Page "Biology and sexual orientation" Paragraph 1

A number of twin studies have attempted to compare the relative importance of genetics and environment in the causation of sexual orientation.

In a 1991 study, Bailey and Pillard found that 52 % of monozygotic ( MZ ) brothers and 22 % of the dizygotic ( DZ ) twins were concordant for homosexuality.

' MZ ' indicates identical twins with the same sets of genes and ' DZ ' indicates fraternal twins where genes are mixed to a similar extent as non-twin siblings.

In 2000 Bailey, Dunne and Martin found similar results from a larger sample of 4, 901 Australian twins.

Self reported zygosity, sexual attraction, fantasy and behaviours were assessed by questionnaire and zygosity was serologically checked when in doubt.

They found 20 % concordance in the male identical or MZ twins and 24 % concordance for the female identical or MZ twins.

A meta-study by Hershberger ( 2001 ) 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.