There's a saying that a woman becomes a mother when she sees that positive pregnancy test, but a man becomes a father in the delivery room.
For men, the moment when you first come face to face with your child isn't just a life-altering emotional experience, but also a physiological one—thanks to dropping testosterone levels and rising prolactin and oxytocin levels.
Although this may make new dads feel like their hormones are all over the place, it's for a purpose, says anthropologist Lee Gettler, a researcher with the University of Notre Dame's Center for Children and Families. "Based on a number of lines of evidence, these hormonal changes in fathers seem to reflect that evolution has shaped men's biology to help them respond to the demands of parenthood," said Gettler in a 2014 talk. "Our research facilitates men's understanding of their own 'built-in' biological parenting capabilities, which is highly applicable to the day-to-day lives of millions of men."
According to a 2011 longitudinal study from Gettler's team, new fathers' testosterone levels fall by about 40% in the first month of parenthood, which seemed to be linked to their paternal sensitivity and attachment.
"Our findings suggest that human males have an evolved neuroendocrine architecture that is responsive to committed parenting, supporting a role of men as direct caregivers during hominin evolution," the researchers said in their discussion, explaining fathers with higher levels of testosterone were less responsive to infant cries and reported feeling less sympathy.
While testosterone levels drop, studies show the estrogen levels of expectant fathers begins to rise in the weeks before the baby's due date and continues to stay elevated for months afterward. Along with that, new dads experience an average 20% rise in their prolactin levels in the first month.
While this hormone is commonly associated with promoting lactation among mothers, researchers believe the purpose for dads is promoting the development of paternal behaviors. As authors of a 2016 study published in the Journal of Postgraduate Medicine said, "The prolactin level was found to be correlated with father-infant interaction in a social context, and fathers with higher prolactin profiles were found to be more responsive to baby cues."
The 'love hormone' goes into overdrive
Among new moms, oxytocin goes into action to facilitate birth and breastfeeding. But among new dads, the triggers have more to do with "stimulatory play," such as when they pull the baby up to sit or are able to get them to giggle. This, again, plays a "significant role in establishing a sense of fatherhood during the infant's first growth stages," says Ruth Feldman, adjunct professor at the Child Study Center at Yale University School of Medicine and author of several studies on the topic.
Feldman's research found that mothers tend to experience oxytocin boosts from moments of loving physical contact while fathers got a surge in their "love hormone" more from play. That means there are more and more opportunities for oxytocin hits as babies grow.
And as the parental relationship deepens, fathers' oxytocin sensitivity rises: According to a study published in the journal Hormones and Behavior last year, dads of toddlers experience spikes to their oxytocin levels simply by looking at pictures of their kids.
In other words: Fatherhood forever changes men's hormones—in the very best of ways.