[southernbelle]

I found some interesting research from twin studies in which they examined homosexual and heterosexual twins.

Twin studies have become a highly debated area of experimentation. Ernest Kallman conducted the earliest twin study. He found a 100% concordance between monozygotic (or identical) twins (MZ), and only a 12% concordance for dizygotic (or fraternal) twins (DZ). Although discredited with methodological problems, the early experiment paved the way for a much-publicized team to conduct their twin studies.

J. Michael Bailey and Richard Pillard also studied the gayness between MZ twins, DZ twins, and non-related adopted brothers. They examined how many of the sample population examined were gay and how many were straight. They found that 52% of MZ twins were both self-identified homosexuals, 22% of DZ twins were so, and only 5% of non-related adopted brothers were so. This evidence, repeated and found to be true a second time, showed to the biological camp that the more closely genetically linked a pair is, the more likely they both are to exhibit gay or straight tendencies. Later experimenters found similar evidence in females.

Source: http://allpsych.com/journal/homosexuality.html