In sports, what’s fair for transgender athletes and their competitors?

At a recent Ivy League swim meet, spectators were split over which side of the pool to look at. 

At one end, several women jackknifed their bodies in flip turns for the final lap of the 500-meter freestyle. Their furious kicks churned the water and left white vapor trails in the sky-blue pool. On the other end, a swimmer named Lia Thomas had already finished and broken the Harvard pool record. In the 7.5 seconds it took for the second-place swimmer to arrive, Ms. Thomas adjusted her swimming cap, took...