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

@article{Kangassalo2011PrenatalIO,
  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},
  year={2011},
  volume={9}
}
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… 

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References

SHOWING 1-10 OF 66 REFERENCES

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

TLDR
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

TLDR
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
TLDR
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.

TLDR
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
    Psychoneuroendocrinology
  • 2005
...