identified, and research is continuing in this area. Although researchers generally believe that sexual orientation is not determined by any one factor but by a combination of genetic, hormonal, and environmental influences, with biological factors involving a complex interplay of genetic factors and the early uterine environment, they favor biological models for the cause. There is considerably more evidence supporting nonsocial, biological causes of sexual orientation than social ones, especially for males. Scientists do not believe that sexual orientation is a choice, and some of them believe that it is established at conception. Current scientific investigation usually seeks to find biological explanations for the adoption of a particular sexual orientation. Scientific studies have found a number of statistical biological differences between gay people and heterosexuals, which may result from the same underlying cause as sexual orientation itself. Genetic factors Genes may be related to the development of sexual orientation. A twin study from 2001 appears to exclude genes as a major factor, while a twin study from 2010 found that homosexuality was explained by both genes and environmental factors. However, experimental design of the available twin studies has made their interpretation difficult. In 2012, a large, comprehensive genome-wide linkage study of male sexual orientation was conducted by several independent groups of researchers. Significant linkage to homosexuality was found with genes on chromosome Xq28 and chromosome 8 in the pericentromeric region. 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." It was the largest study of the genetic basis of homosexuality to date and was published online in November 2014. However, in August 2019, a genome-wide association study of 493,001 individuals concluded that hundreds or thousands of genetic variants underlie homosexual behavior in both sexes, with 5 variants in particular being significantly associated. They stated that in contrast to linkage studies that found substantial association of sexual orientation with variants on the X-chromosome, they found no excess of signal (and no individual genome-wide significant variants) on Xq28 or the rest of the X chromosome. Hormones The hormonal theory of sexuality holds that just as exposure to certain hormones plays a role in fetal sex differentiation, hormonal exposure also influences the sexual orientation that emerges later in the adult. Fetal hormones may be seen as either the primary