Longitudinal Study of Human Male Reproductive Ecology: Biological and Behavioral Responses to Changing Social Roles and Impacts on Offspring and Relationship Quality

Among species with biparental care, there is increasing evidence that testosterone (T) helps modulate male reproductive and life history trade-offs. High T is thought to motivate male-male competition and pursuit of mating, with low T orienting males toward offspring care. Recent research from this study team demonstrated that T declines significantly when men become first-time fathers and multiple studies have found that T is lowest when fathers perform childcare. These results are in agreement with the presumed role of T as a moderator of reproductive strategies. However, no study has evaluated whether declines in T have downstream effects on men’s behaviors as fathers and partners or whether fathers facultatively adjust T and caregiving based on varying childcare demands, such as when allocaregivers assist parents. It is also unknown whether fatherhood/marriage-driven T declines influence relationship quality or child well-being. In total, it is presently unclear whether the large changes in T initiated by partnering and fatherhood have the anticipated effects on outcomes likely to contribute to fitness. This study proposes to test for these downstream effects of changing T in collaboration with a study in the Philippines that has followed men since birth, including longitudinal measures of hormone, relationship and fatherhood data since 2005. As one of the largest ongoing studies of male reproductive ecology and physiology, this project offers unique opportunities to explore whether the marriage- or fatherhood-initiated declines in T have the behavioral and somatic effects predicted by models of male life history, and whether these changes in turn predict relationship quality or child development.