Breast development extension adrenocorticotropic hormone (ACTH)), and thyroid hormones such as thyroxine (and by extension thyroid-stimulating hormone (TSH) and thyrotropin-releasing hormone (TRH)) also play permissive but less well-understood/poorly-characterized roles in breast development during both puberty and pregnancy, and are required for full functional development. Leptin has also been found to be an important factor in mammary gland development, and has been found to promote mammary epithelial cell proliferation. In contrast to the female-associated sex hormones, estrogen and progesterone, the male-associated sex hormones, the androgens, such as testosterone and dihydrotestosterone (DHT), powerfully suppress the action of estrogen in the breasts. At least one
Fraternal birth order and male sexual orientation studies have demonstrated that the fraternal birth order effect does not operate through social or postnatal (e.g., learning or rearing or environmental) mechanisms; rather it is biological in nature and operates prenatally. It is also known that the fraternal birth order effect on male sexual orientation is unaffected by family dynamics, living with older brothers or learning. Fraternal birth order and male sexual orientation A correlation between fraternal birth order and male sexual orientation has been suggested by research. Ray Blanchard identified the association and referred to it as the fraternal birth order effect. In several studies, the observation is