If you're American with an average-sized head, your noggin is likely bigger than your ancestor's was seven generations ago.

That's the implication of research by Richard Jantz and colleagues at the Forensic Anthropology Center at the University of Tennessee.

The scientists analyzed about 1,500 skulls, dated from various years in the 1800s and 1900s. The researchers focused on Caucasian individuals because there weren't as many skulls from blacks and Hispanics. Although their conclusions are about white Americans, there is no reason to believe these patterns don't pertain to other races, Jantz said. More research just needs to be done.



Over the last 150 years or so, it appears that skulls got narrower from side to side by about 5 to 7 millimeters, and higher from top to bottom by an average of nearly 10 millimeters, Jantz said. And the overall size of the head has, on average, increased by an amount equivalent to the size of a tennis ball, he said.

Jantz speculates that there are several factors in the American environment that help contribute to this trend:

For one, there is far more nutrition than ever before in the United States. The problem of obesity - of over-nutrition - is more of a concern than undernourishment. People's work has become less physically-intensive, and even manual labor is often assisted by machines in some way.

Because of cars, people walk less. Because of vaccines, epidemic diseases that once killed many children no longer do.

Mothers who might not have survived childbirth two centuries ago are now living, thanks to medicines and procedures such as Caesarean sections - their children are surviving, too. And people don't often choose mates with the very same ethnicity – particularly not from the same part of Europe as their own ancestors – as they would have before.

"That has resulted in opening up the gene pool," Jantz said.

Brain size is probably correlated with skull size, so this means Americans' brains may also be getting bigger, but this has yet to be proven. The next step for researchers would be to see if certain parts of the brain have expanded, which might give an indication of whether there are particular abilities that have been enhanced as a result of this change over time.

The researchers did not directly measure the circumference of the head, so it's hard to say whether this trend has had any effect on hat size.

Is the increase in skull size part of a larger pattern of the human body growing in general? It's true that people are on average taller today than in the 1800s, but height has plateaued. What about our heads?

"I think the skulls will continue to change, and I think the direction depends on the direction the environment goes," Jantz said. "Obviously there have to be limits on how narrow or on how tall it could get, so I don't know if I want to predict how much longer this trend will continue."

The researchers hope to get this research published. They presented their findings at the American Association of Physical Anthropologists meeting in April.