J. Michael Bailey and Richard Pillard also
studied sexual orientation between identical, fraternal twins, and non-related adopted brothers. They examined how many of
the sample population examined were homosexual and how many were heterosexual. They found that 52% of identical twins were
both self-identified homosexuals, 22% of fraternal twins were so, and only 5% of non-related adopted brothers were so. This
evidence, repeated and found to be true a second time, may suggest that genetics play an overriding role in sexual orientation
over upbringing or the nurture aspect of life.
Later experiments have found the possibility
of homosexuality being an X-linked trait. Dean Hamer genetically examined 40 DNA samples from homosexual men. He found that
there was a “remarkable concordance” for 5 genetic markers on section of the X-Chromosome called Xq28. Hamer hypothesized
upon examining the family trees of the same men that on each subject’s mother’s side, there were markedly larger
numbers of homosexual men, all stemming through the maternal lineages. This observation, along with his discovery on Xq28,
led to his findings to reinforce the “homosexual gene study”.