The relationship between biology and sexual orientation is a subject of research. While scientists do not know the exact cause of sexual orientationthey theorize that a combination of genetic, hormonal, and social factors determine it. Biological theories for explaining the causes of sexual orientation are favored by scientists  and involve a complex interplay of genetic factors, the early uterine environment and brain structure.
A number of twin studies have attempted to compare the Concordance rates for homosexuality importance of genetics and environment in the determination of sexual orientation. Self reported zygositysexual attraction, fantasy and behaviours were assessed by questionnaire and zygosity was serologically checked when in doubt.
Other researchers support biological causes for both men and women's sexual orientation. A study of all adult twins in Sweden more than 7, twins  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 experienceswhile influences of shared-environment variables such as familial environment and social attitudes had a weaker, but significant effect.
Women showed a statistically non-significant trend to weaker influence of hereditary effects, while men showed no effect of shared environmental effects. The use of adult twins in Sweden was designed to address the criticism of volunteer studies, in which a potential bias towards participation by gay twins may influence the results.
Biometric modeling revealed that, in men, genetic effects explained. Corresponding estimates among women were. Although wide confidence intervals suggest cautious interpretation, the results are consistent with moderate, Concordance rates for homosexuality genetic, familial effects, and moderate to large effects of the nonshared environment social and biological on same-sex sexual behavior.
Twin studies have received a number of criticisms including self-selection bias where homosexuals with gay siblings are more likely to volunteer for studies.
Nonetheless, it is possible to conclude that, given the difference in sexuality in so many sets of identical twins, sexual orientation cannot be attributed solely to genetic factors. Another issue is the recent finding that even monozygotic twins can be different and there is a mechanism which might account for monozygotic twins being discordant for homosexuality. Gringas and Chen describe a number of mechanisms which can lead to differences between monozygotic twinsthe most relevant Concordance rates for homosexuality being chorionicity and amniocity.
Monoamniotic twins share a hormonal environment, but can suffer from the 'twin to twin transfusion syndrome' in which one twin is "relatively stuffed with blood and the other exsanguinated". Chromosome linkage studies of sexual orientation have indicated the presence of multiple contributing genetic factors throughout the genome.
In Dean Hamer and colleagues published findings from a linkage analysis of a sample of 76 gay brothers and their families. Gay brothers who showed this maternal pedigree were then tested for X chromosome linkage, using twenty-two markers on the X chromosome to test for similar alleles.
This was popularly dubbed the " gay gene " in the media, causing significant controversy. A later analysis by Hu et al. In the full sample they did not find linkage to Xq Results from the first large, comprehensive multi-center genetic linkage study of male sexual orientation were reported by an independent group of researchers at the American Society of Human Genetics in Significant linkage was also detected in the pericentromeric region of 8, overlapping with one of the regions detected in the Hamer lab's previous genomewide study.
The authors concluded that "our findings, taken in context with previous work, suggest that genetic variation in each of these regions contributes to development of the important psychological trait of male sexual orientation". Female sexual orientation does not seem to be linked to Xq28,   though it does appear moderately heritable. In addition to sex chromosomal contribution, a potential autosomal genetic contribution to the development of homosexual orientation has also been suggested.
In a study population composed of more than participants, Ellis et al. They also found that "unusually Concordance rates for homosexuality proportions of homosexual males and homosexual females were Rh negative in comparison to heterosexuals.
As both blood type and Rh factor are genetically inherited traits controlled by alleles located on chromosome 9 and chromosome 1 respectively, the study indicates a potential link between genes on autosomes and homosexuality. The biology of sexual orientation has been studied in detail in several animal model systems.
In the common fruit fly Drosophila melanogasterthe complete pathway of sexual differentiation of the brain and the behaviors it controls is well established in both males and females, providing a concise model of biologically controlled courtship. Without the gene, the mice exhibited masculine sexual behavior and attraction toward urine of other female mice. Those mice who retained the gene fucose mutarotase FucM were attracted to male mice. In interviews to the press, researchers have pointed that the evidence of genetic influences should not be equated Concordance rates for homosexuality genetic determinism.
According to Dean Hamer and Michael Bailey, genetic aspects are only one of the multiple causes of homosexuality. InNature published an article with a genome wide association study on male sexual orientation.
The research consisted of 1, homosexual men and 1, heterosexual men. Levay's research suggested that the hypothalamus of gay men is different from straight men.