Same-sex attraction in Iran appears to cluster within families and is linked to lower numbers of biological children in some groups, according to research published in Human Nature.
From an evolutionary perspective, same-sex sexual orientation has often been treated as a puzzle because it is associated with reduced direct reproduction but continues to appear across cultures and historical periods. Prior research suggests that genetic factors contribute to same-sex attraction, with family, twin, and molecular genetic studies showing that same-sex attraction can run in families.
However, much of this work has focused on Euro-American samples, especially cisgender gay men (men whose gender identity matches the sex they were assigned at birth), leaving major gaps in what is known about other cultural contexts and gender identities.
Mostafa Sadr-Bazzaz and colleagues examined this question in Iran, where little empirical work has tested whether same-sex attraction shows familial clustering or whether it is associated with lower reproductive output. The study was motivated by the need to broaden research beyond Western samples. By focusing on Iranian participants, the researchers were able to ask whether patterns documented in other populations also appear in a Middle Eastern context, where social and cultural conditions around sexuality and gender are very different.
The study included data from 1,534 Iranian adults recruited across three data-collection efforts between November 2021 and September 2024. Participants were recruited through online survey links distributed on platforms popular among Iranians, including X (formerly Twitter), Instagram, and Telegram; through snowball sampling (where current participants recruit future participants from among their acquaintances) in Mashhad and Tehran; through a gender clinic in Mashhad; through researchers’ social networks; from hospital staff and students; and from public social spaces such as coffee shops and malls. A later recruitment wave also used online posters targeting Iranians with diverse sexualities or genders.
Participants reported their sex assigned at birth, gender identity, whether they identified as transgender or transsexual, and their sexual attraction during the past 12 months using a 7-point Kinsey-type scale (a scale ranging from exclusively heterosexual to exclusively homosexual). Based on participants’ reported gender identity and sexual attraction, they were grouped according to whether they identified as cisgender or transgender, male or female, and whether they were attracted to men, women, or both.
They also reported demographic information, including age, education on a 7-point scale, and financial status on a 5-point scale. To assess the familial occurrence of same-sex attraction, participants indicated whether they had any same-sex attracted brothers, sisters, nephews, nieces, or maternal or paternal female or male kin. They also reported whether they had children and how many biological and adopted children they had.
The findings provided evidence that same-sex attraction clustered in some family lines. Among males, cisgender androphilic men (men assigned male at birth who are attracted to men) were more likely than cisgender gynephilic men (men assigned male at birth who are attracted to women) to report having at least one same-sex attracted male relative on both the maternal and paternal sides of the family. However, the study did not find significant group differences in the rates of same-sex attracted brothers or sisters among male participants.
Among females, cisgender ambiphilic women (women assigned female at birth who are attracted to both sexes) reported more same-sex attracted sisters than cisgender androphilic women. Cisgender ambiphilic and cisgender gynephilic women were also more likely than cisgender androphilic women to report having a same-sex attracted female relative on the maternal side of the family.
The reproductive-output findings were clearer for males than females. Cisgender gynephilic males reported more biological children than cisgender androphilic males, transgender androphilic males, and cisgender ambiphilic males. In other words, male same-sex attraction and ambisexual attraction were associated with lower numbers of biological children.
Among females, the overall group difference was weaker: only transgender gynephilic females had significantly fewer biological children than cisgender androphilic females. Cisgender gynephilic and cisgender ambiphilic females did not differ significantly from cisgender androphilic females in having offspring.
Of note is that the sample may not be representative because the participants were generally young, educated, and relatively financially comfortable, and many were recruited from Tehran and Mashhad through network-based methods. Some groups were also unequal or small, especially the group of cisgender gynephilic females, which may have limited the study’s ability to detect some effects.
Overall, the findings suggest that familial clustering and reduced reproductive output associated with same-sex attraction are not limited to Western samples, while also showing that these patterns may differ by sex, gender identity/expression, and kinship line.
The research, “The Paradox of Same-Sex Sexual Orientation: Evidence for Higher Familial Occurrence and Lower Reproductive Output in Iran”, was authored by Mostafa Sadr-Bazzaz and Paul L. Vasey.