Sexual orientation (heterosexuality, homosexuality, bisexuality) is one of the most debated and methodologically sensitive topics in neuroscience. In this field, the findings point not to a single “sexual orientation center,” but to a complex biological substrate formed by interacting brain regions, genetic factors, and prenatal hormonal effects.

1. The hypothalamus and INAH‑3

One of the most frequently discussed structures in relation to sexual orientation is INAH‑3 (Interstitial Nucleus of the Anterior Hypothalamus 3), located in the hypothalamus. In some postmortem brain studies, the average volume of INAH‑3 in heterosexual men has been reported to be larger than that of women and homosexual men. These studies suggest that INAH‑3 may be associated with typically “male‑typical” sexual behaviors and that sexual orientation may have a biological component. However, it is not clearly known whether this difference is the cause of orientation, a consequence of it, or an indicator parallel to some third biological variable; therefore one must be cautious when making any claims about causality.

For this reason, instead of saying “the biological basis of heterosexuality directly depends on INAH‑3,” it is more consistent with the current scientific perspective to say that “sexual orientation appears to be associated with the structural properties of certain brain regions, such as INAH‑3 in the hypothalamus; this suggests that sexual orientation may have a strong biological component.”

2. Other brain structures: anterior commissure and hemispheres

Another structure proposed to be related to sexual orientation is the anterior commissure, a fiber bundle connecting the two cerebral hemispheres. Some early studies reported size differences in the anterior commissure between homosexual and heterosexual individuals. However, later studies with larger samples did not consistently replicate these differences. For this reason, in the current literature, the relationship between the anterior commissure and sexual orientation is considered controversial, and there is insufficient evidence to regard this structure as a reliable biomarker of sexual orientation.

In addition, studies examining asymmetry between the cerebral hemispheres (right and left hemispheres) have suggested that the right hemisphere may be slightly larger in heterosexual men, whereas women and some homosexual individuals may show a more symmetric pattern. However, these findings do not show full consistency across all studies and may be influenced by methodological and sampling differences. Therefore, these asymmetry data provide interesting clues about the biological basis of sexual orientation, but they are not, by themselves, decisive or diagnostic.

3. How do these differences arise?

Neuroscience research generally supports the view that such structural brain differences largely emerge during the prenatal (before birth) period. Two main biological axes are usually highlighted here:

• Prenatal hormonal effects: The levels of androgens (e.g., testosterone) to which the fetus is exposed during critical developmental windows may influence the “masculinization” or “feminization” of certain brain regions. In animal studies, experimental manipulation of prenatal hormones has been shown to alter adult sexual behavior and partner preference. In humans, direct experimental intervention is not possible, so this relationship is examined mostly through indirect indicators (such as certain endocrine disorders, twin studies, finger‑length ratios, and play preferences). The findings suggest that hormones may have some influence on sexual orientation, but that this influence is limited, varies from person to person, and may differ by sex.
• Genetic and epigenetic factors: Twin and family studies indicate that sexual orientation is not purely an environmental phenomenon and that it carries a certain heritable component. However, a single “gay gene” has not been identified; instead, it is thought that many genes with small effects, in combination with the prenatal hormonal milieu and other biological and environmental factors, create a “risk/profile” that shifts probabilities. Epigenetic mechanisms (for example, chemical marks that affect gene expression) are also among the potential candidates in this process, but the data in this area are still emerging.

Within this framework, sexual orientation is neither simply the product of “a single point in the brain” nor merely a “learned behavior.” A more realistic approach is to understand sexual orientation as a multifactorial trait that emerges from genetic predisposition, prenatal hormonal effects, early brain development, and likely lifelong biopsychosocial interactions.

4. A cautious, summary formulation

In light of all these data, the biological basis of sexual orientation can be expressed more cautiously and scientifically as follows:

• Sexual orientation appears to be associated with structural differences observed in certain brain regions, such as INAH‑3 in the hypothalamus; this suggests that sexual orientation may have an important biological component, but current evidence is not sufficient to define a single, decisive “center.”
• Other brain markers, such as the anterior commissure and cerebral hemisphere asymmetries, have been found in some studies to be related to sexual orientation, but subsequent research has not always replicated these results consistently; therefore these findings should be interpreted with caution.
• Genetic and prenatal hormonal influences are strong candidates for the biological processes involved in the emergence of sexual orientation; however, the available data point to a complex network of multiple interacting influences that shift probabilities, and they do not provide a “simple explanation” at the level of a single gene, hormone, or brain area.

If you want, I can also produce a shorter, more popular‑science version of this English text while keeping the cautious scientific tone.