American heads are getting larger although it's unclear why that is happening, forensic anthropologists at the University of Tennessee, Knoxville, said.

Examining 1,500 skulls dating back to the mid-1800s through the mid-1980s, the researchers found U.S. skulls have become larger, taller and narrower as seen from the front and faces have become significantly narrower and higher, a UT release reported Wednesday.

The researchers said it cannot be determined whether the changes are 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," researcher Lee Jantz said.

"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."

Skull height has increased 6.8 percent since the late 1800s, while body height has increased 5.6 percent and femur length has only increased about 2 percent, the anthropologists said.

The research only assessed Americans of European ancestry because they provided the largest sample sizes to work with, researcher Richard Jantz said.

Changes in skeletal structure are taking place in many parts of the world, he said, but tend to be less studied although research has uncovered shifts in skull shape in Europe.

Still, he said, those changes are not as dramatic as those seen in the United States. (UPI)