For years, scientists have been sounding the alarm about the rapid depletion of the world’s resources. But “peak oil” could be but a blip compared to another energy source that researchers say is running low—say hello to “peak sperm.”

Last month, the European Science Foundation issued an ominous report warning that at least one in five men age 18 to 25 is "subfertile." The group of scientists asserted that sperm counts and sperm quality have been dropping consistently in the developed world for the last 50 years. Men just aren’t producing little swimmers like they used to.

The idea of sperm in decline is controversial. The first research to tip us off to the problem was a 1992 Danish study reporting that sperm counts had dropped by 1 percent every year since 1938. In the following years, other studies yielded mixed results. But more recently, the links between male fertility and the environment is being taken very seriously. Groups like the Center for Reproductive Epidemiology at the University of Rochester Medical Center are adding weight to the idea that sperm is taking a hit, possibly from environmental chemicals. Although the center’s director Shanna Swan concedes it is hard to know for sure, she put the yearly sperm drop at 1.5 percent in the U.S. and 3 percent in Europe and Australia.

We tend to see men's sperm as either working or not working and assume that the story ends there.

While battling global fertility decline may not fall under the purview of most couples’ daily concerns, the problem of vulnerable sperm quality is nevertheless changing the rules of the copulation game. Findings this year have shown that a man's lifestyle—the choices he makes—could have an impact on his sperm, and in turn, the health of his future child. That's a revolutionary idea, because we tend to see men's sperm as either working or not working and assume that the story ends there.

But sperm may in fact be more complicated than that. The idea that toxins in our everyday environment could knock down the numbers is unsettling. But beyond that, if a man's health and typical day-to-day habits affect his future offspring in subtle ways, it challenges our assumptions about the male contribution to baby making.

My husband and I were having dinner with a friend not too long ago—he and his wife have been trying to get pregnant for a while now, and so far it's not working. He lamented that her age could be the culprit (she's in her late 30s), and that maybe the window had closed.

"Are you still smoking?" my husband asked him.

Our friend cocked his head. "Why, does that affect fertility?"

He hadn't really thought about how his semi-regular cigarette habit could be affecting his sperm. But looking back, the fact that he missed the memo on smoking and sperm wasn't really the point—it was his shock at the notion that when going into baby-prep mode, his own health status, not just his wife's, could be in the spotlight.

When it comes to the possibility of environmental toxins affecting sperm, the usual suspects are involved. This summer, for example, researchers from Harvard and Michigan Schools of Public Heath, along with the CDC, reported a relationship between bisphenol-A (BPA) and sperm counts. BPA, found in hard plastic items like food containers, could tinker with reproduction because it's an estrogen-like "endocrine disruptor." We've known it to be true for male rodents exposed to high levels of the chemical, but this year's findings are the first foray into human territory. Men with the highest levels of BPA in their urine had 23 percent lower sperm concentrations than did men with the lowest exposure to BPA. And this fall, a study of Chinese factory workers also found that, on average, those with the highest exposure to BPA had the lowest sperm counts.

Swan’s studies have also shown that male sperm quality is related to where you live, with men in some rural, agricultural areas having overall reduced sperm quality compared to men living in urban settings. The men in her studies in agrarian communities also had higher concentrations of certain pesticide metabolites—and the higher the pesticide traces, the poorer the sperm quality.

Even men who assiduously avoid microwaving their food in take-out containers and avoid eating pesticide-sprayed fruit have reason to be concerned. A finding this fall in Nature opened up a bigger sphere of possible sperm-influence when scientists showed that male diet before baby-making could affect his offspring's health. Through chemical changes in the sperm, male mice who were fattened up with unhealthy foods prior to mating were more likely to have babies who had problems with glucose regulation and insulin resistance—hallmarks of type 2 diabetes.

For me, it calls up images of my single male friends who are breezing through their 30s, drinking, partying, eating what they choose. I've never once heard them talk about protecting their fertility or making health choices for their future offspring—a concern women who want babies are naturally expected to entertain.

If it's true that sperm are influenced by health habits and the environment, it changes the mating game. We pay exquisite attention to the inner workings of female reproductive health. Women are expected to prepare their bodies and pay attention to a litany of advice—acupuncture, special teas, yoga poses—when things aren't going as planned. But our stance toward men is decidedly simple: Unless there's a major hiccup and a savvy fertility specialist is called on, advice for dads-to-be tends to stop at don't soak in hot tubs or wear tight underwear.

When I talked to my husband about the data on male fertility, his first reaction was, "Wait, so we're losing that part, too?" He pointed out that through pregnancy, childbirth, and the newborn months, dads tend to feel less important and unsure of what their role is. But making the baby, that's territory they feel confident in. The idea of fertility slipping seemed to cut into some deeply held ideas about masculinity.

But as we talked, I realized he was actually a little excited by the notion that, on the flip side, men could play a bigger part in preparing to conceive. It reminded me that when we first visited our OB/GYN before we started trying to get pregnant, my husband kept asking him if there was anything he could do to help along the process of making a healthy baby. Should he eat certain things, or do certain exercises maybe? The doctor and I looked at each other and laughed—so sweet that he wanted to be part of it. Then we went back to focusing on me.

Maybe my husband was on to something after all.

Heather Turgeon is a psychotherapist and science writer. She authors the column The Science of Kids for Babble.

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