The Court of Arbitration for Sport in Lausanne, Switzerland, has ruled that women with very high testosterone levels — far above the normal range — cannot compete against other women in races from 400 meters to one mile unless they take drugs to suppress production of the hormone.

The ruling prevents Caster Semenya, 28, an elite runner and Olympic champion from South Africa, from competing in those races because her testosterone levels are naturally very high. She had challenged attempts to disqualify her from racing as a woman.

The science underpinning that decision is complicated, raising difficult questions about biology, fairness and gender identity.

What is testosterone? Where does it come from in women?

It’s a hormone, an androgen, that has a variety of effects on the body. Women and men produce testosterone, but women don’t make nearly as much.