For example, a meta-study by Hershberger (2001)
[5] compares the results of
eight different twin studies: among those, all but two showed MZ twins having much higher concordance of sexual orientation than DZ twins, suggesting a non-negligible genetic component. Two additional examples:
Bailey and Pillard (1991) in a study of gay twins found that 52% of monozygotic (MZ) brothers and 22% of the dizygotic (DZ)
twins were concordant for homosexuality.
[6] Also, Bailey, Dunne and Martin (2000) used the Australian twin registry to obtain a sample of 4,901 twins.
[7] Self reported
zygosity, sexual attraction, fantasy and behaviours were assessed by questionnaire and zygosity was
serologically checked when in doubt. MZ twin concordance for homosexuality was found to be 30%.