12.7 Proximate environmental causes Part 2: Fraternal birth order and the uterine environment

What is the fraternal birth order effect?

In males, it appears to be that number of older brothers alters the likelihood of same-sex preferences later in life. Specifically, more older brothers (not sisters) is associated with homosexuality in men (not women). This is called the fraternal birth-order (FBO) effect in the scientific literature, and the evidence for the FBO effect is compelling. Simply, homosexual men, on average, have more older brothers than do heterosexual men, a difference that is not seen in homosexual versus heterosexual women.

A logical response to this finding would be to wonder whether growing up with older brothers somehow led more men to develop same-sex sexual preferences in adolescence, or if there was something about the uterine environment experienced pre-birth that made homosexuality in successive male offspring more likely.

Reflection Question: How would you address these differences scientifically?

Psychologist Anthony Bogaert was interested in the FBO effect and whether it was due to exposures in the uterus during fetal development, or somehow due to the impact of growing up with older brothers. He tested this by analyzing sexual preferences in several groups of men, including one sample of men raised in step- or adoptive families. That is, he was able to compare rates of homosexuality in men raised with their older brothers, and those raised apart from their older, biological brothers. He found that only biological older brothers were associated with male homosexuality, regardless of the amount of time spent with those older brothers. Being raised with older step-brothers was not associated with male homosexuality. Bogaert used these data to suggest that it is uterine conditions, not how a person is raised, that is associated with same-sex sexual preferences in men.

Consider the data represented in the following two figures. [1]

What do these graphs show? Do these findings lend support to the fraternal birth-order effect?

 

Figure 12.8 Siblings and Sexual Orientation: Males. The average number of siblings of each type, for both heterosexual (in blue; n=225) and homosexual (in orange; n=385) men. Why is it interesting homosexual men have more older brothers than heterosexual men? Does this finding support the fraternal birth-order effect?

 

 

Figure 12.9 Siblings and Sexual Orientation: Females. The average number of siblings of each type, for both heterosexual (in blue; n=192) and homosexual (in orange; n=162) women.

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Check out this brief and accessible summary of research about the fraternal birth-order effect.

Caution: These differences in uterine influences on male and female homosexuality also illustrate a key point: male homosexuality and female homosexuality appear to have different causes, thus we should be careful not to transfer the findings of research on men to the reality of sexuality in women. Further, male and female homosexuality are likely influenced by multiple factors.

  1. from review of data in: Blanchard, R., Dickey, R. & Klassen, P. The relation of birth order to sexual orientation in men and women. (1998). doi:10.1017/S0021932098005112 ↵

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Introduction to the Evolution & Biology of Sex Copyright © by Katherine Furniss and Sarah Hammarlund is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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