Transgender hormone therapy (male-to-female) males is associated with higher serum estrogen. Both estrogen and testosterone help to stimulate bone formation, especially during puberty. Estrogen is the predominant sex hormone that slows bone loss, even in men. In spite of the induction of breast development, transgender women who undergo HRT do not have an increased risk of breast cancer. Only a handful of cases of breast cancer have ever been described in transgender women who have undergone male-to-female HRT. This is in accordance with research in cisgender men in which gynecomastia has been found not to be associated with an increased risk of breast cancer,
Fraternal birth order and male sexual orientation sexual orientation via the fraternal birth order effect (for example, homosexual men with no older brothers) owe their homosexuality to other causes such as certain polymorphic genes (colloquially called "gay genes") or prenatal hormone levels during fetal development. Both direct and indirect evidence have demonstrated that the mechanism by which the fraternal birth order effect operates is prenatal in timing and biological in nature, rather than postnatal in timing or psychosocial in nature. Indirect evidence was found first when it was discovered that the fraternal birth order effect interacts with birth weight. Then, Bogaert (2006) produced direct evidence that the