Question: I've heard that new dads can become hormonal – just like new moms. I swear that's what's happening with my partner. Is that true or is it a myth?  

It's true – and there is science to prove it. 

Becoming a father doesn't just affect a new dad's sex life and sleep patterns: fatherhood also affects a guy's biochemistry in far-reaching ways – and he, in turn, affects his new baby's biochemistry in equally powerful ways. 

Researchers at Bar-Ilan University in Ramat-Gan, Israel, studied 80 couples and found that oxytocin levles  rose in both new fathers and new mothers following the birth of a baby. These biochemical changes were reflected in measurable behavioral changes (as compared to single, childless people).  

Dads with high levels of oxytocin were likely to play with their babies while moms were more likely to gaze at their babies, touch their babies, and talk to their babies in a sing-song voice. 

This latest study builds on two earlier studies (conducted in 2000 and 2002 by researchers at Memorial University in Newfoundland and Queen’s University in Kingston). 

• The 2000 study revealed that an expectant father’s blood levels of prolactin (the hormone responsible for breast-milk production) rises by about 20 percent during the three weeks before his partner gives birth; and that blood levels of the stress hormone cortisol levels rise to levels double what they were earlier in the pregnancy. At the same time, levels of testosterone—the hormone that underlies competitive, aggressive behavior—dips during the three weeks following the birth.
• The 2002 study concluded that a father-to-be’s estrogen levels rise 30 days prior to the birth. (Estrogen increases the brain's sensitivity to oxytocin, boosting nurturing behaviors.) Contact between partners allows for this biochemical meeting of minds—that pheromones (chemical substances) given off by the woman’s body as birth approaches would trigger the father-to-be that it’s time to start switching into daddy mode.

Babies also reap the hormonal benefits of having nurturing dads, starting from the earliest days of life. Researchers at McGill University Health Centre in Montreal found that the prefrontal cortex of the brains of California mice who were deprived of access to their fathers from age 3 days until they were weaned at 30 to 40 days of age did not respond normally to the hormone oxytocin or to feel-good neurotransmitters such as serotonin and dopamine.  

The mice pups who had been deprived of the early influence of a father also showed signs of being socially withdrawn. When given the opportunity to engage with other mice, they ignored their potential playmates. This result was consistent with the what they had observed while examining the mice's brains: the prefrontal cortex of the brain is the part of the brain that is associated with social interaction and personality expression. 

So it's not just new moms who get hormonal: dads are also profoundly influenced – even at a biological level -- by the experience of becoming parents.