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Identical twin studies are a useful mechanism for assessing the role of genes and environment. Twin studies have also found that among twins with differing sexual orientations, homosexual twins were significantly more gender nonconforming than their heterosexual co-twin, and that this was noticeable from a young age. [29] Bailey states:
Of the pairs of identical twins in which one twin is homosexual, the other twin, despite having the same genome, only has a 20-50% chance of being homosexual as well. [12] This leads to the hypothesis that homosexuality is created by something else rather than the genes.
Bailey carried out early twin studies on homosexuality, finding higher rates of concordance for sexual orientation in identical twins than fraternal twins, suggesting genes or shared prenatal environment have influence on sexual orientation. [17]: 88 Bailey has argued that male sexual orientation appears unaffected by socialization.
The fraternal birth order effect has been described by one of its proponents as "the most consistent biodemographic correlate of sexual orientation in men". [5] In 1958, it was reported that homosexual men tend to have a greater number of older siblings (i.e., a 'later/higher birth order') than comparable heterosexual men and in 1962, these findings were published in detail. [6]
A twin study from 2001 appears to exclude genes as a major factor, [53] while a twin study from 2010 found that homosexuality was explained by both genes and environmental factors. [58] However, experimental design of the available twin studies has made their interpretation difficult.
A twin study compares the concordance rate of identical twins to that of fraternal twins. This can help suggest whether a disease or a certain trait has a genetic cause. [3] Controversial uses of twin data have looked at concordance rates for homosexuality and intelligence. Other studies have involved looking at the genetic and environmental ...
The power of twin designs arises from the fact that twins may be either identical (monozygotic (MZ), i.e. developing from a single fertilized egg and therefore sharing all of their polymorphic alleles) or fraternal (dizygotic (DZ), i.e. developing from two fertilized eggs and therefore sharing on average 50% of their alleles, the same level of genetic similarity found in non-twin siblings).
MTFS is a twin study established in June 1989 with 1300 same-gendered twin pairs age 11 or 17, with an additional cohort of 500 such pairs recruited around 2004. Twins were born between 1972 and 2000. [1] All twins born in Minnesota at that time were eligible to participate using birth registry data.