Researchers Examine Familial Aggregation and Reproductive Outcomes Associated With Same-Sex Attraction

Same-sex attraction appears to cluster within some Iranian families and is associated with lower reproductive output among certain groups of men, according to new research published in Human Nature. The findings extend previous evidence beyond Western populations, where most genetic and family studies of sexual orientation have traditionally been conducted.

From an evolutionary perspective, same-sex sexual orientation has long attracted scientific interest because it is generally associated with lower direct reproductive output while remaining consistently present across cultures and historical periods. Previous family, twin, and genetic studies conducted primarily in Europe and North America have suggested that heritable factors contribute to sexual orientation, but relatively little comparable research has been performed in non-Western populations.

Expanding Sexual Orientation Research Beyond Western Populations

Lead author Mostafa Sadr-Bazzaz and co-author Paul L. Vasey investigated whether patterns previously observed in Western countries could also be identified in Iran, where legal, religious, and cultural attitudes toward sexual orientation differ substantially. Empirical research on sexual orientation in the Middle East remains limited because of stigma, legal restrictions, and safety concerns for participants.

The researchers collected data from 1,534 Iranian adults between November 2021 and September 2024 across multiple recruitment waves. Participants completed anonymous online surveys distributed through widely used social media platforms in Iran, including X, Instagram, and Telegram, as well as through snowball sampling in several major cities.

Additional participants were recruited through a gender clinic in Mashhad, healthcare workers, university students, and individuals approached in public locations such as cafés and shopping centers. Later recruitment phases also included targeted online advertisements aimed at individuals with diverse sexual orientations and gender identities to improve representation of underrepresented groups.

How Sexual Orientation and Family History Were Assessed

Participants reported their sex assigned at birth, current gender identity, and whether they identified as transgender or transsexual.

Sexual attraction during the previous 12 months was assessed using a 7-point Kinsey-type scale, ranging from exclusively heterosexual to exclusively homosexual attraction.

Based on these responses, participants were categorized according to sex assigned at birth, gender identity, and predominant pattern of sexual attraction, including attraction primarily toward men (androphilic), women (gynephilic), or both sexes (ambiphilic).

Participants also provided demographic information, including age, educational attainment (7-point scale), and perceived financial status (5-point scale).

To evaluate familial aggregation, participants reported whether they had any same-sex-attracted brothers, sisters, nephews, nieces, or maternal and paternal relatives.

To assess reproductive outcomes, participants indicated whether they had biological or adopted children and, if so, the number of each.

Evidence of Familial Aggregation

The findings suggest that same-sex attraction clustered within certain family lines, particularly among men.

Cisgender androphilic men (men assigned male at birth who are primarily attracted to men) were significantly more likely than cisgender gynephilic men to report at least one same-sex-attracted male relative on both the maternal and paternal sides of their family.

However, the researchers found no significant differences between these groups in the reported number of same-sex-attracted brothers or sisters. This suggests that familial aggregation was more evident among extended family members than among immediate siblings within this sample.

Among women, somewhat different patterns emerged.

Cisgender ambiphilic women reported more same-sex-attracted sisters than cisgender androphilic women. In addition, both cisgender ambiphilic and cisgender gynephilic women were more likely than cisgender androphilic women to report at least one same-sex-attracted female relative on the maternal side of the family.

The authors note that these findings resemble observations from some previous Western studies suggesting that maternal female relatives may contribute to familial patterns of same-sex attraction among women.

Differences in Reproductive Outcomes

Associations between sexual orientation and biological reproduction were more pronounced among men than among women.

Cisgender gynephilic men reported significantly more biological children than cisgender androphilic men, transgender androphilic men, and cisgender ambiphilic men.

These findings are broadly consistent with previous research from Western populations reporting lower biological fertility among men who are primarily attracted to men.

Among women, reproductive differences were less pronounced.

Only transgender gynephilic women reported significantly fewer biological children than cisgender androphilic women. No statistically significant differences in biological offspring were observed between cisgender gynephilic, cisgender ambiphilic, and cisgender androphilic women.

The researchers suggest that these sex-specific findings may reflect differences in social expectations, reproductive opportunities, relationship formation, and family planning within the Iranian cultural context.

Study Limitations

The authors emphasize several important limitations.

The study sample was not nationally representative. Participants were generally younger, more highly educated, and financially better off than the broader Iranian population, with many residing in large urban areas such as Tehran and Mashhad.

Some participant groups, particularly cisgender gynephilic women, were relatively small, reducing statistical power and potentially limiting the ability to detect more subtle associations involving familial aggregation or reproductive outcomes.

Additionally, the study relied on self-reported information, which may be influenced by recall bias or reluctance to disclose sensitive personal information despite anonymous participation.

Broader Implications

Despite these limitations, the findings provide evidence that familial aggregation of same-sex attraction and differences in reproductive outcomes are not restricted to Western populations.

The authors argue that the results are consistent with the possibility that biological and familial influences on sexual orientation operate across diverse cultural settings. At the same time, they emphasize that patterns differ according to sex, gender identity, and family lineage, indicating that the underlying mechanisms are likely to be complex.

Further research involving larger and more representative samples from additional non-Western populations will be needed to better understand how genetic, developmental, cultural, and social factors interact in shaping sexual orientation and reproductive outcomes.

The study, titled "The Paradox of Same-Sex Sexual Orientation: Evidence for Higher Familial Occurrence and Lower Reproductive Output in Iran," contributes to the growing body of cross-cultural research examining the biological and social factors associated with sexual orientation.