Nature versus nurture: it’s one of the oldest debates out there. How much of our personality are we born with, and how much develops in response to our experiences? Modern genetics is making it possible to examine these kinds of questions in ever greater detail, and one Harvard researcher is finding genes at work in complex behaviors. Take, for example, the burrows built by wild deer mice. One species digs a simple burrow that looks kind of like a sock, with an entrance tunnel and a nest chamber