Edited by Michael S. Gazzaniga, University of California, Santa Barbara, CA, and approved August 2, 2013 (received for review March 25, 2013)Life History Theory posits a trade-off between mating and parenting effort, which may explain some of the observed variance in human fathers’ parenting behavior. The current study tested this hypothesis by measuring aspects of reproductive biology related to mating effort, as well as paternal nurturing behavior and the brain activity related to it. Both testosterone levels and testes volume were negatively correlated with paternal caregiving. In...…Edited by Michael S. Gazzaniga, University of California, Santa Barbara, CA, and approved August 2, 2013 (received for review March 25, 2013)Life History Theory posits a trade-off between mating and parenting effort, which may explain some of the observed variance in human fathers’ parenting behavior. The current study tested this hypothesis by measuring aspects of reproductive biology related to mating effort, as well as paternal nurturing behavior and the brain activity related to it. Both testosterone levels and testes volume were negatively correlated with paternal caregiving. In...WW…