Prenatal Influences on Sexual Orientation: Digit Ratio (2D:4D) and Number of Older Siblings

  title={Prenatal Influences on Sexual Orientation: Digit Ratio (2D:4D) and Number of Older Siblings},
  author={Katariina Kangassalo and Mari P{\"o}lkki and Markus J. Rantala},
  journal={Evolutionary Psychology},
Prenatal androgen levels are suggested to influence sexual orientation in both sexes. The 2D:4D digit ratio has been found to associate with sexual orientation, but published findings have often been contradictory, which may partly be due to the large ethnic diversity between and within studied populations. In men, number of older brothers has been found to correlate positively with homosexuality. This phenomenon has been explained with a maternal immune reaction, which is provoked only by male… 

Figures from this paper

Fraternal Birth Order and Extreme Right-Handedness as Predictors of Sexual Orientation and Gender Nonconformity in Men

There was evidence for a relationship among handedness, sexual orientation, and CGN, and mediation analysis revealed that handedness was associated with sexual orientation directly and also indirectly through the mediating factor of CGN.

The Relationship Between Digit Ratio (2D:4D) and Sexual Orientation in Men from China

We examined the relationship between 2D:4D digit ratio and sexual orientation in men from China and analyzed the influences of the components used to assess sexual orientation and the criteria used

Birth weight and two possible types of maternal effects on male sexual orientation: a clinical study of children and adolescents referred to a Gender Identity Service.

Findings support the existence of two maternal immune responses influencing male sexual orientation and possibly also cross-gender behavior and identity.

The molecular mechanisms of sexual orientation and gender identity

Detecting and Correcting for Family Size Differences in the Study of Sexual Orientation and Fraternal Birth Order

Two fraternal birth order studies published in the last 5 years were surveyed for failures to find a statistically significant excess of older brothers for the homosexual group and suggested that the mean family size of the homosexual groups was smaller than that of the heterosexual comparison groups.

Increased birth rank of homosexual males: disentangling the older brother effect and sexual antagonism hypothesis

The theoretical and statistical tools to study FBOE and AE independently or in combination were developed and applied on new individual data, and on various available published data (two individual datasets, and all relevant aggregated data).

Fraternal Birth Order, Family Size, and Male Homosexuality: Meta-Analysis of Studies Spanning 25 Years

The fraternal birth order effect is the tendency for older brothers to increase the odds of homosexuality in later-born males and in homosexual subjects with masculine versus feminine gender identities is compared and evidence for the reliability of the effect was consistent.

Gender Nonconformity and Birth Order in Relation to Anal Sex Role Among Gay Men

The results suggest that the fraternal birth order effect may apply to a subset of gay men who have a bottom anal sex role preference and that this subgroup is more gender-nonconforming, but there were no significant associations between fraternalBirth order and gender nonconformity at the individual level.

Comparison of Partner Choice between Lesbians and Heterosexual Women

Studies comparing preferred partner selection between homosexual and heterosexual women have shown that homosexual women exhibit patterns of choice that resemble both the heterosexual men and

A Life History Approach to the Female Sexual Orientation Spectrum: Evolution, Development, Causal Mechanisms, and Health

Life history theory provides a biobehavioral explanatory framework for nonheterosexual women’s masculinized body morphology, psychological dispositions, and their elevated likelihood of experiencing violence, substance use, obesity, teenage pregnancy, and lower general health.



The ratio of 2nd to 4th digit length and male homosexuality.

  • RobinsonManning
  • Psychology, Medicine
    Evolution and human behavior : official journal of the Human Behavior and Evolution Society
  • 2000

Finger-Length Ratios in Female Monozygotic Twins Discordant for Sexual Orientation

Differences in 2D:4D ratio suggest that MZ twins share virtually the same genes, and lower ratio in lesbians is a result of differences in prenatal environment.

2nd to 4th digit ratio and a universal mean for prenatal testosterone in homosexual men.

Finger Length Ratio (2D:4D) and Dimensions of Sexual Orientation

A continuous neurohormonal sexual differentiation of the brain, most notably for women, that overrides categories and results in varying dimensions of sexual orientation is hypothesized.

Are 2D:4D finger-length ratios related to sexual orientation? Yes for men, no for women.

  • R. Lippa
  • Psychology, Medicine
    Journal of personality and social psychology
  • 2003
Findings add to evidence that prenatal hormonal factors may be linked to men's sexual orientation as well as ethnic groups, after ethnicity was taken into account.

Sexual orientation and the second to fourth finger length ratio: a meta-analysis in men and women.

It is found that heterosexual women had higher (more feminine) left- and right-hand 2D:4D than did lesbians, but there was no difference between heterosexual and gay men.

Fluctuating asymmetry, second to fourth finger length ratios and human sexual orientation

  • Q. Rahman
  • Psychology, Biology
  • 2005