Seth Mnookin says scientists believe that broken-down old sperm is making us stupider:

For sociological and environmental reasons, men are living longer and having children when they’re older. That, combined with the fact that we’re living in an era of diminished pressure from natural selection, means that Stefansson and his colleagues may have identified the single biggest factor in the ongoing development of the human genome: new mutations caused by old sperm. Mutations can be beneficial, but they are much more likely to be harmful—which means the changes will be overwhelmingly negative.

“This is really scary,” Alexey Kondrashov, an evolutionary biologist at the University of Michigan, said last Thursday from Moscow. In a Nature commentary that accompanied the Decode study, Kondrashov raised the spectre of an inevitable “decline in the mean fitness of the population.” As it happens, it’s likely that this is already occurring. If you combine the findings of a 2009 PLOS ONEpaper that examined the link between advanced paternal age and a decline in social and exploratory behaviors with Decode’s results, you get a scenario that is as alarming as it is plausible.

“Diagnoses of autism and schizophrenia is one thing, but [older fathers may have] a perfectly normal [child] in the sense that there may not be a diagnosis, but his IQ is 108 instead of 110,” Kondrashov says, noting that this hypothesis tracks with results from his own research on fruit flies. “This means this is a problem that will lead to very severe consequences for society over several generations.” Over the past several decades, these changes were likely masked by improvements in the environment—“getting rid of lead paint, fewer infections, less malnutrition, whatever”—but moving forward, Kondrashov says that he “strongly suspect our gene pool gets worse.”

See, even our scientists are losing their ability to conjugate verbs properly.

Mnookin suggests that we might see governments proposing that teenage boys save their strapping young non-degenerate sperm so that they can one day conceive Master Race babies in a test tube with their hot wife’s eggs. Mnookin doesn’t quite put it that way, but that’s what he means.