Their bed nucleus of the stria terminalis should be twice as large as a woman’s, and that’s what guided their gender identities. Not that I’m a biological determinist, just a strict physicalist with no belief in metaphysical choice superceding determinism, but a lot of times the brain’s development has recursive feedback loops such that smaller choices early on can alter the size of brain structures along with sex hormones and the development environment in the womb or even outside of it for a while, the earlier the more significant. All I know is that the size of the bed nucleus of the stria terminalis is pretty consistently twice the size in men as it is in women regardless of the gender assigned at birth.
Yep, my point being that a trans man (AFAB) and a cis man (AMAB) both have their bed nucleus of the stria terminalis roughly the same size, twice the size of a woman’s. A woman’s is half the size of a man’s, regardless of whether she’s trans (AMAB) or cis (AFAB). So regardless of what the gender is assigned at birth, the relative size of the bed nucleus predicts the gender that the individual feels most comfortable as.