The relationship between the environment and sexual orientation is a subject of research. In the study of sexual orientation, some researchers distinguish environmental influences from hormonal influences, while other researchers include biological influences such as prenatal hormones as part of environmental influences.

Scientists do not know the exact cause of sexual orientation, but they theorize that it is the result of a complex interplay of genetic, hormonal, and environmental influences. Most published scientific works on the subject do not view sexual orientation as a choice.

Evidence for the impact of the post-natal social environment on sexual orientation is weak, especially for males. There is no substantial evidence which suggests parenting or early childhood experiences influence sexual orientation, but research has linked childhood gender nonconformity and homosexuality.

Often, sexual orientation and sexual orientation identity are not distinguished, which can impact accurately assessing sexual identity and whether or not sexual orientation is able to change; sexual orientation identity can change throughout an individual's life, and may or may not align with biological sex, sexual behavior or actual sexual orientation. Sexual orientation is stable and unlikely to change 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 for men. The American Psychological Association distinguishes between sexual orientation (an enduring attraction) and sexual orientation identity (which may change at any point in a person's life). Scientists and mental health professionals generally do not believe that sexual orientation is a choice.

The American Psychological Association states that "sexual orientation is not a choice that can be changed at will, and that sexual orientation is most likely the result of a complex interaction of environmental, cognitive and biological factors...is shaped at an early age...[and evidence suggests] biological, including genetic or inborn hormonal factors, play a significant role in a person's sexuality". They say that "sexual orientation identity—not sexual orientation—appears to change via psychotherapy, support groups, and life events". The American Psychiatric Association says that individuals may "become aware at different points in their lives that they are heterosexual, gay, lesbian, or bisexual" and "opposes any psychiatric treatment, such as 'reparative' or 'conversion' therapy, which is based upon the assumption that homosexuality per se is a mental disorder, or based upon a prior assumption that the patient should change his/her homosexual orientation". They do, however, encourage gay affirmative psychotherapy.

The influence of hormones on the developing fetus has been the most influential causal hypothesis of the development of sexual orientation. In simple terms, the developing fetal brain begins in a "female" typical state. The presence of the Y-chromosome in males prompts the development of testes, which release testosterone, the primary androgen receptor-activating hormone, to masculinize the fetal brain. This masculinizing effect pushes males towards male typical brain structures, and most of the time, attraction to females. It has been hypothesized that gay men may have been exposed to lower levels of testosterone in key regions of the brain, or had different levels of receptivity to its masculinizing effects, or experienced fluctuations at critical times. In women, it is hypothesized that high levels of exposure to testosterone in key regions may increase likelihood of same sex attraction.

Supporting this are studies of the finger digit ratio of the right hand, a supposed marker of prenatal testosterone exposure (however subsequent research has been critical of using digit ratios as a marker). Lesbians on average were found to have significantly more masculine digit ratios, a finding which has been replicated numerous times in studies cross-culturally, but sometimes contradicted, such as when ethnicity is taken into account. While direct effects are hard to measure for ethical reasons, animal experiments where scientists manipulate exposure to sex hormones during gestation can also induce life long male-typical behavior and mounting in female animals, and female-typical behavior in male animals.

Maternal immune responses during fetal development are strongly demonstrated as causing male homosexuality and bisexuality. Research since the 1990s has demonstrated that the more male sons a woman has, there is a higher chance of later born sons being gay. During pregnancy, male cells enter a mother's bloodstream, which are foreign to her immune system. In response, she develops antibodies to neutralize them. These antibodies are then released on future male fetuses and may neutralize Y-linked antigens, which play a role in brain masculinization, leaving areas of the brain responsible for sexual attraction in the female-typical position, or attracted to men. The more sons a mother has will increase the levels of these antibodies, thus creating the observed fraternal birth order effect. Biochemical evidence to support this effect was confirmed in a lab study in 2017, finding that mothers with a gay son, particularly those with older brothers, had heightened levels of antibodies to the NLGN4Y Y-protein than mothers with heterosexual sons. J. Michael Bailey has described maternal immune responses as "causal" of male homosexuality. This effect is estimated to account for between 15-29% of gay men, while other gay and bisexual men are thought to owe sexual orientation to genetic and hormonal interactions.