Hormonal changes in first-time human fathers in relation to paternal investment

In many biparental species, males experience hormonal changes across the transition to fatherhood that prepare them for their new caregiving role. In humans, cross-sectional comparisons have revealed hormonal differences between fathers and nonfathers, however it is not clear when hormone levels change in new fathers, how such changes relate to paternal involvement and attachment, or even whether men with particular hormonal profiles are more likely to become fathers. In this longitudinal study, we aimed to elucidate the trajectory of hormonal changes in new fathers with greater temporal resolution than previous studies, and to provide further insight into the direction of causality between hormone levels and paternal behavior by asking whether hormone levels predict behavior at subsequent time points, or vice-versa. We recruited a sample of 51 first-time expecting fathers and measured levels of plasma testosterone, oxytocin, vasopressin and cortisol at four time points spanning 4-5 months gestation through 4 months postnatally, and we compared these changes to those found in a control sample of 57 adult male nonfathers. We also examined the concurrent and lagged relationships between hormone levels and fathers' self-reported paternal behaviors. From early in the prenatal period, fathers showed lower levels of both testosterone and vasopressin compared with nonfathers, and lower levels of these hormones during the prenatal period predicted greater postnatal paternal investment in the mother and child. Similar to what is known for human mothers, oxytocin levels increased across the gestational period in new fathers, but oxytocin levels were not associated with greater self-reported paternal involvement or attachment. Finally, cortisol levels did not differ between fathers and nonfathers, and were not predictive of paternal involvement or attachment. Our findings raise the possibility that low levels of vasopressin and testosterone found in expecting fathers anticipate a shift in life history strategy toward greater investment in parenting.