His baby daughter melts his heart. Scientists say she's changing his brain, too.

It was only 10 a.m., but Dalton Hessel had been awake for hours caring for his newborn, Mia.

Hessel, 28, sat on the couch with his wife, watching their daughter lie on the garden-themed playmat during tummy time. The baby's eyes darted back and forth between the shapes and colors before her attention was finally caught by a carrot dangling in her face.

As she reached for the soft toy, Mia paused and skillfully rolled over onto her back. Hessel was in awe. He had just witnessed one of her first milestones.

If it hadn’t been for his eight-week paternity leave, the teacher from Hayward, Wisconsin, would have been going over simple math problems with his second graders at this hour.

“It was such a cool moment,” Hessel said. “Those memories you make and the bond you form during that time is going to be foundational to how you raise your child.”

What happened in that moment struck him at his core, as such moments do for many parents, and it turns out something may have also been happening for Hessel on a neurological level. New fathers who spend large chunks of time with their infants experience brain changes (akin to what mothers experience) that better prepare them for fatherhood, according to a growing body of research. The research provides compelling evidence that taking paternity leave could give fathers time to trigger those changes.

“That caregiving experience is what makes the brain changes come online,” said Darby Saxbe, professor of psychology at the University of Southern California. “That’s where the argument of family leaves comes in – if you give (fathers) more opportunities to interact with their infants, you’ll see more of these changes.”

How a father's brain changes

There’s a long-held belief that women have an innate, maternal instinct that helps them better adapt to motherhood.

Whether or not that instinct exists, it turns out that fathers may be able to develop this ability, without the same flush of hormones and the experience of pregnancy, by simply spending time with their infant.

Saxbe and her team of researchers studied brain scans of women before conceiving and two months after giving birth and found structural changes in the new mothers’ brains.

Pregnancy appeared to affect two parts of the brain: the cortex, or the top layer of the brain, and the subcortex. The cortex is linked to reasoning, thinking and decision-making, and the subcortex is associated with emotional processing.

Researchers also found the women's brains had reduced gray matter, or the middle portion of the brain that connects brain structures, which meant their overall brain volume had been reduced.

“You would think that losing gray matter volume may be a bad thing, but evidence suggests it could be adaptive because it represents streamlining passages (for) faster and more efficient processing,” Saxbe said.

Saxbe’s team at the University of Southern California and their colleagues in Spain’s Instituto de Investigación Sanitaria Gregorio Marañón compared these results with scans from 40 men taken during their partner’s pregnancy and after their baby was 6 months old.

The scans showed similar brain changes among fathers, changes that were not found in a control group of 17 childless men, according to their findings published in a 2022 issue of the journal Cerebral Cortex.

Although the changes were more varied and subtle in men's brains than in women's, the pattern of change was more pronounced among Spanish men, who generally have more generous paternity leave than fathers in the U.S.

Saxbe said fathers who spent more time with their children showed a greater reduction in gray matter.

The hours a father logged with an infant made the difference, she said: “It seems that time with baby is an important part of remodeling the brain.”

How brain changes translate to paternal behaviors

Saxbe and her team’s research built upon studies that found similar patterns in mice.

Scientists at NYU Langone Medical Center linked certain parental behaviors to brain cells that make a signaling chemical called dopamine, which is known to create feelings of satisfaction, according to a 2018 study published in Neuron.

The NYU researchers studied female mice and their brain activity as they interacted with their babies, focusing on a region near the front of the brain that's key to retrieval, according to study author Dayu Lin, a professor of psychiatry, neuroscience and physiology at NYU Langone Health.

Researchers found cells in this part of the brain were the most active when mothers located their babies and when they brought them back to the nest. They artificially stimulated these cells in mice that hadn't given birth, and found they retrieved the babies like the mother mice. When researchers blocked this brain activity, both mother and pup-less mice stopped retrieving the little ones.

In research that has yet to be published, Lin said NYU Langone Health scientists are seeing similar brain activity and pup retrieval behaviors in male mice.

“Our ongoing work suggests that with extensive exposure, (males) can also switch,” she said. “Repeated exposure (to pups) in mice studies is very critical for the emergence as well as the maintenance of parental behaviors.”

Paternity leave in the US

Some fathers say they've experienced this surge of joy the researchers tapped into in brain studies.

Kasean Kitson doesn't think his bond with his daughter, Eleanor, would have been the same were it not for the eight weeks of paternity leave he took last year.

"I don't think I would have had the same relationship with her. ... All the little things, I would have missed so much," said Kitson, 32, who lives near Greensboro, North Carolina. "It’s the happiest I’ve ever been my whole life, not having to worry about anything else except for Eleanor."

The U.S. is one of the few high-income countries that does not uniformly offer paid parental leave. The Family and Medical Leave Act guarantees only 12 weeks of unpaid, job-protected time off for both parents of a newborn or newly adopted child.

The federal law applies only to people who work at companies with at least 50 employees. And even if time off is permitted, most new parents can’t afford to give up a steady income for three months. About 66% of people eligible for leave under the act don’t take it because they can’t afford to, according to a 2020 report from Abt Associates, a government and business research and consulting company.

Only 11 states and the District of Columbia have passed laws that ensure paid family and medical leave: California, Colorado, Connecticut, Delaware, Maryland, Massachusetts, New Jersey, New York, Oregon, Rhode Island and Washington.

New Hampshire guarantees coverage for state employees and enacted a voluntary law that provides paid leave for private-sector employees who purchase coverage. Vermont has taken a similar approach, but neither of these states guarantees workers a right to paid leave.

In addition to brain changes, research shows paternity leave can improve relationships between men and their partners, lead to better health and developmental outcomes for children, increase fathers' engagement and bonding, and narrow the gender wage gap by increasing employment and pay opportunities for mothers.

Hessel returned to work the first full week of December. Although he missed his second graders, he wouldn’t have traded the time he spent with Mia for anything in the world.

Without that time, he wouldn't be as confident in interpreting her cry cues: when she needs changing, feeding or burping. Their connection wouldn't have been the same.

If he could live those months over, he'd take paternity leave without a doubt, he said, because “you don’t get that time back.”

Send tips to Adrianna Rodriguez: adrodriguez@usatoday.com

This article originally appeared on USA TODAY: Paternity leave more than changing diapers. It can change your brain.