From an interview with Lee Gettler, the researcher who discovered that testosterone drops in new fathers:
What we see, specifically, in our research is that men have the biological capability to “attune” to fatherhood, responding to the transition to fatherhood with large declines in testosterone. We also see that fathers who are the most involved with physically taking care of their children have the lowest testosterone.
This does not mean that this pattern and its predicted relationships to behavior will occur in every man everywhere, but simply that, within a normative range of variability, male human reproductive biology has the capacity to respond in this way. Individual personalities and cultural norms can certainly affect, mitigate, even dictate, in some cases, how or if these testosterone patterns come to fruition and how they relate to male’s behavior, if they do at all.
Classic models of human evolution have often emphasized the role of men as hunters and providers, without much (or any) attention given to men’s possible contributions to childcare. Our findings help revise this model by showing that human male biology can specifically adjust to the demands of fatherhood and suggest an important role for fathers’ direct care of their children in that accommodation process.