The physical capacity of the sexes is different, and top-end females tend to be only as capable as the lower-end males. The males in the Marine study averaged 178 pounds, with 20 percent body fat, whereas females were 142 pounds, with 24 percent body fat. The top 25 percent of females in anaerobic power overlapped with the bottom 25 percent of males; the top 10 percent of females in anaerobic capacity overlapped with the bottom 50 percent of males.

The physical disadvantage meant that women were more likely to be fatigued and suffer stress fractures. Women were six times more likely to be injured in entry-level training than males.

The rejoinder to such inconvenient facts is always that the Russians and the Israelis deployed or deploy women in combat. But this is much too simplistic. Lyudmila the sniper was an exception. According to a study for the School of Advanced Military Studies at Fort Leavenworth, even under the extreme pressure of the Nazi invasion, women were only 8 percent of the Red Army, and largely served as medics or otherwise in medical care.

As for the Israelis, women initially fought with the Haganah guerrilla force prior to the creation of the Jewish state. But they were pulled back over time. “Generally,” the Fort Leavenworth study notes, “because of their comparative lack of physical strength, commanders employed women in defensive operations whenever possible.” Today, as the New York Times notes in a report on gender integration of the Israeli Defense Forces, “it remains rare for women to kill or be killed.”