The transition to fatherhood beyond early infancy: Hormonal, behavioral and psychological changes.

Previous cross-sectional studies have documented hormonal differences between human fathers and nonfathers. However, the timing and persistence of these changes remain unclear. We previously reported that first-time expecting fathers exhibited lower plasma testosterone (T) and arginine vasopressin (AVP), as well as dynamic changes in oxytocin (OT) across portions of the prenatal and early postpartum period compared with partnered nonfather controls. By four months postpartum, however, these group differences were no longer significant. Here, we extend these findings longitudinally into late infancy and early childhood. After controlling for relevant confounds, plasma T, OT, AVP and cortisol levels did not differ significantly between fathers and nonfathers at either one or two years postpartum. Nevertheless, individual differences in paternal hormone levels continued to predict paternal behavior and relationship functioning. At one year postpartum, lower T was associated with greater paternal positive engagement and less frustration with the child, whereas higher cortisol was associated with lower paternal positive engagement. At two years postpartum, higher OT was associated with stronger father-infant attachment. Both fathers and their partners reported declining relationship quality across this period, and mediation analyses suggested that higher T levels mediated the association between reduced time spent with partners and poorer relationship quality. These findings suggest that hormonal changes accompanying the transition to fatherhood may be transient at the group level, while individual variation in paternal hormones continue to relate meaningfully to caregiving, attachment, and relationship functioning into early childhood.