You get born as gay or you become gay, scientifically?

1. Nature is polygenic and measurable but not deterministic

Large genetic studies and reviews show there is heritability to same-sex attraction, but it is polygenic—many genes each make small contributions—so DNA cannot predict who will be gay or straight and there is no single “gay gene” ^{[1] [6]}. Twin and family studies consistently find genetic influence on sexual orientation, but estimates vary and genetics explains only part of the variance, leaving substantial room for non-genetic factors ^{[7] [8]}.

2. Prenatal biology: hormones, brain development and the prenatal window

Experimental and clinical work points to prenatal hormonal environments as important contributors: atypical androgen exposure correlates with differences in later sexual orientation in some clinical cases (for example congenital adrenal hyperplasia in genetic females) and with morphological markers thought to reflect fetal hormone exposure ^{[2] [9]}. Reviews emphasize that many environmental effects implicated in sexual orientation are intra‑uterine—organizational hormonal effects on the developing brain during critical periods are a leading explanatory mechanism ^{[4] [8]}.

3. The fraternal birth‑order and maternal immune hypotheses

One of the most replicated findings is the fraternal‑birth‑order effect: each older biological brother increases the probability a later‑born son will be gay, an effect best explained by a maternal immune response to male‑specific antigens during successive male pregnancies; empirical work has found higher antibody titers to neuroligin 4 (NLGN4Y) in mothers of gay sons in some studies ^{[4] [5]}. While not universal and subject to replication limits across cultures, the fraternal birth‑order effect is a robust signal of a prenatal, non‑genetic influence ^{[10] [3]}.

4. Brain, behavior and early gender nonconformity as correlates

Neuroanatomical differences and childhood gender nonconformity are repeatedly observed associations: some studies report differences in hypothalamic structure and consistent patterns of early‑life gender‑nonconforming behavior among people who later identify as gay or lesbian—findings that are interpreted as evidence sexual orientation is often established before social experience could plausibly cause it ^{[11] [3]}. Nevertheless, these are correlates that help locate when development matters, not causal proofs of a single pathway ^{[11] [7]}.

5. What environmental explanations lack—and what “non-shared” means

Strong claims that upbringing, parenting or social learning determine orientation are not supported by the bulk of empirical literature: systematic reviews find weak hypotheses for postnatal social determinants, and instead emphasize non‑shared environmental influences—factors that differ even between identical twins, many of which appear to be prenatal ^{[3] [4]}. That said, “non‑shared environment” is a catch‑all in behavioral genetics and does not identify specific mechanisms; researchers acknowledge limitations in pinpointing precise prenatal processes ^[8].

6. Scientific limits, policy implications and contested rhetoric

Researchers caution against oversimplified slogans like “born this way” or purely “chosen,” because they gloss over scientific complexity even while such claims carry political weight; some scholars argue evidence strengthens the view that sexual orientation is largely biologically programmed, while others stress multifactorial causation and interpretive humility ^{[4] [6]}. Importantly, the absence of a single genetic determinant does not imply voluntariness, and the literature notes conversion claims lack evidence and that orientation is difficult to alter by postnatal manipulation ^{[1] [4]}.