Did the 20th century make us big-headed? Maybe so, since forensic anthropologists at the University of Tennessee in Knoxville found that white Americans' heads are getting bigger and bigger...in size, that is.

The researchers studied about 1,500 skulls that dated from the mid-1800s through the 1980s. They noticed that the skulls gradually became larger, taller, and narrower. As a result, faces have become longer.

"The surprising thing is the skull size increase has not been documented in modern Americans," researcher Dr. Richard Jantz told The Huffington Post. "We might have suspected that that was happening but this documents it ... The shape of the skull has also changed rather dramatically. In fact, shape change has been more dramatic than size change.”

Specifically, the researchers found skull size in white men has grown by 200 cubic centimeters, which is about the volume of a tennis ball. Skull height, from the base to the top of men’s heads, has increased by 8 millimeters--so about 0.3 inches. Among white women, skull size has grown by 180 cubic centimeters and height has increased by 7 millimeters.

Overall, skull height has grown 6.8 percent since the late 1800s. That's a bigger percentage than the growth of body height, which has increased 5.6 percent. And comparatively, femur length has only increased about 2 percent. The researchers said that skull shape in Europe has also changed, but not as drastically as in the U.S.

The research team now is trying to pinpoint a cause for this growth.

“The varieties of changes that have swept American life make determining an exact cause an endlessly complicated proposition,” researcher Dr. Lee Meadows-Jantz of the University of Tennessee’s Forensic Anthropology Center said in a written statement. “It likely results from modified growth patterns because of better nutrition, lower infant and maternal mortality, less physical work, and a breakdown of former ethnic barriers to marriage. Which of these is paramount we do not know.”

In a nutshell, lifestyle changes or evolution could be why some Americans are getting big heads, so to speak. The researchers also didn’t rule out obesity.

“Increased calories, nutritional adequacy and decreased activity have probably been important factors,” Dr. Richard Jantz told The Huffington Post. "We know that changes how the body works."

The researchers pointed out that the U.S. population is maturing sooner. They said the bone structure of the skull is fusing much earlier for girls at age 14 and boys at age 16. In the past, the bone was thought to fuse around age 20. Richard told World Science he has no rea­son to be­lieve these changes in the skull have stopped.

The Jantzes said they only studied white Americans because they provided the largest sample size. Their research was presented April 14, 2012 at the annual meeting of the American Association of Physical Anthropologists in Portland.