The Scientist Who Scans Fathers' Brains: The Parenting Checklist You NEED Before You Have Babies!

The Diary Of A CEO with Steven Bartlett

In this episode of 'The Diary Of A CEO,' Steven Bartlett interviews Dr. Darby Saxby, a neuroscientist who explores the profound changes in fathers' brains after

Key takeaways

• Fatherhood leads to brain shrinkage in the mentalizing network and visual areas, enhancing social cognition and emotional processing efficiency.
• Brain volume reduction is not a loss but an optimization—like a director's cut of a film, removing unnecessary pathways for better focus on parenting.

Main topics

• Neuroscience of fatherhood
• Brain changes during parenthood

Notable quotes

Fathers are facultative—optional but extremely useful. They're really useful for humanity.

Volume reduction is not a deficit; it's the brain becoming more efficient, building better pathways for communication.

Conclusion

Parenthood transforms the brain in ways that enhance empathy, focus, and purpose. While both mothers and fathers experience structural

Transcript preview

Speaker 2 (0:00) You've discovered something about the brain I think is quite surprising. So Speaker 3 (0:03) we found that in fathers the brain is actually shrinking and I'll explain why. But also there are studies that find that both men and women who report having had more kids have markers of a younger looking brain age. Speaker 2 (0:15) And then also typically men Speaker 3 (0:17) when they do this show about a 25 % drop in testosterone. The next studies of single women have found that, you know, if you show them a series of body types, the dad bod is the most popular. And Speaker 2 (0:29) that's proven by science. So if I go out and get a dad bod, you're telling me the science says I'm going to be more attractive. Yeah, go Speaker 3 (0:34) for it. We'll see Speaker 2 (0:34) what my fiance says. The other thing that's worth talking about is... Have you done any studies on that? Speaker 3 (0:40) Yes. That is one of the most robust findings in parenthood research. And my lab found actually that... Speaker 2 (0:46) Wow. Speaker 3 (0:47) So I've always been really interested in families and becoming a parent is one of the biggest transformations that you can experience. But I'm concerned about the declining birth rate. And I think that we've managed to market it as an undesirable choice. Speaker 2 (1:01) According to the CDC and Pew Research data, the U.S. total fertility rate just hit an all-time recorded low. Speaker 3 (1:07) Yeah, I mean, the transition to parenthood can be really challenging. You are sacrificing yourself. It can be very isolating. You're losing your sleep. But becoming a parent, you are then benefiting from this sense of connection and purpose you will rediscover your priorities in a way that will ultimately bring a lot of value so Speaker 2 (1:23) if all goes well next year i will be a dad and i would like you to give me some advice on becoming a father based on all the research that you have so Speaker 3 (1:30) great fathers are made not born there's a lot you can do i'm gonna write it down love it so first Speaker 2 (1:39) Guys, I've got a favour to ask before this episode begins. The algorithm, if you follow a show, will deliver you the best episodes from that show very prominently in your feed. So when we have our best episodes on this show, the most shared episodes, the most rated episodes, I would love you to know. And the simple way for you to know that is to hit that follow button. But also, it's the simple, easy, free thing that you can do to help us make this show better. And I would be hugely grateful if you could take a minute on the app you're listening to this on right now and hit that follow button. Thank you so... So, so much. Dr. Darby Saxby, you've discovered something about the brain that I think is quite surprising to a lot of men and women as it relates to what happens to a man's brain when they become a father. And I've got the report here in front of me, but I have a model brain over there. Could you explain to me what exactly it is that you discovered? Speaker 3 (2:32) Yes. So we found that in fathers, and this is similar to what's been observed in mothers, the brain is actually losing gray matter volume. So it is shrinking. So if I had stronger hands, I could probably squish this brain and make it look tinier. Speaker 2 (2:47) But Speaker 3 (2:48) the parts of the brain that are shrinking the most are in what we call the mentalizing network. And that's a part of the brain that helps us understand other people's feelings and intentions. Those areas are kind of concentrated around the midline of the cortex, which is kind of what we're seeing here. And then you also see... volume reduction in the visual areas, which is the back of the brain, which is really important for understanding when you look at an infant's face, what kinds of emotions it's trying to tell you. And so we see changes to those parts of the brain in new dads. In moms, we see changes to these same parts of the brain. We also see changes in moms to what's called the subcortex, which is this part of the brain that kind of controls our gut feelings and our sense of reward and is really strongly linked with our hormones. So moms are kind of changing all across the brain. Dads are mostly changing in this top layer, the cortex, which kind of controls our higher order thinking, reasoning, and executive function. And that tells us something about fatherhood, which is that fathers choose to engage in parenting. It's a sort of cognitively mediated investment that dads are making. Speaker 2 (3:59) What does that mean? Speaker 3 (3:59) So if you think about a mom who's been through nine months of pregnancy, she is flooded with hormones when her baby is born. And she has a really high investment in the baby's survival. Just thinking about the number of calories that it takes to sustain a pregnancy, it's thousands. And so there's a very high floor for maternal investment. And moms are undergoing plasticity all across the brain that reflects that. In dads, there's a tremendous amount of cross-cultural variability. So there are cultures in which dads participate. in child care and are very hands-on with babies from the moment of birth, there are cultures in which there are prohibitions against men picking babies up. Speaker 2 (4:46) You don't pick babies up in the first year in some cultures? Speaker 3 (4:48) You don't pick babies up because it's considered unmanly. Speaker 2 (4:51) Okay. So Speaker 3 (4:52) there's a tremendous amount of variability, and dads calibrate their investment according to what the culture says and what the context looks like. If mom becomes unavailable... dad is often needed to step in and provide more of the care. And so this sort of top layer of brain changes reflects that, you know, dads are modulating their investment depending on what they're learning from the environment. And the word that biologists use for this is facultative. So a facultative adaptation is an adaptation that is optional but extremely useful. You could think about an animal that walks on four legs suddenly having something really important to grab and switching to walk on two legs so that it can use its hands. Hands are really useful for that animal. Fathers are really useful for humanity. Speaker 2 (5:48) But are you saying they're optional? But Speaker 3 (5:50) they vary depending on situation. And I don't want to say that fathers aren't important. They're incredibly important. There are dads who are primary caregivers and even sole caregivers of children. But dads vary, and they vary depending on what is needed for children to thrive in their specific local context. Speaker 2 (6:09) And a woman's brain shrinks as well, but it shrinks a little bit more. Speaker 3 (6:13) Yes, it covers more of the brain. Speaker 2 (6:15) And Speaker 3 (6:16) it also shows a larger across-the-board volume reduction. And I should explain, volume reduction, we often think about deficit. mommy brain, you're forgetting things, volume reduction is not a bad thing. Speaker 2 (6:28) What it means is that the brain is actually working more Speaker 3 (6:32) efficiently because it's building better connected pathways and better channels for communicating information. Is Speaker 2 (6:40) it accurate to say that the brain is focusing? Speaker 3 (6:43) Yes, exactly. So it's reshaping in a way. that enables more efficient communication. It's learning, it's consolidating. So it's kind of like if you think about the director's cut of a movie that has lots of interesting extra scenes and characters, and then you watch it in the theater and it's like this lean, mean two-hour cut, Speaker 2 (7:04) right? Speaker 3 (7:05) Like many of us prefer the theatrical release because it's easier to follow. Speaker 2 (7:09) And by getting rid of certain parts of the brain, that means more energy can be used in the other parts of the brain. Is that broadly accurate? Speaker 3 (7:17) Exactly. So that we're better able to focus on what matters. And when we become parents, what matters is our children and the inputs they're giving us. The Speaker 2 (7:27) interesting thing is the way that you described the part of the brain that the man loses when they become a father made it seem like they're losing empathy. Speaker 3 (7:36) No, they're actually improving in their empathy because they're becoming more efficient processors of social information. So the fact that where we see the biggest changes are in these what we call mentalizing social cognition networks tells us that those networks are really important for dads to use when they become parents. And they're getting a lot of practice putting those networks into use. I Speaker 2 (8:01) thought you said that section of the brain shrunk. Speaker 3 (8:03) Yes. Because the brain is becoming more efficient. Speaker 2 (8:06) So Speaker 3 (8:06) it's losing all the extra pathways and you're sort of building a superhighway for that specific social information. Speaker 2 (8:15) Sounds a bit counterintuitive because I imagined that my brain would grow when I get better at something. Speaker 3 (8:20) You would. But in this case, the brain is actually shrinking. And I know it sort of feels like a puzzle to wrap your head around, but it actually does reflect that the brain is working. better. Speaker 2 (8:31) And is there a relationship between how hands-on the father is and the amount of shrinkage that happens in their brain? Speaker 3 (8:39) Yeah, and my lab found that relationship. So when dads told us before birth that they felt more bonded to their unborn baby, when they wanted to take more time off after work, and when they told us three months after birth that they enjoyed their babies more, they felt more connected and bonded to them, and they were spending more time taking care of their babies. they had more profound brain shrinkage. All of this seems to support the contention that the shrinkage is really adaptive and it's enabling men's adjustment to parenthood. Speaker 2 (9:09) Is there any other biological changes that take place when someone becomes a father? Speaker 3 (9:14) So we also see changes to men's hormones. I talk about this in the book. Men typically show a drop in testosterone. I've got a Speaker 2 (9:23) big thing here. Yeah, Speaker 3 (9:24) let's talk about testosterone. So typically, men, when they're hands-on as fathers, show about a 25 % drop in testosterone. And some of the best studies of this have been done longitudinally in the Philippines. Speaker 2 (9:37) Before we had longitudinal studies, Speaker 3 (9:39) people said, well, maybe lower testosterone men are just choosing to become fathers. And so looking across time lets us tease that out and see, is there something about fatherhood that's actually changing testosterone? And so when you look across decades, you find that testosterone is typically pretty high before pregnancy happens. And it's adaptive to have high testosterone. You know, testosterone has been studied a lot in birds. It's also been studied in rodents and in other mammals. You want high testosterone at the start of breeding season when it's time. to maximize your chances of producing offspring. You want to compete. You want to build status. You want to fight off your rivals. High testosterone is your friend. Once you've accomplished your reproductive goal and you've caused a pregnancy and you want your offspring to survive, you want to be investing and nurturing that new baby. And this is what we saw in the men in the Philippines, that as they were spending more time taking care of kids, they showed a sharper drop. And then you typically see some rebound when the baby is getting older. And again, this is not unique to humans. We see it all over the animal kingdom. Speaker 2 (10:55) So during the pregnancy, something, some chemical tells my body that I need to make less testosterone, even though the baby's not here yet. Speaker 3 (11:04) Yeah, it might just be your proximity to your pregnant partner. Speaker 2 (11:07) Interesting. Yeah. Speaker 3 (11:08) Our lab found expectant dads and expectant moms had correlated levels of testosterone during pregnancy. And the more strong that relationship was, the more dads showed a drop in testosterone after birth. But I want to caveat because I think a lot of men are really concerned about the idea that testosterone might drop. And I think we have a really strong narrative right now that testosterone sort of like equals successful masculinity. Speaker 3 (11:35) Masculinity is not about a particular hormone. It is about a body that is responsive to our goals and that sets us up to meet the demands of our environment. And so having a biology that fluctuates across the lifespan as our reproductive strategy shifts is really adaptive. We want to be flexible. And that doesn't always mean maxed out testosterone. So high levels of testosterone actually decrease our sperm count. and they are bad for our immune system. I Speaker 2 (12:09) think some men might be concerned that dropping testosterone might make them worse in the gym, or it might make them less ambitious or less able to pursue their goals in life. Is there any truth to that? Speaker 3 (12:19) There's definitely a link between testosterone and men's muscle mass and their motivation, their drives. These are not one-to-one relationships. So the human hormone literature is messy, and a lot of these are weak correlations. So testosterone doesn't necessarily equal success at work, for example. And high testosterone can get in men's way in multiple ways. So there are links between men's testosterone levels and relationship satisfaction in the couple relationship. My lab did a study where we found that men who had relatively higher testosterone after their baby's birth had partners who were more dissatisfied in the relationship and reported more intimate partner aggression. So the dads themselves reported more parenting stress. So there's a little bit of a mixed bag of high testosterone and what it looks like. you want is to hit a sweet spot. You don't want testosterone levels that totally crash. My lab has also found that when testosterone is really low after a birth, men have higher postpartum depression symptoms. You don't necessarily want testosterone levels that are sky high either because that can cause problems in your intimate relationship and it can make it hard for you to switch. into nurturing and parenting mode. So the men who seem most successful in our study were men who had mid-range testosterone levels that could be flexible and that could adapt to different life changes. Speaker 2 (13:50) And am I right in thinking that the more time you spend hands-on as a father in those studies that they did on men, the lower your testosterone will go? Speaker 3 (14:00) Yeah. So the longitudinal work in the Philippines found that when men spent more time changing babies, doing hands-on care, sharing a sleeping surface with babies, they had more of a testosterone drop. So the testosterone, the hormones are responsive to the context and to what we're doing. But testosterone does rebound, you know, after birth. So it's not necessarily the case that testosterone is going to bottom out and it's going to stay low forever. Speaker 2 (14:31) You know, you said a second ago that men that had crashes to their testosterone during the pregnancy and thereafter had a high likelihood of experiencing postpartum depression themselves. Are you implying that that means that there's an ideal amount of time and hands-on-ness that a father can spend with their child to make sure their testosterone hits that mid-range point? Speaker 3 (14:52) Yeah, it's Speaker 2 (14:53) a Speaker 3 (14:53) really good question. I mean, I don't know that we know for sure if there's like a dose of parenting involvement that sort of lends itself to an ideal amount of testosterone. It's hard to optimize your way out of these natural changes. I don't want to discourage dads from being hands-on with their kids because they're worried they're going to pay some kind of physiological tax. I think it's more like... Make sure you're protecting your sleep, which we know is linked with overall health. Make sure you're getting time to exercise and to socialize, you know, to participate in activities that bring you joy. Like those are ways to help mitigate your risk that I think will be more successful than trying to hit a particular hormone level. Speaker 2 (15:39) My brother is a year older than me and he has three young kids under the age of seven now. They were actually just here. They've just left. But I noticed a remarkable difference in him when his wife, Julia, got pregnant. Speaker 3 (15:54) What did you notice? Speaker 2 (15:56) Just a total shift in what he wanted out of life. Yeah. And so before growing up, I remember one day seeing him on the computer figuring out which job would pay the most money. And he was very, very smart. He is very, very smart. And he sort of orientated his career towards the job that would make the most money. And then he was successful. He went to LSE, got the job. And then shortly after graduating university, met someone that got pregnant unexpectedly. And I watched my brother go from someone who was like really striving for material goals to someone who suddenly placed the material goal immediately after. How do I retire and spend the maximum amount of my time with my kids? Speaker 3 (16:38) Yeah. Speaker 2 (16:38) His kids are... number one above all anything else and it's something I admire a lot about him but the most remarkable thing as someone that's not gone through that transition because I've not had kids yet is just watching how profound the change was in him in terms of his stated goals and ambitions yeah Speaker 3 (16:53) what Speaker 2 (16:54) is this I'm Speaker 3 (16:55) so glad you said that because that's something I heard from so many of the fathers that I interviewed for the book is that you experience this profound shift in your values and your priorities And a lot of men, you know, if you survey parents, you say, what are the most meaningful priorities in your life? Men are just as likely as women to say, my kids. Speaker 2 (17:16) And Speaker 3 (17:16) I think fathers increasingly are recognizing that the parenting role is really central to their self-concept. And, you know, like we're in a culture where I think it can be hard for men to find a sense of meaning and purpose and connection to other people. Being a parent can give you that sense of meaning and purpose. And so, you know, I think that's a really important framing around, you know, your testosterone level might change or your brain might shrink, like things that maybe sound a little scary and negative, but they're in the service of this larger goal, which is building a more meaningful life. And I think, you know, like a message to men and also to women, to moms and to dads, is anything that's worth doing is hard. Being a parent is really hard. You know, you are sacrificing yourself. You're losing your sleep. You are getting extra wrinkles. Ask me how I know. But you are benefiting from this sense of connection and purpose that, you know, it sounds like your brother has been totally transformed by. And I think we're in a lot of ways like selling young men on this message that you should be optimizing your bank account and your... body fat percentage, and you should be out there hustling as much as possible. And I think that can be a little bit of a hollow message because what we keep learning again and again from the longevity studies is that the quality of men's relationships is the best predictor of their well-being in old age. So men with healthy marriages, men who have positive relationships with their kids, those are the men who age well, not necessarily the men who've maxed out on, you know. riches or the perfect body fat score. Speaker 2 (19:00) It's really a remarkable thing because as you look forward into becoming a parent, you often see the sacrifices you'll have to make. You hear about, okay, you're not going to sleep as much as you used to. You'll hear about the anxieties of being a parent and loving something so much more than yourself and all these things. But that sort of profound psychological, biological, hormonal shift, you kind of just have to believe in it. Speaker 3 (19:24) People Speaker 2 (19:25) like tell you about it as if it's like almost like a religion. I say that because you have to have faith that the profound upside will be worth the very obvious cost that you see parents, you know, the sacrifices they have to make. Speaker 3 (19:39) Yeah, I think that's exactly right. Because I think a lot of the downsides are really visible. Speaker 2 (19:43) Like Speaker 3 (19:43) we've all been on a plane with a crying kid and we've seen the frantic parent trying to calm them down. Speaker 2 (19:48) We've Speaker 3 (19:48) all seen a tantrum in the grocery store, right? Like that stuff is right in your face. upsides are sort of invisible. You know, just like the joy of seeing your child's face as they discover something new. Or, you know, getting to snuggle late at night and feeling this sense of closeness that you've never had, right? That, like, love that's bigger than yourself. And I think it's important to speak about the benefits as well as the downsides because we humans crave connection. And, you know, we crave care. And I think becoming a parent is an opportunity to provide care on a scale that's hard to find in other ways. I Speaker 2 (20:29) also want to hear from you about how the mother's brain changes through pregnancy and beyond. Yeah. Because Julia, so my brother's wife, she was here as well. And she's remarkably different now that she's had these three beautiful children. And I was asking her last night because she was in a sort of corporate world before I was saying. When they grow up and they leave home, which again is probably the worst thing you could probably say to a mother who has three young, beautiful children and who's already anxious that someday they're going to fly the nest. What will then be in the place of these three kids in your life? Would you want to go back to the corporate world? Would you want to do something else? And how has it reordered your sense of meaning? But tell me, how does the mother's brain change? Speaker 3 (21:09) The mother's brain changes really profoundly as well. Moms are losing about two and a half percent of their brain volume from... before pregnancy to after birth. Dads in our studies, we found they lose about 1 % of their brain volume. So similar pattern in similar parts of the brain, but the magnitude is different and there's more variability in the dads. And Speaker 2 (21:30) how did you conduct these studies? What was the methodology? Speaker 3 (21:33) So we recruited a bunch of first-time parents. These were all couples that we recruited during pregnancy. Our only rule was that you had to be cohabiting and you had to be planning to continue cohabiting after the birth because we really wanted to capture the dads and get their experience, you know, being in proximity to their babies. And so we brought them into the lab. We asked them every question we could think of. We sampled their blood, their saliva, their hair. We had them fight with each other on camera. We scanned the men's brains. And then about six months after their baby was born, we brought them back into the lab. Blood, saliva, hair. This time we got a breast milk sample. We got a sample of baby poop. And we had them fight again. And we scanned the dad's brains again. Speaker 2 (22:21) You had them fight again? Speaker 3 (22:22) We had them fight again. We wanted to see how do couples communicate with each other? And does that change across the transition to parenthood? And also, what communication styles are going to predict better? couple level adaptation to becoming parents. How Speaker 2 (22:38) did you get them to fight? I need to make sure I don't do that. Speaker 3 (22:41) So we gave them a whole list of different topics. We said, these are topics that couples commonly argue about, which are hot button issues for you and your partner. And so we had both the mom and the dad fill these out independently. And then we went through and we saw what are the issues that both people are endorsing.