Possible Balancing Selection in Human Female Homosexuality

Area of Science:

• Evolutionary biology
• Human genetics
• Behavioral science

Background:

• Female homosexuality is suggested to have genetic influences.
• Few studies have investigated the evolutionary maintenance of female homosexuality.
• Understanding the population dynamics of non-heterosexual traits is crucial.

Purpose of the Study:

• To explore the role of fecundity and sexual orientation in the families of homosexual women.
• To investigate the potential mechanisms maintaining female homosexuality in the population.
• To compare fitness and prevalence rates between homosexual and heterosexual individuals and their relatives.

Main Methods:

• Utilized multiple recruitment strategies to gather data from 1,458 adult female respondents.
• Compared sexual orientation and fecundity data among 487 homosexual, 163 bisexual, and 808 heterosexual females.
• Analyzed pedigree data for 30,203 relatives to assess familial patterns.

Main Results:

• Homosexual females exhibited approximately four times lower direct fitness compared to heterosexual females.
• Nonheterosexuality prevalence was over four times higher in homosexual women's families (2.83%) than the general Italian population (0.63%).
• Families of homosexual women showed significantly higher pedigree size and relative fecundity on both paternal and maternal sides.

Conclusions:

• Increased fecundity within the families of homosexual women may offset their lower direct fitness.
• This balanced fecundity could explain the maintenance of female homosexuality at a low-frequency equilibrium.
• Familial studies are essential for understanding the evolutionary persistence of traits like female homosexuality.