Dr. Lukas and Dr. Huchard found that the species that evolved infanticide had a few things in common. For instance, the females tended to give birth year-round, rather than at just one time of the year. Another key factor was that males and females lived in groups where females greatly outnumber males.

The scientists concluded that conditions such as these fostered the evolution of infanticide. If only a few males get to mate with females, there will be a lot of other males facing the prospect of dying childless. Natural selection will favor males that can take over groups of females.

But these males are pressed for time. It won’t be too long before another male threatens them with exile. Meanwhile, many females will be busy nursing infants. Killing the offspring of other males can free up females for reproduction and widen the window of opportunity for new males, leading to more offspring for them.

The researchers also noted that infanticide generally did not evolve in species where females only give birth once a year. “There’s no sense for a male to kill the offspring in the previous year, because he has to wait anyway,” said Dr. Lukas.

Dr. Hrdy and other early infanticide researchers not only investigated how infanticide might have evolved. They also suggested that mothers and fathers had evolved defenses against infanticidal males.

Dr. Lukas and Dr. Huchard found support for this idea, too. In species in which infanticide appeared, females gradually mate with more and more males. This behavior may protect infants by introducing uncertainty: Males might be more reluctant to kill infants if one of them could be his own.

“In effect, whenever promiscuity is high enough, it does not pay for males to commit infanticide,” said Carel P. Van Schaik, a primatologist at the University of Zurich. “This new study beautifully confirms the major role of sexual behavior.”