From even before a child is born, men undergo dramatic hormonal changes that can profoundly affect their behaviour — with direct consequences for their child’s wellbeing.
In the months before my son was born, my wife and I attended an ‘active birth’ workshop — training for parents on labour and delivery before a baby arrives — a ‘breastfeeding session’ and a hospital-run antenatal course.
We read a stack of books on pregnancy and babies and browsed countless websites. Our notepads quickly filled with information.
My notes from that time described in detail how women’s bodies prepare for childbirth and motherhood — the hormonal swings, the shifting of organs and the rewiring of the brain.
But no one told me that my own brain and body were also preparing for fatherhood.
When my son was a little over a year old, I first encountered this idea after reading ‘Father Time,’ a book by the primatologist Sarah Blaffer Hrdy.
In it, the author argues that men, too, have the necessary biological wiring to be ‘as protective and nurturing as any devoted mother.’
It piqued my curiosity.
I am keen and committed to being an active father, but I had assumed this was perhaps a cultural choice made by men of our generation.
But Hrdy’s book introduced me to an entirely new academic idea — that the roots of this behaviour actually lie deep in biology, lying dormant and activating at the right time.
After interviewing experts and digging deep into various studies, I reached a simple conclusion: fatherhood transforms a man in ways remarkably similar to the transformation of motherhood.
The more involved a father is in caring for his child, the deeper this transformation. The changes in men’s endocrine (hormonal) and nervous systems show that a caring father is no modern exception but rather a deep biological feature of who he is.
A drop in testosterone
The earliest research into how fathers change physically because of their children came from observing other animals.
These late-20th-century studies found that many male mammals (including other primates), when they actively care for their young, undergo clear hormonal changes.
These include swings in hormones such as testosterone, vasopressin and prolactin, which are usually associated with motherhood.
When the American anthropologist Lee Gettler heard about these findings as an undergraduate in the early 2000s, he became engrossed.
Gettler, director of the Hormones, Health and Human Behavior laboratory at the University of Notre Dame in Indiana, said: ‘I asked my lecturer whether anyone was researching these questions in human fatherhood, and the answer was — no.’
In 2000, two Canadian researchers — Catherine Wynne-Edwards and Anne Storey — published the first study on hormonal change in men.
By the time Gettler began working in the field, it was an established fact that men with children had lower testosterone levels than childless men.
‘But there was a chicken-and-egg problem,’ Gettler explained. ‘Are men with lower testosterone simply more likely to become fathers? Or does the transformation of fatherhood create this pattern of biological change in men?’
To find the answer, Gettler joined scientists on a decades-long project in Cebu City, the Philippines.
In 2005, the team collected saliva samples from 624 men aged 21 who had no partner and tested their testosterone. They tested them again four years later.
They wanted answers to two questions: whether those who had become fathers in the interim would have lower testosterone, and whether it would be lower still for those who spent more time on childcare.
When the results came in, the answer to both questions was ‘yes.’
Those who had become fathers had significantly lower testosterone than childless men.
And the men who had spent more time caring for children had the largest drop in testosterone.
Those who slept in the same bed as their children also had lower levels.
Gettler told me: ‘I think that was the first clear message in scientific research that men have this capacity to prepare for fatherhood.’
In a sense, the biological process itself readies them for childcare.
Their findings are not unique. Several other teams of scientists have also found that a drop in testosterone during a partner’s pregnancy is linked to fathers spending more time with the baby after birth, to clear interest in raising the child and to satisfaction.
The level of this hormone was even linked to men’s responses to a baby’s crying — making them more alert and sensitive.
In 2018, a team from Gettler’s lab concluded that fathers with lower testosterone were more involved in caring for young children.
But when does this happen — before or after birth? That question preoccupied James K. Rilling, director of the Laboratory for Human Social Neuroscience at Emory University in the United States.
Rilling told me: ‘My assumption was that it would happen after the mother gives birth, when fathers spend some time with their babies.’
What they found in the research surprised them.
When expectant fathers were tested four months into their wife’s or partner’s pregnancy, their testosterone and vasopressin — two hormones — were already low.
Rilling, who in 2024 published a book on the science of fatherhood called ‘Father Nature,’ said vasopressin had a similar effect.
Rilling wanted to know why this happens. Do expectant fathers receive some pheromonal signal from their pregnant partners — a chemical reaction between two people that affects behaviour or physical change?
Or is it a psychological change after learning a child is on the way?
We do not yet know this for certain. But what is certain is that the changes extend beyond testosterone.
A wave of the love hormone
Take oxytocin, the so-called ‘love hormone,’ for example.
I remember from the antenatal course that we were encouraged to keep my partner calm and relaxed during labour, so that her oxytocin could flow and the delivery would be easier.
After our child was born, we were told that a huge surge of oxytocin is created at birth, and that breastfeeding raises this hormone again and again, helping to bond mother and baby.
But I did not know that in the first few hours after birth, when he slept on my bare chest, my own oxytocin was rising too.
Many studies around the world have found high levels of oxytocin in fathers — even in those whose children are one to two years old, or who are caring for babies under six months.
It depends on how much time is spent with the children.
For example, fathers who played more with their children and stayed close to them had higher oxytocin.
The same change was even evident when fathers held their newborn for the first time.
Oxytocin boosts our instinct to play the role of father. Rilling explains that you can test this by spraying the hormone into men’s noses and noting what happens.
He said: ‘There’s one study I love. They give fathers intranasal — through the nose — oxytocin while they interact with their baby, and find it helps fathers turn their heads more quickly.’
On the video call, Rilling swung his head from left to right and up and down, looking like an over-enthusiastic father.
Such findings point to a positive cycle with oxytocin — the more the hormone rises, the more likely a father is to engage with his child, and being close to the child can then raise the hormone further.
The more scientists investigate this, the more changes they find in other hormones too.
In a study published in 2025, Rilling and his team found that levels of the hormone vasopressin had fallen in new fathers even before the baby’s birth.
This hormone is linked to male-on-male conflict and to feelings of maintaining dominance.
Another surprising hormone is prolactin. In humans, this chemical is known for its role in lactation and maternal care. But biologists have linked it to paternal care in other animals, including birds, fish and marmosets — a South American monkey known for its paternal instinct. In 2023, a team led by the American clinical psychologist Darby Saxbe tested the prolactin levels of expectant fathers and concluded that those who felt a strong bond with their unborn child had higher levels of the hormone.
Again, prolactin levels before birth determined how involved these fathers would be in caring for the child.
As we have already seen with oxytocin levels, both of these hormonal changes are more pronounced in fathers who care more for their babies.
Darby said: ‘It’s not just new mothers who go through hormonal changes. It seems men show the same kind of adaptation and the same kind of outcomes.’
A man’s second adolescence
Saxbe is examining whether the effects of these hormonal changes leave any mark on fathers’ brains.
She told me: ‘I thought fathers were really a very curious — you might say a special — population, in the sense that they experience the transformations of parenthood without going through pregnancy.’
Birth mothers experience a burst of hormones while carrying a child and another surge during delivery. But their partners’ experience is more subtle.
Saxbe’s book ‘Dad Brain’ is to be published in 2026. She explains: ‘Fathers’ role gives us a chance to separate the effects of pregnancy from the experience of raising a child.’
A few years ago, her research team, with colleagues in Spain, scanned the brains of first-time fathers before and after their child’s birth.
They found neurological changes were taking place. Their brains were adapting to new experiences and information.
Saxbe compares this transformation of fatherhood to adolescence — another crucial period of our development in which our brains must adapt to new challenges, stimuli and ideas.
And in a follow-up study, she found that men who felt a stronger bond with their unborn child, or who planned to take longer paternity leave, showed larger changes in their brains.
In 2026, Rilling reported similar evidence of brain change in new fathers, confirming this neurological transformation.
As with many of the changes in our fatherly brains and bodies, there is one constant here — the more involved you are, the more change comes within you.
Sarah Hrdy, the primatologist who wrote ‘Father Time,’ says all human brains have a latent capacity to raise children — what she calls an ‘alloparental substrate’ — that can be activated in the right circumstances.
In ‘Father Time’ she argues that, as humans evolved into more complex societies, collective care helped humanity flourish.
Having men who could provide primary care for a child was valuable, and so we acquired the ability to do it — which we still retain.
In a 2014 study, a team of Israeli academics led by Ruth Feldman included heterosexual couples in which a woman provided the primary care and the father ‘helped.’
They also included gay male couples raising children without any woman. Their brains were scanned while they watched videos of their children.
Among the heterosexual couples, the brains of the women who were primary carers were stimulated in areas linked to deep instinctive responses, such as the amygdala.
The brains of the supporting men, by contrast, showed more activity in social areas, which could mean they were assessing the situation before taking action.
But the gay men who were primary carers showed the same kind of activation in the amygdala and ‘maternal’ feelings, while also retaining the social element.
Fatherhood was literally reshaping their brains.
Social change
The experts I spoke to, and most of the research in this field, agree that these biological developments of fatherhood should prompt a fresh look at government policy for families.
Saxbe said: ‘Society should work, as an urgent priority, to create opportunities for fathers to build those bonds.’
She said paternity leave can make bonding between father and child easier.
Gettler told me another key change is to involve men from the start — such as attending ultrasounds, going to the doctor and actively keeping in touch with their partner during pregnancy.
He told us: ‘We know this biology potentially begins to take effect from the time of pregnancy, when a family is preparing to welcome their baby.’
Active fathers are a great benefit to families.
In many countries, including Pakistan, Kenya and the United States, mothers whose partners are more active have reported better mental health. And, importantly, children benefit.
A large study published in early 2026, carried out over seven years on 292 families, led US researchers to conclude that the children of fathers more attentive to parenting had better heart health.
Interestingly, the same effect was not seen in mothers.
Gettler said: ‘I think there is room to reflect on how the biological process of fatherhood plays a role as a foundation for building strong and healthy families from the very start.’
বাংলায় মূল প্রতিবেদন পড়ুন · Read the original Bengali report