While the loudest voices in our culture continue to insist that men and women are interchangeable, science is offering more evidence than ever that the two sexes are anything but interchangeable. Nowhere is this uniqueness more apparent than in the distinct and very real transformations that both men and women undergo on becoming parents.

Of course, having children changes a woman—both body and mind—but it turns out men experience changes, too. They may not be as outwardly obvious as those in women, but they are dramatic and life-altering.

Writing at The New York Times, Oxford anthropologist Anna Machin cites a decade of findings on how men are designed to respond physically, emotionally, and neurologically to the arrival of a baby. And those changes, she observes, aren’t the same as those seen in women.

For example, a five-year study of over 600 men in the Philippines found that those who became fathers experienced a significant drop in testosterone—the hormone associated with male aggression, strength, and sex drive.

While this may herald the arrival of the stereotypical “dad-bod”—which I may or may not know something about—it also marks a critical transition in a father’s behavior. Lower testosterone makes way for a cast of new hormones that draw him toward his partner and infant, and encourage him to participate in “caregiving and baby-related household tasks…”

Machin points to studies indicating that the lower a man’s testosterone, the more likely his brain is to experience surges of bonding and reward hormones like oxytocin and dopamine when he spends time with his child. These are partly responsible for that feeling of warmth and contentment new parents know so well—a feeling that helps make up for the sleepless nights and dirty diapers that come with a newborn.

But here’s where things become really interesting, and where science is showing how biologically distinct dads are from moms. A 2012 study by Israeli neuroscientists found that the parts of the brain that light up most dramatically in fathers aren’t those most active in mothers. Machin writes: “For moms, regions closer to the core of the brain—which enable them to care, nurture and detect risk—were most active. But for dads, the parts that shone most brightly were located on the outer surface of the brain, where higher, more conscious cognitive functions sit, such as thought, goal orientation, planning and problem solving.”

This difference between maternal and paternal brain activity, she writes, “may reflect a difference in role, and different but equally strong attachments, between mothers and fathers.”

The old cliché that kids run to mom for hugs and kisses when they scrape their knees, and to dad when they want to play and laugh, may have more basis in biology than many would like to admit.

Remember those reward hormones I mentioned earlier? It turns out moms and dads experience surges of them while engaging in very different activities with their kids. A woman’s oxytocin and dopamine peak when nurturing and comforting her children. For a man, these same hormones peak when he engages in horse-play with his children. Machin writes that wrestling with dad “not only cements bonds between father and child, but also plays crucial roles in a child’s social development.”

All this talk of “roles” and “differences” between moms and dads—in the New York Times, no less!—isn’t politically correct. But reality often isn’t. The roles moms and dads play in reproduction and child-rearing are anything but socially-constructed. As Ryan Anderson is fond of saying, there is no such thing as “parenting.” There’s only mothering and fathering.

Of course, God is gracious in tough situations when either a mom or a dad is absent, but both moms and dads play unique and non-interchangeable roles in a child’s development. Machin attributes this to evolution, but Christians recognize the design of God, who made us male and female, and who inscribed the family into our very bodies.

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