In human studies, the possible long-term effects on behavior of early physical insult or pharmacological agents have received little attention. We present both circumstantial and direct evidence that circumcision of male infants leads to behavioral changes. In some American studies using circumcised infants, reported gender differences may instead be the result of the altered behavior of circumcised males. We suggest that circumcision requires more study in its own right, and that it requires description if not control in all neonatal and infancy studies.