(05) HOMOSEXUALITY'S EVOLUTION AND THE "GAY GENE":
5.1) Sexuality is Hereditary
5.2) Twin Studies
5.3) Brain Structure
5.4) FBO Effect
5.5) Evolutionary Viability
To consider homosexuality as being either determined by a single "gay gene" or a purely psychological choice is an oversimplification. There is no "black gene". The current scientific consensus is that sexual orientation is at least partially determined by hereditary/ congenital factors.
5.1) SEXUALITY DOES HAVE A HEREDITARY COMPONENT:
Kendler et al. 2000, The American Journal of Psychiatry, 157(11), 1843-1846:
"Biometrical twin modeling suggested that sexual orientation was substantially influenced by genetic factors."
Mustanski et al. 2002, Annual Review of Sex Research, 13, 89-140:
"Genetic research using family and twin methodologies has produced consistent evidence that genes influence sexual orientation".
Langstrom et al. 2010, Archives of Sexual Behavior, 39(1), 75-80:
"In men, the full twin model suggested heritability estimates of 39%".
Burri et al. 2011, PLoS One, 6(7), e21982:
"In summary, we found genetic influences on female sexual orientation".
Biology of homosexuality - Epigenetics may play a role. 7:06
Some of the latest research indicates an epigenetic model of sexual orientation development. Epigenetics is the study of the chemical modification of DNA and therefore gene expression through means other than a change in the nucleotide sequence:
Rice et al. 2012, The Quarterly Review of Biology, 87(4), 343-368, DOI: 10.1086/668167:
"Our model predicts that homosexuality is part of a wider phenomenon in which recently evolved androgen-influenced traits commonly display gonad-trait discordances at substantial frequency, and that the molecular feature underlying most homosexuality is not DNA polymorphism(s), but epi-marks that evolved to canalize sexual dimorphic development that sometimes carryover across generations and contribute to gonad-trait discordances in opposite-sex descendants."
5.2) TWIN STUDIES:
Identical twins don't have identical fingerprints or palmprints. This refutes suggestions that homosexuality cannot be prenatally determined because monozygotic (identical) twins do not always have the same sexual orientation:
Jain et al. 2001, Lecture Notes in Computer Science, 2091, 211-217:
"We show that a state-of-the-art automatic fingerprint identification system can successfully distinguish identical twins."
A. Kong et al. 2005, Lecture Notes in Computer Science, 3832, 668-674:
"The experimental results show that we can employ low-resolution palmprint images to distinguish identical twins."
They exhibit a much higher likelihood of sharing the same sexual orientation:
Whitam et al. 1993, Archives of Sexual Behavior, 22(3), 187-206:
"Thirty-eight pairs of monozygotic twins (34 male pairs and 4 female pairs) were found to have a concordance rate of 65.8% for homosexual orientation. Twenty-three pairs of dizygotic twins were found to have a concordance rate of 30.4% for homosexual orientation."
5.3) BRAIN STRUCTURE:
One group of researchers have carried out studies assessing the brain structures of homosexuals and comparing them to heterosexuals of the opposite sex. The findings indicate a correlation between homosexuality and certain brain responses mirroring those of members of the opposite sex (I.E. gay men similar to straight women etc). The third study by this group was designed to determine whether or not this was likely the result of having homosexual interactions (learned) or of biological origin. Newscientist provides a good summary of some similar studies on transsexuals.
The Two Initial Studies:
Savic et al. 2005, PNAS, 102(20), 7356–7361, DOI: 10.1073/pnas.0407998102
Berglund et al. 2006, PNAS, 103(21), 8269–8274, DOI: 10.1073/pnas.0600331103
Savic & Lindstrom, 2008, PNAS, 105(27), 9403–9408, DOI: 10.1073/pnas.0801566105
"The present study shows sex-atypical cerebral asymmetry and functional connections in homosexual subjects. The results cannot be primarily ascribed to learned effects, and they suggest a linkage to neurobiological entities."
5.4) FRATERNAL BIRTH ORDER EFFECT:
The fraternal birth order effect and maternal immune hypothesis is an example of a prenatal, purely biological, environmental effect that correlates with homosexuality in men. Having more older brothers corresponds with a higher likelihood of homosexuality:
Anthony F. Bogaert, 2006, PNAS, 103(28), 10771-10774:
"Only biological older brothers, and not any other sibling characteristic, including nonbiological older brothers, predicted men’s sexual orientation, regardless of the amount of time reared with these siblings. These results strongly suggest a prenatal origin to the fraternal birth-order effect".
R Blanchard, 2001, Journal of Hormones and Behavior, 40(2), 105-114:
"In men, sexual orientation correlates with an individual's number of older brothers, each additional older brother increasing the odds of homosexuality by approximately 33%."
5.5) EVOLUTIONARY VIABILITY:
Some heterosexists point out that homosexuality cannot be genetic because it is not evolutionarily viable:
- Parents can carry genes that are expressed in their offspring, without the parent themselves expressing it.
- Homosexuals can benefit a community, and therefore contribute to its reproductive output indirectly.
- Anthropologists consider "cooperative breeding" to be reasonably common in humanity's history.
- Genes associated with homosexuality could have been passed on and conserved via their relatives.
Two of many ways in which homosexuality may be evolutionarily beneficial:
Camperio Ciani et al. 2008, Archives of Sexual Behavior, 38(3), 393-399:
"Data have emerged suggesting not only an increase in maternal fecundity but also larger paternal family sizes for homosexuals."
Kirkpatrick et al. 2000, Current Anthropology, 41(3), 385-413:
"Same-sex alliances have reproductive advantages, and sexual behavior at times maintains these alliances."