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The Biology of Fatherhood: Rewiring the New Dad Brain

The new dad brain changes SO MUCH when baby arrives into the world.  Let’s explore what’s going on in the brain to transform new dads into mushy, baby cuddle machines…


First, say goodbye to the manly hormone – Testosterone.

Ah, testosterone — the ‘male sex hormone’ responsible for everything from hairy chests to aggression. Aptly named for its testicular production but carefully regulated by control centers in the dad brain.

For animals that rely on dads for help with baby care, studies have shown a steep decline in testosterone production after their offspring arrives into the world. Hello baby, bye bye testosterone.

Human dads show a decrease in testosterone immediately after the birth of their babe. The hormone stays low for up to three months. For the dads who actively participate in childcare (good for you!), testosterone concentrations are even lower than their counterparts taking on a less active child care role.

Also in that study, the sharpest drop in mens’ testosterone occurred in the month after baby arrived. We should note, however, that the researchers could not correct for the potential effects of stress (what do I do with this crying beast?!) or the effects of lack of sleep (what do I do with this crying beast?!). Both of which might independently affect testosterone production.

Interestingly, this study enrolled participants in their single days, long before they had babies. The men with higher testosterone in the years leading up to fatherhood showed a higher likelihood to partner and pop out offspring — suggesting that low testosterone is not a prerequisite for fatherhood but rather fatherhood itself leads to the reduction in testosterone.


dads, Don’t Panic…


Okay, let’s pause for a moment before I scare all childless men away from siring offspring. The change in testosterone is far more subtle than it sounds—dads will not lose their hard-earned chest hair, their muscles will not wilt away, and they will remain a healthy baritone. The dad brain itself is highly responsive to even small changes in testosterone. Generations of evolutionary tuning at work.


low testosterone is not a prerequisite for fatherhood but rather fatherhood itself leads to the reduction in testosterone.


In other species that rely on paternal support, a lower testosterone signal in the dad brain shifts dads to do exactly what dads should do: it helps guide good dad behavior, essentially redirecting new fathers to care for their young and ignore other ‘manly’ priorities that might arise (stop chasing tail and fighting fights and literally, nest). There is a change in motivation.

Next, add in a “mom” hormone – Prolactin.

Another shift in the dad brain to help dads become dads — Prolactin.

As the name suggests, the primary responsibility of the hormone, prolactin, is to promote lactation. This is the hormone that moms have surging through their bodies in the third trimester of pregnancy in anticipation of the new role for their mammary glands. Expecting dads show a similar surge in prolactin in the days leading up to labor.

An abrupt shift in the dad brain can instruct dad to put the chest puffing aside and focus attention on the new delicate creature that they have to keep alive. Dads with lower testosterone and higher prolactin felt a greater ‘need to respond’ when exposed to the sound of a baby crying.


An increase in prolactin helps dads focus on the new delicate creature that they have to keep alive.

Plus a dose of the “love” hormone – Oxytocin.

Oxytocin is often called the “love” hormone due to its role in bonding, empathy, and altruism.

While the research world is still investigating how oxytocin contributes to good dad behavior, one interesting study suggests that baby responds directly to dad’s oxytocin levels. In this study,  dads who inhaled oxytocin (therefore, spiking the oxytocin in their bloodstream) engaged more with their babies — longer gazes, increased interaction, perhaps a belly raspberry or two. The most important part of this? The babies showed a spike in their oxytocin levels to match their spiked dads’ levels.

Changes to your “dad brain” are GOOD!

In the end, all of these changes in your brain and body are GOOD. They kick in your dad instinct. They make you a good partner and a better snuggle monster to your sweet little offspring. Congrats, dad! 

So dads, put down whatever device you are reading this on and go play with your babies!

Are you an expecting dad? Check out our Expecting Partner’s Guide to Preparing For Birth. You can never be too prepared 🙂

Special thanks to my super smart friend, Dr. Rebecca Calisi, who researches how hormones control parental behavior at UC Davis. Pretty badass, right?

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Molly Dickens, PhD

About Molly

Molly has her PhD in Physiology and spent over a decade as an academic research scientist slightly obsessed with the colliding worlds of brain science, hormones, stress and the reproductive system. Nowadays she heads up Content and Community at Bloomlife and edits Preg U. Science is still her jam and she can't help but continue to dive into the research world to find interesting bits about pregnancy and parenting.


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