Craig Kingsley and Kelly Lambert, whose research lead to Katherine Ellison's 2005 book The Mommy Brain, are studying fatherhood in both humans and other animals. They found that when male deer mice are exposed to baby deer mice (even if the babies aren't theirs), motivation and problem-solving areas of their brains grow. Male titi monkeys, which made for life and help care for babies, also experience such changes. In men, the researchers found that levels of prolactin — a hormone that in women influences lactation and in men may increase responsiveness to a baby's cry — increased after the birth of a child. Testosterone, which influences mate seeking and aggression, correspondingly decreased.
However, Michale Lewis's claim that he didn't love his children initially may not be so unusual. Susan Kuchinskas, writing about Kingsley and Lambert's research for Miller-McCune, says, "fatherly love may take time to grow. After all, mom's body and brain have enjoyed a nine-months-long stew of hormones to prepare her for this role, while the overhaul of dad's brain seems to begin only at the appearance of the child."
Kingsley and Lambert's findings suggest a model of parenthood that is different from the essentialist mommy-as-natural-nurturer, daddy-as-natural-dumbass stereotypes promulgated by traditional-family types. Their research indicates that childrearing, for both men and women, is a physical process, and that contact with children actually makes parents better at taking care of them. Kuchinskas writes,
To maximize the physical changes that support parenting, the best thing a prospective father can do is take an active role in birth preparations and be physically close to his partner and their child when the baby is born, snuggling close and inhaling that unique baby smell. Research by Jay Fagan, a professor of social work at Temple University, shows that fathers who get involved in pregnancy seem more committed to their partner and the child after it's born.
Lewis writes about the strangeness of being expected to change diapers, when his dad "didn't talk to him til he was 21." But all those dirty diapers — and the physical closeness that came with them — may have made him a better dad.