Twenty years ago, Ray Blanchard and Anthony Bogaert demonstrated that the probability of a boy growing up to be gay increases for each older brother born to the same mother, the so-called fraternal birth order (FBO) effect. Their first investigation indicated that each older brother increased the probability of being gay by about 33% (1). This startling phenomenon was confirmed in multiple studies based on independent populations totaling over 10,000 subjects, and a meta-analysis indicated that between 15% and 29% of gay men owe their sexual orientation to this effect
This isn’t about people who are trans, but there is a fascinating thing called the Fraternal Birth Order, where every male child a mother has, their likelihood of being gay goes up significantly.