Two female runners competing in a state high school championship this weekend have come under attack because people believe they are transgender and therefore may have an unfair advantage. But the science is far from clear as to whether trans athletes are actually faster or stronger than their cisgender peers, experts say.

The evidence is especially murky for teen athletes, who may be going through puberty or taking drugs to suppress hormones — actions that could improve their performance or hinder it, or have very little impact at all.

As the global debate around transgender athletes’ participation in all levels of sport ramps up — particularly in the United States, which is enduring a wave of anti-trans legislation — a paucity of research on the subject makes it challenging to understand exactly what’s at stake, say experts in sports medicine and gender biology.