By now, it is a truism that men have invested more in their lives as fathers now than ever before. The new fatherhood offers a host of new pleasures and new intimacies. Others have pointed out the not-inconsiderable disadvantages involved in being a dad today.

What is obvious to anyone who has even glanced into the research on fatherhood is how little is actually known about it. Women who are about to become mothers have dozens upon dozens of books to read, filled with bounties of thoroughly collected information. Men who are about to become fathers? They read the books on motherhood in which their presence and responsibilities are mere footnotes.

This gap in the market has recently been admirably filled by Paul Raeburn, and his recent book Do Fathers Matter? What Science is Telling Us About the Parent We've Overlooked. The most important insight is in the title. In his introduction, Raeburn notes that a search for the term "mothers" in the online catalogue of the U.S. National Library of Medicine retrieves about 97,934 studies. The term "fathers" produces only 15,156 results. That proportion, about one study of fatherhood for every six on motherhood, applies across other fields.

Psychological studies, particularly studies of child development, regularly exclude fathers entirely. Kyle Pruett, a psychiatrist at Yale, claims that studies on problems as diverse as ADD, autism, depression and suicide rarely, if ever, mention the roles of fathers. And this despite the fact that fatherhood deeply, deeply matters.

"When we bother to look for the father's impact, we find it—always," Pruett says. "Not looking at the impact of fathers and children on one another has given the entire field (and the best-selling parenting books it produces) a myopic and worrisomely distorted view of childhood development, a view with staggering blind spots."

This all should be obvious. All research onto fatherhood shows that it's a massively potent force, not just in family life but in society generally. The two answers to the question "Do Fathers Matter?" explored by Raeburn are yes, or hell yes.

To me at any rate, this is all the confirmation of common sense. Does anyone who has ever had a father, or missed a father, or been a father doubt that children need them?

And yet I was surprised by how much the scientific research shows that fatherhood matters. The best parts of "Do Fathers Matter?" are on questions of paternal relevance at a biological level.

One of the most startling discoveries to me was that men go through profound hormonal changes during their wives' pregnancies. Nobody told me this when my wife was pregnant. I had heard, as a kind of joke, about "couvade," in which the husbands of pregnant wives suddenly start eating like they are pregnant. What I didn't know is that couvade isn't merely a phenomenon of spoiled Western countries but in the Third World.

In Papua New Guinea men who are waiting for their wives to give birth "retire to bed with unremitting nausea and incapacitating back problems, demand to be looked after, and otherwise raise an emotional fuss during the last months of their wives' pregnancies." So don't feel too bad if you're behaving that way.

It is not just an expecting partner that changes the hormonal realities of men with pregnant partners. One study showed that exposure to nursery blankets, or to films about breastfeeding, or to infants themselves caused significant changes to the levels of the testosterone, cortisol and prolactin in men. When you first hold your infant in your arms, your testosterone level drops 33%. The testosterone levels of fathers remain suppressed. Indeed, the more time you spend with your kids, the less testosterone you have:

"In September 2013, James K. Rilling and his colleagues at Emory University reported in the Proceedings of The National Academy of Sciences that testosterone levels in the blood were inversely correlated with paternal caregiving--that is, testosterone was highest in fathers who devoted less effort to child care, and lowest in those who invested more effort in child care. They also found that the fathers who devoted more resources to their children had smaller testicles."

Everybody knows that fatherhood changes men, and now we know how: Fatherhood makes men's balls smaller. But that's only one of the minor change.

The power of "Do Fathers Matter?" is that it shows how vast and yet submerged those changes are. The power of fatherhood remains mysterious, hidden from us. We do not know exactly how fatherhood changes us. But we know that those changes are written into the most elemental nature of our bodies.

Stephen Marche Stephen Marche is a novelist who writes a monthly column for Esquire magazine about culture.

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