I think I remember reading that Caster has intersex condition ( there are other female athletes also ) that is they have female external sex organs, but often have testes internally, and have XY chromosomes. So they will have been brought up female, but can't get pregnant and have higher testosterone levels than XX females. They used to have gender tests based on chromosomes in the past, but this was deemed unfair as women who had been brought up and thought of themselves as female were being told they were male. It's a bit of a mine field ,should those with intersex condition , who think of themselves as female be denied the sporting opportunities of others? Some say they should only be allowed to compete if they have testosterone suppressing drugs, but that's a whole other debate.