FRIDAY, June 1, 2012 (HealthDay News) -- 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.
The study also found evidence that Americans are maturing earlier. An opening in a certain bone structure of the skull used to close at about age 20, but is now fusing at age 14 in girls and age 16 in boys.
This could be due to rising rates of childhood obesity, which may affect hormones in ways that alter timing of growth and maturation, the researchers suggested.
Jantz said researchers have documented less dramatic shifts in skull shape in Europe.
The study was recently presented at the annual meeting of the American Association of Physical Anthropologists.
Data and conclusions presented at meetings should be considered preliminary until published in a peer-reviewed medical journal.
The American Medical Association provides a close-up look at the human skull.