The first time I held my newborn son, like many fathers I felt like I was being hit all at once with all kinds of overwhelming emotions and the realisation of new responsibilities. Unlike most new fathers, I then grabbed a small plastic tube and collected a sample of my own saliva while my son slept on my chest.
The reason goes back 20 years, to when I was an early-career graduate student, wondering why the only ideas about fathers and human evolution were narrowly focused on how fathers provided resources. Even then, scholars knew that fathers were capable of much more than being a “breadwinner”. Extensive research has shown that when fathers are committed and routinely involved with their children, the children have better outcomes for health, academic achievement, emotional wellbeing and social relationships. What is less commonly known to the general public and policymakers is that human fathers are truly exceptional in these parenting capabilities and motivations. In the overwhelming majority of other mammalian species, fathers do not cooperate with mothers to help raise young. Even among our closest animal relatives, the Great Apes, intensive, consistent, and costly forms of care by fathers are absent.
This suggests that capabilities and motivations of human fathers emerged as a unique and important part of our evolutionary history as a species. But how? One challenge to thinking about this question is that such care leaves no marks in the fossil or archaeological record; there is no “daddy baby carrier” or “fathers’ teaching toolkit” from the ancient past.
I started to consider what other types of questions we might ask to gain insights into the significance of fathers’ caregiving. Scientists have known for decades that in bird species where fathers help to care for their young, bird dads show changes in hormones such as testosterone, prolactin and oxytocin. There were also already hints that partnered fathers had different hormone profiles from those of single men, particularly lower testosterone.
However, most of those existing studies were small and cross-sectional, meaning they were like a snapshot in time of men’s family status and hormones. This kind of research presents a “chicken and egg” problem. Which comes first? Does becoming a partnered father affect testosterone and other hormones? Or are men who have lower testosterone more likely to become partnered fathers?
Lower testosterone indicates you are becoming an involved father
When my wife and I decided to start a family a little over ten years ago, I began collecting my saliva every week. I did this so that I could see how my own testosterone changed over time as we went through my wife’s pregnancy to bringing our first baby home and learning to be new parents. I collected almost 50 of my own saliva samples, and we tested them for testosterone in my lab. My testosterone actually went up early in my wife’s pregnancy, but then dropped dramatically after my son was born.
By this stage in my research, this did not come as a surprise to me, and these biological changes I went through were helping me get ready to “meet the moment” as a new dad. Our son required a lot of us – he was colicky and rarely slept for months and months. It took a great deal of empathy, sensitivity and patience during long hours for us to give him the care he needed. In many ways, the challenges and the needs of my son are an example of why lower testosterone might be helpful for new dads. The way I tracked my testosterone through time also helps point to the way my team and I tackled the “chicken and egg” question about fatherhood and testosterone.
To answer this kind of question, scientists need longitudinal data that rigorously tracks the same men over time, as they move from being young and single without children and experience transitions to partnership and fatherhood. Fortuitously for me, I was able to connect my interest in the biology of fatherhood to a unique and important longitudinal study in the Philippines that would allow for this type of perspective.
In 2011, I led a research paper tracking men over a five-year period.[1] These
men were in their early to mid-twenties – in their reproductive prime. Among men who remained single non-fathers during this period, testosterone levels changed very little. But in the men who went from being single non-fathers to partnered fathers, testosterone levels declined substantially – by an average of around 25%.
Gettler. L.T., McDade, T.W., Feranil, A.B. and Kuzawa, C.W. (2011) Longitudinal evidence that fatherhood decreases testosterone in human males. Proceedings of the National Academy of Sciences of the United States of America 108(39): 16194–99. DOI: https://doi.org/10.1073/pnas.1105403108
Here was our answer to the “chicken and egg” question. It was not the case that men with lower testosterone were more likely to become partnered fathers. Indeed, men who had the highest testosterone as single non-fathers were the most likely to have become partnered fathers five years later. Instead, becoming a father was linked to testosterone levels going down. We found the biggest decreases among fathers with newborns.
We also found that the most committed, involved fathers had the lowest testosterone. Fathers who spent the most time caring for their children – reading to them, feeding them, taking them on walks or to the park – had lower testosterone than fathers who spent very little time caregiving. If fathers increased their caregiving time over the years, their testosterone went down further.[2]
Gettler, L.T., McDade, T.W., Agustin, S.S., Feranil, A.B. and Kuzawa, C.W. (2015) Longitudinal perspectives on fathers’ residence status, time allocation, and testosterone in the Philippines. Adaptive Human Behavior and Physiology 1: 124–49. DOI: https://doi.org/10.1007/s40750-014-0018-9
Changing the narrative on masculinity
When we published our study, I was certainly aware of popular narratives around testosterone. Listen to sports podcasts, or watch shows that have anything to do with men and violence, and you will hear testosterone being thrown around as the driving force behind how men interact with one another. Still, I was not prepared for the extent to which journalists would conflate testosterone and masculinity when reporting our study.
Every time I spoke to a reporter, they would keep trying to bring me back to this talking point: “so becoming a dad turns a man into a woman? The more you care for your child, the less manly you become?” It was jarring, frustrating, and kind of scary how the study was being misconstrued. It showed me the need to work hard at putting out a different kind of narrative about what it means to be a man.
Whenever I talk about my research now, I invite people to consider the question of how men conceptualise their own manhood. Most men who are fathers and partners value those roles for themselves. They see themselves as more manly, not less, if they’re invested in supporting their family, becoming role models and doing their best for their kids. The idea that being a good dad somehow makes you less of a man, just because your testosterone goes down, is incoherent.
Oxytocin also primes fathers to bond
Numerous other research teams have since confirmed our findings about testosterone in different cultural settings.[3] They have also shown that, for some expectant fathers, changes in their testosterone start during their partners’ pregnancy. The partners of men whose testosterone declines more during the pregnancy say they feel better supported in the postpartum period and fathers are more involved with caring for their newly born infants.[4]
Gettler, L.T. (2016) Becoming DADS: Considering the role of cultural context and developmental plasticity for paternal socioendocrinology. Current Anthropology 57(S13): S38–51. Available at: https://www.journals.uchicago.edu/doi/10.1086/686149 (accessed January 2026).
Edelstein, R.S., Chopik, W.J., Saxbe, D.E., Wardecker, B.M., Moors, A.C. and LaBelle, O.P. (2017) Prospective and dyadic associations between expectant parents’ prenatal hormone changes and postpartum parenting outcomes. Developmental Psychobiology 59(1): 77–90. DOI: https://doi.org/10.1002/dev.21469
This story about the biology of fatherhood is not solely about testosterone. Dr Ruth Feldman’s lab in Israel has shown that fathers’ levels of another hormone, oxytocin, are linked to beneficial forms of father–baby contact, such as physical, exploratory play that helps children learn about the world around them. They have also found that fathers with higher oxytocin feel more strongly bonded to their child, compared to fathers with lower oxytocin.
In our research in the Upper Midwest in the USA, my team has shown that fathers’ oxytocin system is primed to respond from the earliest moments of caring. Working with fathers on the child-birthing unit in the minutes around their babies’ birth, we found that fathers experienced large spikes in oxytocin while they held their babies for the first time. Hormonal changes prime fathers to respond to opportunities for caregiving and bonding from the outset.
It was for this experiment that I took my own saliva sample after the first time I held my newborn. As I experienced the hit of overwhelming emotions, it was as if I could actually physiologically feel the oxytocin changing in my body
Hormonal changes help us embrace the demanding transition to parenthood
All the academic knowledge in the world doesn’t prepare you for the day-to-day demands of being a parent. As I said, my son was colicky and slept poorly for many months. It took everything my wife and I had to be calm and nurturing parents in the moments when he couldn’t sleep and would cry for hours. Getting through those challenging early years demanded more sensitivity, empathy and patience from me than I would have imagined possible.
As a biologist and anthropologist, I’d come to understand the role of hormonal changes during fatherhood in the evolutionary story of our species: men who experienced those changes will have tended to do a better job of helping to raise their children, making the children more likely to survive and thrive – so we think that whatever genes predisposed those men to hormonal changes will have spread through the human population long ago.
But it was only as a father myself that I could really translate that academic understanding to a personal level. I experienced for myself how my changing hormones changed me, in a way that prepared me to be a better parent and partner. If you see stepping up to the plate as a father as a defining feature of masculinity, the dip in testosterone that fatherhood brings is not something to be regretted: it actually helps you become the man you are hoping to be.
The true human story of hormones and fathering is one that needs to be shared with more men. Dads who want to embrace the role of caregiver. Dads who think caregiving is not their role. Dads who might be nervous to hold and care for their fragile newborn. They are all men who will be shaped by fatherhood and whose care will shape the ideas and understanding of family life and masculinity for the next generation of men.
