“We’ve evolved these big brains partly to attract mates,” Kanazawa says. “And science is one part of what we do to attract mates.”

It sounds almost too pat — or too weird — to be true. And in fact, Kanazawa came to his unusual theory through a circuitous path. He was studying the behavior of male criminals, and musing on the well-documented fact that their criminal activity increases sharply in adolescence, peaks in the late teens and early 20s, and then drops. Evolutionary psychologists have believed for some time that this spike in criminal behavior was in fact a mate-attracting device, since crime provided men with the wealth and “success” to set them off from their peers. Kanazawa knew about the stereotype of the early-blooming scientist; could it follow the same pattern?

To test his notion out, he selected 280 scientists at random from the 1994 edition of Oxford University Press’s Biographical Dictionary of Scientists. (More than 97 percent of them were male.) He took the date of the main discovery for which the scientist was listed, surmising that it would represent the commonly accepted “peak” of his career. When Kanazawa crunched the data, he found that almost one-quarter of the scientists made their biggest discovery roughly between the ages of 27 and 32, and two-thirds had done so by their mid-30s. By their early 40s, a total of 80 percent had made their breakthrough.

What’s more, if a guy did make it big later in life — a rare event — he often wasn’t married. Half as many unmarried scientists made their major contribution in their late 50s as in their 20s. Married scientists, however, were only 4.25 percent as likely to hit it big in their 50s as in their 20s. For Kanazawa, it was proof that our evolutionary urges are governing our science. (He added pointedly that some studies show testosterone levels drop “precipitously” in new fathers.)

Kanazawa’s paper made headlines in several countries. Older scientists scrambled to declare that they were still alive, kicking, and, for that matter, innovating. Nigel Forteath, a 59-year-old marine biologist in Australia, noted in an interview with a local paper that he didn’t become a world leader in his field — seahorses — until quite later in life. “We’ve just bred half a million of these animals in captivity, having been told it was impossible,” he says. “This idea of needing to attract a mate is just crazy.”

Others pointed out that Kanazawa ignored much more mundane reasons for declining productivity. Saddled with administrative tasks, older tenured academics sometimes barely set foot in the lab, argues Spencer Weart, a historian at the American Institute of Physics in College Park, Md.

“They’re the ones who get dragooned into the committee, doing the work in Washington to get funds,” he says. “If they had a good idea, they’d be handing it off to their grad students and saying, ‘Here — go check this out!”’ (One might add that the tenure system itself, which requires that researchers make a significant discovery early but doesn’t penalize them for resting on their laurels later, may also be a factor.)

Critics of evolutionary psychology were perhaps the most withering of all. “This isn’t a theory based on data, it’s a political idea,” says Brown University biologist Anne Fausto-Sterling scornfully. “The really good biologists, the serious sociobiologists, don’t make these ridiculously broad claims anymore,” she adds.

More subtly, the idea of the young “prodigy scientist” may be a historic relic. Universities faculties in the 19th and early 20th centuries — heavily represented in Kanazawa’s study — were much younger than they are now, and this fact has continuing demographic implications today. As Weart points out, in the 100 years leading up to 1970, the number of scientists doubled every 10 years. So for every scientist who was in his 50s, there were two in their 40s, and four in their 30s. By sheer odds, you’d wind up with more younger stars than older ones.

Dean Keith Simonton, a professor of psychology at UC-Davis who studies age and achievement, was asked to review Kanazawa’s paper before publication and recommended that it not be published. For one thing, he explains, Kanazawa didn’t control for life expectancy: By conflating the achievements of those who died young and those who lived to a ripe old age, Kanazawa’s approach artificially lowers the average age at which achievement peaks.

In his own studies, Simonton has found results that are far more soothing to the late bloomer: On average, scientists produce their first breakthrough in their 20s, another one — their biggest — in their late 30s and early 40s, and one final one in their 50s.

Other theorists suggest that patterns of achievement may have less to do with age than with intellectual style. In his book “Painting Outside the Lines” (2002), the University of Chicago economist David Galenson studied the careers of 100 artists and concluded that their peak was determined by whether they were a “finder” or a “seeker” of their aesthetic. Finders — Picasso, Seurat — usually stumble upon their idea early in life, and thus are labeled prodigies. Seekers— Cezanne, Kandinsky — hunt their ideas down slowly.

In science, Galenson says, different disciplines showcase different styles. Math and physics are highly theoretical and reward “a-ha” thinking, so they attract finders; biology and archaeology are more empirical and reward a lifetime of field work, so they attract seekers — and thus produce older success stories.

Quite apart from the merits of Kanazawa’s theories, though, this debate is unlikely to go away. In our youth-addled culture, nearly everyone is terrified of losing their edge — which is probably why so many people thrilled, or panicked, at Kanazawa’s theory. “The bottom line is, people want to know when they’re over the hill,” Simonton notes. “And they’re asking the question, well, I haven’t done anything yet — but do I still have a chance?”

And Kanazawa himself? He just turned 40, and has a wedding ring on his finger. “Here’s hoping,” he says ruefully, “that I’m an exception to my own rule.”

Clive Thompson covers science, culture, and politics for The New York Times Magazine, Wired, Details, and other publications.