Whatever incredibly dumb things humanity got up to in the 20th century (and there were, as you know, some doozies), we all had at least one thing to crow about: as measured by IQ tests humans were at least steadily getting smarter.

The steady uptick in average IQ scores is known as the Flynn effect, and it lasted for decades. Basically wherever scientists looked they found a rise of intelligence of about three IQ points per decade.

But recent research has worrying news: this trend appears to be reversing.

Humanity is now officially getting dumber.

It probably shouldn't worry us if some pocket of the population saw a decline in IQ as things like education and diet affect IQ and these factors can vary from one group or time to another. But according to this new study it doesn't appear to be some small segment of the population whose IQ is going down. It appears to be the entire nation of Norway.

When scientists from the Norway's Ragnar Frisch Centre for Economic Research analyzed some 730,000 IQ tests given to Norwegian men before their compulsory military service from 1970 to 2009, they found that average IQ scores were actually sinking. And not just by some miniscule amount. Each generation of Norwegian men appear to be getting around seven IQ points dumber.

And as PsyBlog points out, this isn't even the first study to find that the Flynn Effect has reversed, though it may be the most convincing to date.

The million dollar question: why?

That's pretty horrifying news for fans of progress, but it also begs one incredibly important question: Why? What's causing IQ scores to start heading in the wrong direction?

You might first wonder if it's genetic. Maybe some change in the makeup of a particular group being studied has caused the decline (crudely, you could call this the 'dumb people have more babies' hypothesis). But that seems to be ruled out by the new research, which shows that even within single families IQ has declined. Marginal Revolution blogger economist Tyler Cowen sums up what that means: "In other words, we have started building a more stupidity-inducing environment."

So we know that the culprit is nurture rather than nature (or, sorry xenophobes, migration), but scientists are still baffled as to what exact aspect of modern life is driving the decline. Some have proposed that our tech obsession might be to blame, but as the decline started in the 1970s, well before everyone spent their days staring at screens, that can't be the whole story.

Other proposed explanations are unhealthy modern diets, increasingly trashy media, or a decline in the quality of schooling or the prevalence of reading.

The issue could even be down to a technical detail of IQ tests. Scientists make a distinction between crystallized intelligence (all the stuff you've been taught and remember) and fluid intelligence (your ability to learn new stuff). IQ tests generally measure crystallized intelligence more, so changes in schooling that de-emphasize memorization might be driving a decline in scores. If this explanation is true, students remain as smart as ever (just way more reliant on Google).

The bottom line, however, is that the cause of the decline remains a mystery. Whatever it turns out to be, however, we should all probably start worrying about what our sedentary, screen addicted, junk food-munching lifestyles might be doing to our brains.