Birth Order and Sibling Sex Ratio Predict Same-Sex Sexual Attraction in Iranian Cisgender and Transgender Adult Males and Females.

This research examined if sibship composition (birth order and sibling sex ratio) was associated with sexual orientation among Iranian participants who varied in their gender identity/expression. Previous research shows that androphilic males tend to have more older brothers, a pattern known as the fraternal birth order effect (FBOE). Recent findings have also highlighted the possible relationship between number of older sisters and male androphilia, a pattern known as the sororal birth order effect (SBOE). Evidence on possible influence of sibship composition on female gynephilia is still emerging. In this study, sibship data for Iranian cisgender gynephilic males (n = 239), cisgender ambiphilic males (n = 106), cisgender androphilic males (n = 315), transgender androphilic males (n = 105), cisgender androphilic females (n = 269), cisgender ambiphilic females (n = 110), cisgender gynephilic females (n = 52), and transgender gynephilic females (n = 256) were analyzed. Compared to gynephilic males, a significant FBOE was found in ambiphilic and both androphilic male groups, and a significant SBOE was found in cisgender androphilic males. Compared to androphilic females, a significant SBOE was found in transgender gynephilic females, and a suggestive trend toward a FBOE was found in cisgender gynephilic females. Our study provided further empirical support for the association between older biological brothers and sisters and androphilia in later born males. Our study also contributed to limited literature on the association between older biological sisters and gynephilia in later born females, particularly, those who are relatively more masculine and identify as transgender.