influence upon adult sexual orientation or as a co-factor interacting with genes or environmental and social conditions. For humans, the norm is that females possess two X sex chromosomes, while males have one X and one Y. The default developmental pathway for a human fetus being female, the Y chromosome is what induces the changes necessary to shift to the male developmental pathway. This differentiation process is driven by androgen hormones, mainly testosterone and dihydrotestosterone (DHT). The newly formed testicles in the fetus are responsible for the secretion of androgens, which will cooperate in driving the sexual differentiation of the developing fetus, including its brain. This results in sexual differences between males and females. This fact has led some scientists to test in various ways the result of modifying androgen exposure levels in mammals during fetus and early life. Birth order A significant volume of research has demonstrated that the probability of a male growing up to be gay increases with each older brother he has from the same mother. Known as the fraternal birth order (FBO) effect, scientists attribute this to a prenatal biological mechanism – specifically a maternal immune response to male fetuses – since the effect is only present in men with older biological brothers, and not present among men with older step-brothers and adoptive brothers. This process, known as the maternal immunization hypothesis (MIH), would begin when cells from a male fetus enter the mother's circulation during pregnancy. These cells carry Y-proteins, which are thought to play a role in brain masculinisation (sex-differentiation) during fetal development. The mothers immune system builds antibodies to these Y-proteins. These antibodies are later released on future male fetuses and interfere with the masculinization role of Y-proteins, leaving regions of the brain responsible for sexual orientation in the 'default' female-typical arrangement, causing the exposed son to be more attracted to men over women. Biochemical evidence for this hypothesis was identified in 2017, finding that mothers with a gay son, especially those with older brothers, had significantly higher levels of anti-bodies to the NLGN4Y Y-protein than mothers with heterosexual sons. The effect becomes stronger with each successive male pregnancy, meaning the odds of the next son being gay increase by 38–48%. This does not mean that all or most sons will be gay after several male pregnancies, but rather, the odds of having a gay son increase from approximately 2% for the