(CNN) -- It took less than half a second for Jeremy Hooper to fall in love with his little girl. He was with her mother for 24 hours in the hospital while she gave birth, and even cut the umbilical cord.
He remembers waking up the first night his daughter cried, begging for a bottle.
"She looked up at me, and our eyes connected," he said. "I thought, 'Here we go, kid.'"
Hooper and his husband of five years adopted their daughter, Savannah, last fall. Although they are not biologically related, Hooper says "it was love at first sight."
Hooper is one of a growing number of fathers who are acting as primary caregivers for their children, with or without maternal involvement. The number of stay-at-home dads has doubled since 1989, to 2 million in 2012, according to Pew Research Center. Single-father homes are also on the rise: 8% of homes are now headed by a single father, up from 1% in 1960.
Our brains are ready for this cultural shift, researchers say. Scientists from Bar Ilan University in Israel recently published a study showing that the brains of dads who are highly engaged in their infants' lives are activated in the same way mothers' brains are during pregnancy. This maternal neural network allows mothers to react quickly and instinctively to their child's needs.
"There's something really strong that makes (mothers) bond with the infant right when they're born," said lead researcher Ruth Feldman, a professor of psychology at Bar Ilan University and an adjunct professor at Yale University. "Fathers need a little more work for that to happen."
Dads' brains are 'particularly plastic'
Feldman's study is the largest on the parental brain to date, with more than 90 mothers and fathers participating in the research. Feldman and her team wanted to find out how our brains operate differently as parents, and, more specifically, how men's brains are altered in response to fatherhood.
Researchers know from previous studies that the amygdala becomes highly active in mothers after giving birth. What they didn't know was if -- and if so, how -- fathers' brains change.
Feldman studied three groups of parents: heterosexual primary caregiving mothers, heterosexual secondary caregiving fathers, and homosexual primary caregiving fathers. The researchers videotaped the parents playing with their children at home and later played the videos back to the parents while they were in an MRI tube, along with videos of other infants to compare the brain responses.
The researchers found that the brains of primary caregivers and secondary caregivers showed different responses while the parents watched videos of their infants.
The brains of the primary caregiving mothers demonstrated a strong response in the amygdala, which regulates emotional reactions and allows mothers to bond quickly with their newborn. Secondary caregiving fathers' brains, on the other hand, showed a strong response in the neural network that regulates social cognitive processing
However, in the primary caregiving fathers -- the 48 homosexual fathers who were parenting without maternal involvement -- the researchers discovered that both paternal and maternal networks were activated. In other words, these primary caregiving fathers demonstrated a brain response similar to mothers who had carried their babies to term through pregnancy, yet also showed brain responses similar to those of secondary caregivers.
Amongst all fathers, both primary and secondary caregivers, time spent taking care of their child correlated with greater amygdala activation.
"It shows that the amygdala can be sensitized through months of pregnancy, but also through active parenting," Feldman said. "Fathers' brains seem to be particularly plastic."
The new fatherhood
As societal norms continue to evolve, Feldman and others say the definition of what it means to be a father is rapidly changing.
A mother's role in child rearing has been fairly consistent across cultures and throughout history, Feldman says, while fathers' involvement has varied widely. More than anything, fathers are now playing a greater role in rearing their offspring, a relatively recent development in Western society.
It's a fascinating shift, said James Morris, an assistant professor of psychology at the University of Virginia who studies the neurobiological basis of social functions.
"We can (now) study parenting without a female being involved, which is a relatively new phenomenon in nature," he said. "It's well known that (neurobiological changes) will happen when you become a mother, but we don't know this for males."
Morris emphasized that scientists didn't need brain imaging to confirm that fathers can be fantastic primary caretakers.
"That said, the study is important because the neurobiological processes underlying parenting behavior are not well understood," he said.
Morris, however, cautioned against drawing conclusions based on the differences between the gay men in the study and their heterosexual secondary caregiving counterparts.
Gay male fathers were used as test subjects because researchers wanted to see what happens to fathers' brains when there are no mothers around, not because these men were homosexual.
"The differences found between men in this study cannot necessarily be attributed to the fact that one group is gay," Morris said in an e-mail. "These findings might be useful when considering other social dynamics (like) situations where fathers may take more of a 'primary' role in parenting than has been considered 'traditional' in the past."
Although Morris praised the study as "well-grounded," he also questioned its strong distinction between the so-called maternal pathway and paternal pathway. The distinction is probably much looser than that, he said.
"I would say there are probably some interesting differences between males and females, but I don't know if we could say that one group is more emotional than the other, or more cognitive than the other," Morris said.
Morris added that to further perfect the study, researchers would need to perform the tests before and after parents have children to show that these changes occurred after the birth of their child.
For now, primary caregiving fathers like Hooper say they are heartened by the researchers' findings.
"It does thrill me as a parent because I think we're doing a pretty nice job raising our little girl," Hooper said. "I love that we're having this conversation."
Researcher Feldman was not particularly surprised by the findings, saying she expected to discover some degree of plasticity in the male brain.
"What was surprising was how neatly and how beautifully these things worked together," she said.