found that same-sex behavior was explained by both heritable factors and individual-specific environmental sources (such as prenatal environment, experience with illness and trauma, as well as peer groups, and sexual experiences), while influences of shared-environment variables such as familial environment and social attitudes had a weaker, but significant effect.
A number of twin studies have attempted to compare the relative importance of genetics and environment in the determination of sexual orientation.
In a 1991 study, Bailey and Pillard conducted a study of twins recruited from "homophile publications", and found that 52% of monozygotic (MZ) brothers (of whom 59 were questioned) and 22% of the dizygotic (DZ) twins were concordant for homosexuality.
Other researchers support biological causes for both men and women's sexual orientation.
They studied 289 pairs of identical twins (monozygotic or from one fertilized egg) and 495 pairs of fraternal twins (dizygotic or from two fertilized eggs) and found concordance rates for same-sex attraction of only 7.7% for male identical twins and 5.3% for females, a pattern which they say "does not suggest genetic influence independent of social context".
They also found that "unusually high" proportions of homosexual males and homosexual females were Rh negative in comparison to heterosexuals.