This piqued my curiosity. I am a resolute believer in active fathering, but I had imagined this was a cultural decision by my generation of men. Hrdy's book, however, introduced me to an entire academic field saying that our approach is rooted in biology, just dormant and waiting to be triggered.

After interviewing Hrdy and other experts and delving into the studies, I came to a simple conclusion: fatherhood changes men in ways that echo how motherhood transforms women. The more involved a father is with their baby's care, the deeper this transition becomes. These shifts in our endocrine and neural system show that the nurturing father is not a modern aberration, but a deeply rooted biological trait.

The earliest research on how fathers are physically changed by babies came from observations of other animals. These late 20th-Century studies found that many mammalian males – including other primates – show clear hormonal shifts, including rises and drops in hormones like testosterone, vasoprin and prolactin, typically associated with motherhood, as they engage in active parental care.

When the American anthropologist Lee Gettler, then an undergrad student, heard about these findings in the early 2000s, he was hooked.

"I asked [my lecturer] whether anyone was studying these questions in human fathers, and the answer at that point was largely no", says Gettler, now the director of the Hormones, Health, and Human Behavior Laboratory at the University of Notre Dame in Indiana.