How Hormones Shape Sexual Development

Hormones guide the development of sexual characteristics in the brain and body through a sequence of chromosomal, gonadal, and steroid signals that begin early in fetal life. This episode lays out the core biological mechanisms, drawing on established findings from behavioral endocrinology to explain how testosterone, estrogen, and their derivatives produce typical male or female phenotypes while also noting documented exceptions and external influences.

From Chromosomes to Gonads and Morphology

Development begins with chromosomal sex, determined by the presence of two X chromosomes or an X and a Y. The Y chromosome carries genes such as SRY that promote testis formation and encode Müllerian-inhibiting hormone, which suppresses female reproductive structures. In the absence of these signals, ovaries typically develop. Gonadal sex then leads to hormonal sex: testes produce testosterone, which the enzyme 5-alpha reductase converts to dihydrotestosterone. Dihydrotestosterone drives the growth of primary sexual characteristics, including external male genitalia, during embryonic stages. A natural experiment in the Dominican Republic illustrates this step. Individuals lacking functional 5-alpha reductase are born with female-appearing external genitalia despite XY chromosomes and internal testes; at puberty, rising testosterone produces secondary male characteristics, a pattern