User:Plantbasederick/Fraternal birth order and male sexual orientation

However, the numbers of older sisters, younger brothers, and younger sisters have no effect on those odds.

A 2020 re-analysis that replicated the FBO effect but took family size and other previous constraints into account further confirmed Blanchard's findings.

Family size differs per country but the implication is that the larger a family size becomes the greater the commonality for homosexuality among men becomes.

With regards to the brain, Simon LeVay in 1990 cut the hypothalamus of 19 homosexual men who died of AIDS and 16 heterosexual men, 6 of whom also died of AIDS, into distinct pieces and measured the cell groups within them. Six women of unknown sexuality were also included. What he found was that the cell group named INAH3 was more than twice as large in the men when compared to the women. Most importantly, INAH3 was also between two and three times larger in the straight men than in the gay men. There seemed to be no significant difference between volumes of INAH3 in the gay men and in the women which indicates a form of sexual dimorphism between male sexual orientation and the relation they have towards sex.

Later, another study found the fraternal birth order effect may be limited to only moderately right-handed men, as extreme right-handers also did not display a fraternal birth order effect.