Modern science ignored fathers for decades. The neuroscience of how motherhood impacts women has been documented across a handful of studies, and thousands of papers have been published that detail how it impacts women psychologically, socially, and biochemically. But, until recently, we weren’t even sure that having children makes men happy—let alone how neuroscience figures into it all. Even now, as more researchers are taking an interest in the male experience of family, undisputed facts remain few and far between. “There’s some conflicting work out there,” explains Margaret Kerr, a psychologist who studies the emotional experiences of parents at the University of Wisconsin-Madison. “Some say overall life satisfaction declines, others that it stays the same, and some work says it increases. So… that’s not super helpful.”

That said, in recent years “not super helpful” has begun to give way to “authoritative” if not “definitive.” New work on the biological and psychological impacts of fatherhood is starting to paint a picture of the measurable changes men experience when they become fathers — everything from shifts in physiology to changes in priorities. What has become clear is this: Fatherhood is a condition, both chronic and acute. And the nature of that condition is finally coming into focus.

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“Dads have long been seen as ‘along for the ride’, while mothers go through all sorts of physical changes that help them prepare to be parents,” says Lee Gettler, a biological anthropologist who studies family systems at the University of Notre Dame. “We now know that there is a biology of fatherhood.”

There was a non-cultural reason for the initial misconception. Males contribute to child-rearing in only about five percent of mammalian species, mostly rodents and New World monkeys, like capuchins and marmosets. Because scientists have never met a rodent they didn’t want to slice into thin slices, much of the best work on how fatherhood changes mammalian brain structures comes from experiments on Prairie Voles (Microtus ochrogaster), which happen to be some of nature’s most attentive fathers.

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Studies have shown that, when a Prairie Vole is born, new neurons sprout up in its proud father’s hippocampus, a brain region involved in learning and memory, and those neurons grow surplus dendritic spines, which help brain cells connect with one another. Both of these changes probably exist to help papa voles identify and remember their offspring.

It’s trickier to track structural changes in the human brain, but neurobiologist James Rilling, who studies the anthropology of fatherhood at Emory College, is trying. In 2014, Rilling and colleagues hooked a group of fathers and non-fathers up to an fMRI brain scanner. When they showed the men photographs of toddlers, the fathers’ brains activated in the regions dedicated to reward processing—non-fathers, less so. When the researchers showed the men sexually provocative images, however, non-fathers’ brains outshined fathers’ brains on the fMRI readout. Whatever is going on in fathers’ brains, this study suggests that it impacts a man’s priorities— tamping down desires for sexual conquest and replacing interest in cooing at toddlers.

“Fathers find the child stimuli to be more rewarding or salient, while non-fathers find the sexual stimuli more rewarding,” Rilling told Fatherly. “One can imagine that it would be adaptive for the father to orient his efforts and energy toward his family rather than sex outside the family.” Indeed, this shift from conquest to caretaking may be one reason why early humans developed such complex brains. When our ancestors shifted toward semi-monogamy millions of years ago, this made active fatherhood far more likely and, in turn, influenced fathers to hunt for their families. Scientists suspect this led to a protein boost that gave our brains an evolutionary edge.

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Oxytocin may be a key hormone behind these priority shifts. Oxytocin is known for its role in social bonding, and studies have shown that it’s a key chemical involved in strengthening the bonds between parents and children. In Prairie Voles, the reward centers of the brain start to produce more oxytocin receptors when fatherhood is nigh, rendering the rodents extremely sensitive. Here, the jump to humans is less difficult to make. One 2010 study that asked new fathers to take supplemental oxytocin found that those dads interacted with their children more intensely than usual, and were quicker to respond to their infants’ cries and vocalizations.

“People traditionally thought oxytocin was just a maternal hormone,” Rilling says. “Now it’s becoming clear that it’s involved in paternal caregiving as well.”

The other key hormone that throws fathers for a loop is testosterone. “When men go from being single and not having children to being married fathers, their testosterone declines by around a third,” Gettler says. “Men with newborns show the biggest drops…these changes in testosterone start to occur for some U.S. men during the mother’s pregnancy, before baby even arrives.”

Gettler’s research suggests that fathers with lower testosterone (and higher levels of prolactin, yet another hormone involved fatherhood) are more likely to soothe crying babies and engage with their children. He has also found that fathers who take the most active roles in childcare have lower testosterone levels—although men with exceedingly low T may suffer depression, and consequently take on fewer childcare responsibilities. “There is a sweet spot in the middle for dads’ testosterone that helps facilitate nurturant, sensitive caregiving,” Gettler says.

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It seems, then, that dads are cocktails of chemicals, brains pitching and yawing to accommodate their new roles as providers and caregivers. That action can do a number on a man’s psychology. “Becoming a father involves a change in role and identity,” Marcy Carlson, a sociologist who studies families at the University of Wisconsin-Madison, told Fatherly. “One suddenly has responsibility for raising another human being, and most men aspire to fulfill the various aspects of the role—providing, teaching, guiding, nurturing, supporting. So men see themselves and what they want to do with their time, money, and attention differently.”

Although research on how men emotionally respond to fatherhood is still in its infancy, Kerr is currently studying the self-reported emotions of new fathers as part of a large, yet unpublished body of work. Although the final results are still unavailable, a handful of themes have already emerged from the interviews. “We ask about joy, pain, difficulty, happy and sad feelings, guilty and proud feelings, being angry, being annoyed,” she says. “We find that fathers get a lot of joy and also pride from teaching things to their children, or watching their children meet new milestones and learn to do something for the first time.”

The strongest negative emotions, Kerr says, are pain and guilt, usually surrounding the need to go to work or take time for self-care, such as going to the gym or going out for a date night. “They sort of feel bad doing those things,” she says. “Even though they recognize that it’s important.” Many dads also experience deep emotional pain because they feel that their children are less attached to them than they are to their mothers. Often, that’s due to a simple biological imperative—infants need their mothers’ breastmilk to stay alive, and it’s unrealistic for fathers (especially those who work full-time) to expect that small children will bond with them in the same way that they bond with their mothers. But this is cold comfort to dads who feel forgotten. “A lot of toddlers have a preference for the mom, and that can be hard for the dad,” Kerr says.

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“Dads really want to have that same level of bond with their kids.”

This desire to engage with one’s children may be why involved fathers tend to have more positive psychological outcomes than non-resident or less present dads. Fathers who do not live with their kids have lower earnings on average, and are more likely to struggle with depression. For non-resident dads fatherhood is often many, many emotional valleys; precious few peaks.

“Emotionally, becoming a father certainly brings great joy, but the nature of the relationship with the child’s mother can have a big impact on how fathers feel about fathering and how involved they will be,” says Carlson, who has studied the consequences of childbearing outside of marriage as part of the Fragile Families Study. “The more that a father identifies with the father role early on, the more he is shown to be involved and engaged as a father over time.”

The early emotional downsides of fatherhood can be compounded by a handful of stark postnatal realities. Both men and women tend to struggle with intimacy after they become parents, and argue more often. They get less sleep, experience higher levels of stress and, with mom benched, dad often needs to pick up the slack around the house. It doesn’t help that sex isn’t happening at all in those first few weeks—and, in some cases, quite a while after that.

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“It’s important for couples, including dads, to realize that most people struggle, particularly after the first child,” William Bradford Wilcox, sociologist and director of the National Marriage Project at the University of Virginia, told Fatherly. “A dad’s struggle is not an indicator that he’s doing badly. It’s just a natural side-effect of becoming a dad. His relationship with mom is changing.”

But new fatherhood isn’t all bad, from a sociological perspective. “Men who are living with their kids work harder and make more money,” Wilcox says. “We see a ‘married father premium’ in the research.” Unlike the testosterone spike which attenuates over time, studies suggest the “married father premium” never quite wears off. One Harvard study found that active fathers have greater life-time earnings than single men, even after controlling for outside factors. “Economics leads to certain kinds of family structures, and certain kinds of family structures lead to economics,” Wilcox says. “Men feel a sense of responsibility, to work harder and earn more.”

Unfortunately, the psychological deck is often stacked against new fathers. We know that dads are the most emotionally healthy when they have time to bond with their kids, but the realities of the workplace make that difficult. One straightforward solution is for dads to take paternity leave. Studies have shown that men are hesitant to take paternity leave, even when it’s offered, and this is a mistake. “Being there for the first few weeks helps the dad learn the diaper change and feeding schedules, so he can be engaged instead of having to catch up after work,” Kerr says. “It’s extremely beneficial for bonding, partnership, and the well-being of the whole family.”

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Men can also maximize the positive impacts of fatherhood by catching a few naps during the day, and setting aside date nights to reconnected with their partners. Engaging in mindful parenting, taking time to focus on the best parts of being a dad, can also do wonders. “Spend some time each day thinking about things that went well with your child, and reliving those moments by sort of putting yourself back into that memory,” Kerr suggests.

Gettler agrees with this advice in theory, but is realistic about the challenges of maintaining one’s mental health as a parent. “Sleep, diet, psychosocial stress, and exercise all probably matter in terms of reducing some of the potentially negative effects,” he says. “It is easy to say that dads should aim for healthy habits but much harder for any of us, mothers or fathers, to put into practice with little ones in the picture. I have two kids who never sleep, and my wife and I both have stressful careers. I wish I had some magical solution — for myself!”