The heads of white Americans have become larger, taller and narrower since the 19th century, a new study says.

The forensic anthropologists at the University of Tennessee, Knoxville, did not pinpoint a reason for the changing head shapes or know whether it is primarily due to evolution or lifestyle changes.

"The varieties of changes that have swept American life make determining an exact cause an endlessly complicated proposition," Lee Jantz, coordinator of UT's Forensic Anthropology Center, said in a university news release.

"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," Jantz said. "Which of these is paramount we do not know."

The team examined 1,500 skulls dating from the mid-1800s to the mid-1980s and found that the average height from the base to the top of the skull in men increased by eight millimeters (0.3 inches), while skull size grew by 200 cubic centimeters, a space equivalent to a tennis ball.

In women, skull height increased by seven millimeters and skull size increased by 180 cubic centimeters.

Overall, skull height has increased 6.8 percent since the late 1800s, compared with a 5.6 increase in body height and a 2 percent increase in femur (thigh bone) length. The researchers also noted that skull height has continued to change while the increase in body height has recently slowed or stopped.

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