Family life is fascinating–whether the family involved is made up of humans, monkeys, or hippos. Recently I’ve been exploring the complexities of mammal family life, and I’ve been thinking about what this research can and cannot tell us about our own experiences in families.

Last week in the New York Times, I wrote my column about some intriguing research on what happens when monkey mothers nurse their babies. Their milk doesn’t just deliver nutrients. It also has messages–different levels of hormones–that influence how babies develop, both physically and psychologically.

This week I’ve taken a darker turn. My column is about why male mammals sometimes kill unrelated infants.

It may sound strange, but the “why” of infanticide has a lot in common with