Biology and sexual orientation in the context of "Homosexuality"

Homosexuality is romantic attraction, sexual attraction, or sexual behavior between people of the same sex or gender. It also denotes identity based on attraction, related behavior, and community affiliation.

Along with bisexuality and heterosexuality, homosexuality is one of the three main categories of sexual orientation within the heterosexual–homosexual continuum. Although no single theory on the cause of sexual orientation has yet gained widespread support, scientists favor biological theories. There is considerably more evidence supporting nonsocial, biological causes of sexual orientation than social ones, especially for males. A major hypothesis implicates the prenatal environment, specifically the organizational effects of hormones on the fetal brain. There is no substantive evidence which suggests parenting or early childhood experiences play a role in developing a sexual orientation. Scientific research shows that homosexuality is a natural and normal variation in human sexuality and is not in and of itself a source of negative psychological effects. Major mental health organizations overwhelmingly reject conversion therapy as ineffective, scientifically unsupported and potentially harmful.

Biological determinism, also known as genetic determinism, is the belief that human behaviour is directly controlled by an individual's genes or some component of their physiology, generally at the expense of the role of the environment, whether in embryonic development or in learning. Genetic reductionism is a similar concept, but it is distinct from genetic determinism in that the former refers to the level of understanding, while the latter refers to the supposed causal role of genes. Biological determinism has been associated with movements in science and society including eugenics, scientific racism, and the debates around the heritability of IQ, the basis of sexual orientation, and evolutionary foundations of cooperation in sociobiology.

In 1892, the German evolutionary biologist August Weismann proposed in his germ plasm theory that heritable information is transmitted only via germ cells, which he thought contained determinants (genes). The English polymath Francis Galton, supposing that undesirable traits such as club foot and criminality were inherited, advocated eugenics, aiming to prevent supposedly defective people from breeding. The American physician Samuel George Morton and the French physician Paul Broca attempted to relate the cranial capacity (internal skull volume) to skin colour, intending to show that white people were superior. Other workers such as the American psychologists H. H. Goddard and Robert Yerkes attempted to measure people's intelligence and to show that the resulting scores were heritable, again to demonstrate the supposed superiority of people with white skin.

Sexual fluidity is one or more changes in sexuality or sexual identity (sometimes known as sexual orientation identity). Sexual orientation is stable for the vast majority of people, but some research indicates that some people may experience change in their sexual orientation, and this is more likely for women than men. There is no scientific evidence that sexual orientation can be changed through psychotherapy. Sexual identity can change throughout an individual's life, and does not have to align with biological sex, sexual behavior, or actual sexual orientation.

According to scientific consensus, sexual orientation is not a choice. Although no single theory on the cause of sexual orientation has yet gained widespread support, scientists favor biological theories, especially for explaining male sexual orientation. Research over several decades has demonstrated that sexual orientation can be at any point along a continuum, from exclusive attraction to the opposite sex to exclusive attraction to the same sex.