0162 Tbilisi, Georgia
123
Arch Sex Behav (2016) 45:1705–1711
DOI 10.1007/s10508-016-0742-2
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