The Dad Bod Theory with Lee Gettler

Lee Gettler So when I was in high school, I was a cross country and track runner. So I was pretty skinny 15 year old. And my closest friends were all on the tennis team, so they were also pretty skinny adolescents. And our dads were all sort of burly Midwestern guys, right, so barrel chests and big forearms and things like that, kind of like Popeye. So, we would sit around in the cafeteria and talk about how much bigger and stronger our dads were. We would talk about this a lot, kind of over and over, and we came up with this idea that we called “the theory of man strength”, which was that when men became new dads, their chest expanded and their forearms got big. And we thought it was because men were carrying their kids around in sort of the physicality of carrying that weight around all day long, you know, moving your babies and toddlers around, contributed to these kind of big physical changes in their bodies that helped explain why our dads looked so different than how we looked as skinny teenagers.

Michael Feigelson They probably weren't aware of it at the time, but beneath all the joking around was a serious scientific question. What actually happens to men's bodies when they become fathers? And as it turns out, one of the skinny guys from that lunch table in rural Minnesota would end up spending his whole career trying to find an answer.

This podcast begins with a simple idea: when a baby is born, a parent is also born. And in that transition, just like their babies, parents' brains and bodies team up to help them learn and adapt at high velocity, developing a set of skills and perspectives valuable for their kids and for society as a whole.

This is Birth of a Parent. And today's guest, Lee Gettler, is an anthropologist at the University of Notre Dame. And one of the world's leading researchers on the biology of fatherhood. He has spent the last two decades studying what fatherhood does to men's bodies, including his own.

Michael Feigelson Is it something you shared with other people or did this stay at your lunch table like in your group?

Lee Gettler It really just kind of stayed as a joke between my friends and me until I went to graduate school and my research on how men's testosterone changes got into the news and was on social media and things like that and then my high school friends joked with me that I built my entire career off of this theory of man strength that we had come up with. I mean the funny thing, the theory of man strength helped, I think, propel me into this area, but I essentially found something that was the opposite of the theory of man strength.

Michael Feigelson How do you get from there to actually studying male hormones? What is the moment where you say, oh, I think I'm gonna research this topic?

Lee Gettler Well, that definitely was not one moment or some sort of seamless journey for me at all. I grew up in a family and in a place where I had no idea what anthropology was. Anyway, I fell in love with anthropology and how it approaches questions about what it means to be human. And as I was kind of moving through the early years of being in college, I begged and pleaded my way into this advanced biological anthropology class that was kind just focused on different major themes that were happening in that area. But one of them was about non-human primates, these little monkeys that live in Central and South America, which are some of the only other primates besides humans where the dads are really involved with taking care of their babies.

Michael Feigelson Owl monkeys?

Lee Gettler Owl Monkeys are one, marmosets. And some of those scientists who studied these monkey species in Central and South American where the dad's are really involved had started to look at their hormones. Like testosterone and another hormone called prolactin, and had shown that when these dads were more involved with taking care of their babies, their hormones changed. So their testosterone might be lower or their prolactin might be higher. So I went to my professor and asked whether anyone was asking these types of questions about hormones in human dads. And the answer was largely “no”. This was in the early 2000s. There wasn't really a lot of research at that time. And, I got really excited about that at that time and continued to kind of focus on this topic with my professor. I, you know, took an independent study with him to think about how you might ask these questions about humans, wrote kind of like a grant proposal that was designed around how do we look at how men's hormones might change during their partner's pregnancy and what does that mean for how emotionally or physically close they are with their partner. And he was really, really excited about this and wanted me to go to graduate school to get my PhD, but that was just not something that felt possible for me, in part because of my background. So I thought that was kind of a crazy idea.

Michael Feigelson After a short stint as a teacher, Lee went to graduate school, just like his professor had suggested, where he became one of the first researchers to seriously ask what happens inside men's bodies when they become fathers. And that investigation kept leading him to three hormones in particular: testosterone, prolactin, and oxytocin.

Michael Feigelson In the simplest sense, can you tell me first what is testosterone known to do as a hormone? What does it do?

Lee Gettler What do people think that it does, or what does it actually do?

Michael Feigelson Tell me both.

Lee Gettler Okay, so what does do, I mean, before boys hit puberty, there are not major differences between boys and girls, and when they hit puberty, both male and female bodies start to produce differing levels of reproductive steroids, and in males, they start to produce a lot more testosterone, and that does contribute to the growth spurts that they experience during puberty and how much muscle they put on. So testosterone does help build up muscle and strength. And it's one of the the drivers of why you see shifts between males and females on average and and how strong and how much muscle they have during puberty. In humans and in other species is related to competitive behaviors, and for humans testosterone does help motivate competition between boys with other boys or men with other men. It makes men and women more oriented towards social status and dominance, but it is actually pretty weakly linked to aggression. One of the primary assumptions in the general public is that testosterone drives male aggression and there really is just not good evidence for that. But it is related to these other types of behaviors where sometimes aggression can come out.

Michael Feigelson Competition, social status, dominance.

Lee Gettler Dominance.

Michael Feigelson And it does help with the building of muscle physiologically speaking. And then same question for prolactin, which you mentioned, what does prolactin do?

Lee Gettler So prolactin, if you break down its name, pro-lactin it's because it helps promote lactation. So it's often thought of as a breastfeeding hormone. It's also produced in men and women who are not breastfeeding or parents. So it is a hormone that exists in our bodies in general. And it's this really, really ancient hormone. So, not to get too deep into the weeds and sort of like vertebrate evolutionary history, but prolactin predates the existence of breastfeeding. Okay. So fish have prolactin and fish don't breastfeed.

Michael Feigelson And this is how you know a biological anthropologist has fully taken over the conversation. You start with testosterone and two minutes later, you're discussing emotionally available fish dads.

Michael Feigelson Well, what if fish use prolactin for?

Lee Gettler Well, so one of the things that fish use prolactin for is in fish species where dads help to take care of their young when they're doing things like how fish dads help take care their babies, like if the eggs have been laid on the lake floor and they're helping to fan them with their fins to keep things off of the eggs, their prolactin is higher.

Michael Feigelson So it's prolactin, a caregiving hormone. Is that what you're saying? What's its function other than breastfeeding?

Lee Gettler I think a simple way of thinking about it is prolactin is a caregiving hormone.

Michael Feigelson After studying primates during college, Lee was ready to test these ideas in humans. And he was about to stumble onto a study that would change the way scientists think about fatherhood.

Michael Feigelson I wanna talk about the 2011 study, which got a lot of attention, and became a real landmark piece of work. So the first question is, who were you studying and why?

Lee Gettler So I went to do my PhD about these questions around the biology of fatherhood at Northwestern University in Chicago and when I was in graduate school I got connected to another scientist who had been working on this really long-running project in the Philippines. And there had been some really important studies that had shown that married men in the US and married fathers in the U.S had lower testosterone than men who were single non-fathers. But there was a chicken and an egg question of were lower testosterone men more appealing to women? Were they more sensitive? Were they more warm in a relationship, and so they were more likely to get married or become dads? Or did the transition to getting married and having kids lead to testosterone going down. And to answer that, no one really had the right kind of data to look at this. You needed this long-term data where you tracked people through time. And so this project in the Philippines had those kinds of data. And it was also a really good cultural place to ask questions about the biology of fatherhood and how men's hormones might be related to what they were doing in families.

Michael Feigelson Why was it a good place culturally? What were these dads in the Philippines doing that other dads maybe weren't?

Lee Gettler Okay, so over the past few decades in the United States, if you look at average levels of how much time dads are spending with their kids and spending taking care of their young kids, that time has gone up substantially over the last few decades. Dads are spending more time watching their young kids, taking care of them, changing diapers, things like that. The exact same pattern has played out in the Philippines. So you have seen this same type of shift in how a lot of the dads in this study are engaging with their babies and their young kids and taking on more of a shared role as caregivers, which traditionally was not the role for dads in Filipino families. But there was still a lot of variation. There were some dads who were still following the more traditional provider or breadwinner model. And then there were some dads, who were really, really involved and were even the primary caregivers to their kids, and then dads in between. And so if you were a scientist like me, who was really interested in these questions about how men's biology might change as they make this transition to being a new dad, and how it might be linked to what they're doing in their families, then you need dads who are doing different kinds of things to look at how the hormones are related to that.

Michael Feigelson So it was a combination of the fact that there was variability in dad's involvement, and that there was data over a long period. This made it possible to study how the amount of caregiving shaped the reaction in men's bodies and to see how these changes evolved over time.

Michael Feigelson I wanted to ask if we could divide this up into time periods and have you explain what's happening hormonally to men in different periods. First, from being single to going into a relationship, what's happening with their testosterone?

Lee Gettler Okay, so when we are asking these questions about how men's testosterone is changing, the men are in their 20s. And if they go from being single to being married, their testosterone drops a middle amount between, if men stay single, non-fathers, and they're in their twenties, their testosterone basically doesn't change across that time period. And the men who become married and don't have kids are kind of in the middle. So, let's say around 10%, their testosterone goes down around 10% if they make that transition to just being married.

Michael Feigelson Just being in a relationship.

Lee Gettler Being in a committed long-term relationship. So either being married or living with your partner.

Michael Feigelson In a long-term committed relationship, 10%. Now this particular man's wife becomes pregnant. What happens to his testosterone next?

Lee Gettler For some men during their partner's pregnancy, their testosterone starts to go down. The more men's testosterone goes down during their partners' pregnancy, the more involved they are after their babies are born. So in terms of those hands-on caregiving behaviors of carrying your baby around and taking them on walks and helping to change diapers, men whose testosterone goes down more during their partnership pregnancy are more involved with those and are more engaged as dads.

Michael Feigelson Remember, this was the 2010s, and up until that time, there was very little research about how becoming a dad affected men's hormones. We tend to think a pregnancy is something that happens inside women's bodies, but Lee's research suggested that for some men, their partner's pregnancy can help to kickstart their biological transition into fatherhood, even before the baby is born.

Michael Feigelson What about a situation where a guy doesn't know he's becoming a dad?

Lee Gettler Then there's no biological change there because it has nothing to do with having sex with someone and then getting someone pregnant. If you are not kind of engaged and committed to the role of being a dad, and in this case, aware of the fact that you're going to be a dad then you're not gonna see these biological changes. It's about the investment and involvement in that role of becoming a dad.

Michael Feigelson So this isn't biology responding to reproduction. It is biology responding to involvement and once the baby arrives the changes become much bigger.

Lee Gettler In our data, in the study in the Philippines, men who have newborns at home have a huge drop in testosterone, like 50%. So their testosterone is dropping dramatically compared to where they were when they were single non-fathers five years earlier. It's this massive change.

Michael Feigelson A fifty percent drop, that is not a subtle effect. Lee is describing men's bodies reorganizing themselves during early fatherhood. But as we heard earlier, how much the body changes depends on how much men get involved. And there's another part to it too.

Lee Gettler These big drops in testosterone for dads, like you in the Netherlands, Mike, or me in the United States, I mean, we're living in contexts where oftentimes we're not super physically active. There's a lot of calories available to us, many of which are not super healthy in the form of things like fast food or really sugary foods and drinks, things like that. So when you combine that sort of environment with these big changes in testosterone, it can lead to men gaining more body fat when they become new dads. Sort of like the ‘dad bod’ phenomenon.

Michael Feigelson What would you say to, you know, an expecting dad? Is it, so you're saying if I take care of my baby, like I'm gonna get less healthy and put on weight. What would say to him?

Lee Gettler I would say that there's nothing inevitable about ‘dad bod’. In the Republic of Congo, working with these dads who are living out in the rainforest and are literally carving out their lifestyle and getting resources for their family from the rainforest, they’re climbing trees and they're paddling canoes on the river and they are doing like really intense physical activities all day long, they eat a really healthy diet that comes from the rainforest, and there's no dad bod there. These guys look like professional athletes and like CrossFit athletes. They all have six packs, but we know that in one of these communities the dads who are most involved with taking care of their kids and like teaching them about the rainforest, they have lower testosterone, just like we see involved dads having lower testosterone here, but there's no dad bod.

Michael Feigelson Okay, so full disclosure, this last question about dad bod was a personal one. And here I was thinking that Lee would tell me, it's the hormones, it's inevitable. But in fact, he told me I need to get more exercise. Fair enough.

Michael Feigelson And the difference in this trajectory between men who are, I don't know, very involved versus not very involved, like what's the range of difference that you can see?

Lee Gettler We do know that the dads who are the most involved, spending the most time with their kids, have meaningfully lower testosterone, about twenty percent lower than the dads who are reporting basically not spending any time doing hands-on caregiving or helping to be kind of responsible for their children in that caregiving way where they're directly engaging with them.

Michael Feigelson When I first learned about the impact of dad's involvement on testosterone, I found it fascinating, but I wasn't exactly surprised. But the deeper Lee looked, the more fatherhood started to resemble a full body biological transition involving two hormones that I didn't expect to be part of the story. One known for its involvement in childbirth and the other for its link to lactation.

Michael Feigelson What about the same sort of trajectory, but for other hormones? So say oxytocin, prolactin, what do you see in how it changes when men get into relationships, their wife gets pregnant, the child is born, et cetera?

Lee Gettler When we look at hormones like testosterone, we can get really good levels, a good idea of what a dad's testosterone looks like from their spit, okay? There are other hormones that you cannot look at in spit, you really need blood and prolactin is one of them. Because of the sort of cultural cache around testosterone and the fact that you need blood to look at prolactin, there's just a lot less research on prolactin. Both in new dads and married men, in males in general. I think again, partially because it's thought of as a breastfeeding hormone. So we don't have longitudinal data on men's prolactin in the same way we do for testosterone, but we were able to show that if you compare men whose partners are pregnant to men who have babies at home, who have infants, and then dads who have older kids, it's dads who had babies at homes who have the highest prolactin in that study in the Philippines. So there's a pretty complimentary or similar trajectory, in this case, prolactin is going up, not down. But if you think about it in comparison to what's happening with testosterone, we see that men have this really dramatic drop in testosterone when they have babies at home. And with prolactin, we see that prolactin is going up during that same time period or it's higher.

Michael Feigelson And oxytocin?

Lee Gettler We can look at oxytocin in spit or in blood, can also look at it in pee. So there's lots of different ways to get at oxytocin. But oxytocin is a hormone that your body produces really quickly, so it can spike up really quickly. One of our friends and colleagues, Ruth Feldman, her lab has done really, really great work on oxytocin with moms and dads and kids. And she's shown that dads have higher oxytocin, sort of like the set point of their oxytocin is higher again in those first years of having a baby at home. And then our lab and other teams have shown that new dad's oxytocin will spike up when they're doing things like holding their babies or playing with their babies. And so it's a very responsive hormone.

Michael Feigelson Scientific breakthroughs don't always happen in laboratories. Sometimes they begin in much messier places and sometimes it means arriving at the maternity ward with a Ziploc bag full of spit tubes.

Lee Gettler Yeah, so my son and I, my son's now 11. Around the time that my wife Laura was pregnant with him, we were starting to think about doing this study with the local hospital. We wanted to sort of test, what would this look like if you asked dads to spit in tubes while their baby's being born and then after they've held them for the first time, like sort of how intrusive might this be during this really significant kind of intimate moment for families. And so my son, Leif and I were the first test cases in this. So I brought my sort of Ziploc bag full of little plastic tubes to the birthing unit. And after Leif was born, he did skin-to-skin care with Laura. And then it was my turn. And so I spit into a tube before I had Leif and before he got onto my chest. And then I did skin-to-skin care with him. And I really remember just how overwhelming that moment was in a really positive way of just feeling this flood of emotions and love and commitment in ways that I couldn't have imagined as a dad who wasn't capable of being pregnant and didn't have this kind of physical connection with the baby with Leif in that same way.

Michael Feigelson For years, scientists had mostly studied bonding through mothers and babies. Now, Lee was measuring it inside himself in real time.

Lee Gettler And so we were able to show that especially for dads who were first time fathers and holding their new babies for the first time, their oxytocin like skyrocketed when they held their babies for the time.

Michael Feigelson From your spit, you can see that.

Lee Gettler Yeah, we could look at it from my spit or another new dad’s spit. My testosterone went down really substantially, really, really dropped a lot in those moments where I was holding Leif for the first time. I mean, I remember when he was on my chest and his little hands were moving and he was, he was rubbing my chest and kind of moving his fingers through the hair on my chest and for me, it felt like, I can still remember all of the little tiny details of that interaction and that's exactly what oxytocin does, is it like shines a spotlight on these really significant social moments, including when you're a dad holding your baby for the first time. And so it really brought to life for me the importance and significance of what this flood of hormones is doing from those earliest moments of dads interacting with their babies.

Michael Feigelson This is where Lee's research starts to challenge a very old idea about fatherhood, that men are wired to provide, but not to care. And in fact, the more they care, the more the biology responds.

Michael Feigelson What are the activities that dads do or can do that make these hormones change?

Lee Gettler Okay, so the way I think about this is that the hormones and what dads are doing in families, it's a two-way street, right? So we know that there are biological changes that help lead dads to engage in these kinds of activities. I also think that dads being more involved helps kind of continue these biological changes. And so you're seeing a feedback loop between the two, but it's holding babies, including when they're crying. It's being with them and reading to them. It's changing their diapers. It's taking them out on walks. It's when you're out on a walk and you're in the park or you're on a trail in the forest, it's talking to them about all of the various things they're seeing in the forests, the different trees and the way the wind feels on their face. So it's like teaching them about the world. And this biology helps dads do all of those things in a warm, sensitive way.

Michael Feigelson Can you give an example of a moment in your own fatherhood where you were doing something and were thinking to yourself, “thank God my biology prepared me for this, otherwise I wouldn't be able to handle it”.

Lee Gettler Yeah, I think when Leif was a baby, my oldest, was the moment when I really felt that kind of most acutely. So, Leif never slept when he was a baby and he had colic so he would cry uncontrollably for what felt like hours but for long periods of time he would just cry and cry, and there's nothing when a baby has colic, there's essentially nothing you can do to kind of calm them down and stop them from crying you just have to be patient and calm and try to help them through it. And my wife Laura was a registered nurse when Leif was a baby and she worked the night shift. So after a few months, she went back to working the nightshift on the neonatal intensive care unit. And so I was at home with him all night by myself while he was having these uncontrolled crying episodes and wasn't sleeping. And being with Leif and him just crying and crying and crying, it's heartbreaking, it's frustrating, it's really challenging. And those were definitely moments where I'm sort of trying to dance and bounce with this baby to do anything to try to get him to stop crying, where I felt like if it weren't for the fact that I know that my testosterone dropped really dramatically after he was born, I might not be able to handle this. Like my biology really had kind of equipped me for these extreme moments where it wasn't easy to be calm and patient and sensitive and you're really as an exhausted parent being kind of pushed to your limits. So I think it really did give me some kind of sense of reassurance of like, okay, I got this. I can handle this. Even though my wife isn't here, I can handle this as a dad because I'm kind of equipped to do this too.

Michael Feigelson If involved fatherhood changes men so profoundly, I became curious, what did those changes actually look like in my own dad? So I called someone who had a front row seat to his transition into parenthood.

Michael Feigelson Hello?
Mom Michael. Hi.
Michael Feigelson Yeah. Hi, where are you?
Mom Well, I'm walking right now, you wouldn't believe what I'm doing, it's not the exercise. I'm looking for my keys.
Michael Feigelson Oh yeah, which keys?
Mom Oh, don't ask.
Michael Feigelson So I'm talking to a scientist who studies the biology of fatherhood, and he found that when men take care of babies, their testosterone goes down.
Mom Oh, really?
Michael Feigelson Yeah, does that make sense to you?
Mom As I recall, we didn't miss a beat even when we became a father.
Michael Feigelson You want to elaborate on that?
Mom Do I have to elaborate?
Michael Feigelson Do you wanna?
Mom Isn’t it clear?
Michael Feigelson Yeah, I guess I guess it's clear. Did you see any changes in him after I was born?
Mom Well, yeah, he became more accessible. He was more present. He was involved. He was amused. He was happy. He enjoyed your babyhood. He was always very, very fond of babies. Very warm.
Michael Feigelson What do you think babies brought out in him that bigger humans didn't?
Mom Maybe a softness that wasn't present before as much, yeah.

Michael Feigelson My mom wasn't talking about hormones. She was talking about a person, a father who had become more present, more engaged, more available. And as Lee's research suggests, those kinds of changes don't just matter for children. They may turn out to be good for fathers too.

Lee Gettler In the long term, we have really good evidence that being married and having kids is a positive for long-term health outcomes for men in terms of, you know, living longer lives, having lower risks for some forms of kind of chronic diseases and being happier and sort of feeling like life is more fulfilling in the long term as you kind of move into those, those later years of life. There's really good evidence that being an involved committed dad kind of sets you up for these healthier long-term trajectories.

Michael Feigelson So, the picture is a little counterintuitive. In the short term, involved fatherhood can be exhausting. Less sleep, more stress, more demands. But over the long run, the same biological and behavioral changes seem to push men toward healthier trajectories.

Lee's description of how the biology of fatherhood prepares us for the hardest moments of parenting is comforting, but what about the joyful ones? Might it also help us experience the joy of fatherhood at a deeper level?

Michael Feigelson Give me one of those other kinds of examples, like an activity where you're helping him discover the world or helping your daughter discover the world and you felt like, “my biology is awesome”.

Lee Gettler One of my favorite moments is actually when both of the kids learn to ride their bike for the first time. So I have videos of me with them, both of them doing that and being the like, like running alongside them and having the hand on the back of the seat and then taking it off and putting it back on and you know, just experiencing that moment with them where you're trying to protect them but also you're super excited that they're getting to do this really awesome thing for the 1st time. And that is definitely the exact kind of moment that oxytocin I wish someone would do a study of this exact thing, but I'm pretty confident that in those moments, like that's when oxytocin spikes up, right? Cause you're-

Michael Feigelson You didn't have your spit kit with you?

Lee Gettler No, I didn't have my spit kit with me at that specific time. So, when my daughter was riding her bike for the first time, I think just running alongside her and being with her in that moment and being so excited for her and sort of experiencing that through her eyes, that's, I mean, that's empathy, essentially. And what we know is that oxytocin helps you be a more empathic person, especially in a really positive, exciting moment like that.

Michael Feigelson His daughter experiences the thrill of riding a bike for the first time, and somehow, her dad experiences part of it too.

Michael Feigelson In the Gettler household, if I'm your daughter, your son, and I got a big event, like I'm gonna run a race, and “dad, we gotta go”. Do they say, “don't forget your spit kit?”

Lee Gettler No, they, I don't think we've actually ever introduced the formal phrase spit kit in the house, but there are definitely times where like, I can specifically remember in the last year or so being on the first floor of our house and my daughter and Laura being upstairs and Rowan yelling down the stairs like, “hey dad, I just cut my toenails. Do you need them?” Because in my lab we look at hormones and all different kinds of things, including fingernails and toenails. And so my family is very accustomed to both providing things like spit and toenail to me, but then also asking, “I got this thing from my body, do you want it?”

Michael Feigelson Inside the Gettler household, all of this had become normal. Hormones, spit samples, toenails. But outside the house, Lee's research was landing very differently. Not everyone was as excited and happy about his findings because once you start talking publicly about testosterone, you're no longer just talking about biology. You're talking about masculinity. You're talking about culture.

Archival clip A new study comes out of the Philippines that shows that a man's testosterone levels take a dive after he becomes a parent and drop even further the more he gets involved in caring for his children. Because when he's caring for the children, the wife cuts off his cojones.
Archival clip Fathers who slept on the same surface as their children showed the lowest evening testosterone.
Archival clip Haha, beta males, caring about your kids.
Archival clip They're saying that he's a physiological hormonal change when the wife has a baby, and then the hormones, the estrogen increases, the testosterone drops when he's nurturing. It's like, but I don't understand. It's, like, if I went bowling yesterday, am I going to grow testicles? It doesn't matter.
Archival clip So, look, just to be safe, I make my kids sleep out in a tent in the backyard. Ridiculous. I don't want them anywhere near me.
Archival clip You're fine in the Testosterone Department, Jake. Ha ha ha!
Archival clip Haha, god damn right I am! Okay.

Michael Feigelson The funny thing is, because we're talking testosterone, almost all of the coverage treats this as a story about men becoming less manly, but Lee sees something else entirely.

Michael Feigelson You kind of go back to your theory of man strength, you say it's allowing you to be strong, to be manly in a different way.

Lee Gettler Yeah, I definitely think that's the case. The idea that testosterone going down as a dad is somehow going to prevent you from being the role model you want to be for your kids, or to be tough and strong or a protector, there's not a conflict between these things. And if anything, what the changes in the biology are doing is allowing you to be involved from the start, so you develop stronger bonds with your kids, which is then what allows you to be a better role model for them because the reason that they look up to you as a parent is because of the strength of that bond.

Michael Feigelson When Lee and his friends joked about man strength in high school, they imagined strength as something physical. But the deeper Lee went into the biology of fatherhood, the more he started to see a different kind of strength emerging. Not strength as power or dominance, but strength as connection and patience.

Michael Feigelson One of the ideas is that becoming a parent isn't just training for how we care for our kids, but also gives us skills and abilities that we can use. In other parts of our life. And I wanted to ask, is there something about this biological change that you think makes men better at things outside of their role as fathers, at work or in friendships, for example?

Lee Gettler For me, I feel like becoming a dad has really helped me be more empathetic, to be more patient, to be calm, to be emotionally generous. I think, really, in my 20s, before I was a dad, I could be a jerk, frankly. And I definitely was not especially empathetic to maybe some of the people that I was closest to. I mean, I think an example of this is. Someone who used to be a fellow professor with me at Notre Dame. I was very, very close to him when I was in my 20s, and we would get into a lot of fights and conflicts when I was in my twenties because I kind of couldn't meet him in the middle. And he has definitely told me that after I became a dad our relationship changed because I was much better at being able to understand his kind of mental states and emotional wellbeing and the things that allowed us to kind of better connect as friends.

Michael Feigelson So he was kind of like the you were more of a jerk before you became a dad?

Lee Gettler He's too nice to say that, but that's definitely how he would characterize it if we gave him true serum.

Michael Feigelson The longer we talked, the more the science started bleeding into ordinary life and also showed up in some unexpected ways.

Michael Feigelson If a new father was listening to this and feeling like things have really changed in him, but can't quite name it, what would you tell this guy to pay attention to?

Lee Gettler I think new dads, you know, you might be experiencing emotions in ways that are new and different from how you felt comfortable kind of experiencing them, like feeling a greater response to small things in your life, like a profound moment of seeing something for the first time with your kid and maybe that bringing like tears to your eyes in ways that that might not have been something you ever thought would be kind of typical for you before you were dad. So I bring up that specifically, like for me, when my kids were little, what I'm thinking of specifically is when we watched the movie Inside Out for the first time. And I started crying when Bing Bong the elephant is sacrificing himself to get them out of the memory pit. And my kids were pretty little then. I remember eating dinner afterwards and the kids being confused why I was crying about Bing Bong the Elephant. And Laura was like, well, you know, your dad is really sensitive and that movie makes him think about you and it makes him think of his own dad, because my dad died when Leif was a baby, but crying at a Pixar movie, or getting emotional about Bing Bong the Elephant is not something I would have done when I was in my more jerky phase before becoming a dad in my 20s. And I think these changes that men are experiencing biologically do contribute to kind of changing the way they experience the world. It's sort of like that type of experience of shifting how you experience the word is reflective of this broader kind of super power that you now have to be more attentive to these important critical moments that that happen with your kids, and interpreting them and experiencing them in ways that you wouldn't have been capable of previously.

Michael Feigelson There is this cultural meme about ‘dad bod’, commentary on the fact that many men put on weight when they become fathers. But what Lee's research offers is an alternative story about what ‘dad bod’ really means. A story about the versatility of men's bodies, the ability to reorganize, to prepare them to connect to their babies and their partners in a new way.

Lee and his high school friends may have been wrong in their theory of how fatherhood builds muscle. But that doesn't mean they were wrong that fatherhood builds strength. On that point, I think their cafeteria conversations were way ahead of their time.

In fact, I think that Lee's research actually does a lot to prove that fatherhood does make men stronger. Perhaps not more dominant or more competitive, but stronger nonetheless.

So strong that they're able to let another tiny person reorganize their inner life. That they can take care of a colicky baby every night for a year. So strong that they can tap into the deepest well of joy available in the human experience just by watching their daughter ride a bike for the first time.

This is also a theory of man strength.

Lee showed us how fatherhood changes dads from the inside out. Next week on Birth of a Parent, Victoria Long explores what happens when two developing minds begin to sync up.

Vicky Leong It was exhilarating because I think we had done something that no other team at that point had been able to do, which was empirically established that you can measure synchrony between a child and an adult.

Michael Feigelson If you'd like to support this series, the best way is simply to follow Birth of a Parent on Spotify, Apple Podcasts, or wherever you listen. It really helps more people discover the show. Thanks for listening.