Why are American heads getting bigger?

Americans shouldn't let this go to their heads — but recent forensic research suggests that their heads are getting bigger. National Geographic recently reported the discovery that skulls of white Americans have significantly increased in size since 1825, allowing for a tennis ball's worth of new brain. But why? Scientists are stumped.

To make this discovery, University of Tennessee anthropologist Richard Jantz had to measure hundreds of skulls of white Americans born between 1825 and 1985. His findings suggest that the typical American head has grown by about a third of an inch (8 mm) over the course of that time - a paltry 160 years. It might not seem like much, but the added space can accommodate a significant amount of new brain.

It's very likely that this effect is not exclusive to Americans and that the trend extends out to other races and ethnicities.

Now, before you get all excited and jump to the conclusion that Americans are getting smarter, it would be wise to do some science.

According to Jantz, research shows a bigger cranium doesn't necessarily mean more intellect. Indeed, while the discovery might lead some to speculate that Americans are getting smarter, there is no correlation between brain size and intelligence. What matters to cognitive function is how the brain is organized. The issue brings to mind the pseudoscientific field of craniometry, as exposed by Stephen Jay Gould in his now classic, The Mismeasure of Man.

What's particularly interesting about Jantz's work, however, is that it overturns a much broader evolutionary trend toward smaller heads. As National Geographic reports:

Beginning with the dawn of the first Homo species, human skulls evolved to be increasingly bigger until about 30,000 years ago, when head size plateaued.

And about 5,000 or 6,000 years ago, when agriculture took off in earnest, skulls began shrinking. The cause of the shrinkage is a mystery, but scientists have tentatively fingered more efficient brain wiring and easier access to food and safety — the idea being that people no longer had to be especially smart to survive (aka the Idiocracy theory).

Jantz's research indicates that this trend is now reversing. As for an explanation, he contends that there are likely multiple factors at play:

Jantz cautioned that American life has changed in too many ways to pinpoint a single cause for the skull growth.

"I am absolutely certain, however, that it's due to the unparalleled environment that we now live in," he said.

"Americans drive cars, vaccinate their children-and an excess of food is now a bigger problem than undernutrition, among many other things. It's almost as if we're conducting an experiment on ourselves to see how we'll respond to a totally new environment."

Other scientists, such as Texas demographer Corey Sparks, argues that we shouldn't jump to conclusions and make wild guesses. Moreover, he doesn't feel that the small increase in head size is significant.

It's clear that further research into this issue will be required before any definitive conclusion can be made. For now, Americans can simply enjoy knowing they have big heads.

Image via Shutterstock / Ilya Andriyanov.