The heads of white Americans are getting bigger, say forensic anthropologists at the University of Tennessee.
They've become taller and narrower as seen from the front, and faces have become significantly narrower and higher. And this isn't simply because people have got bigger overall, as skulls appear to have grown disproportionately.
The researchers don't know for certain why this is.
"The varieties of changes that have swept American life make determining an exact cause an endlessly complicated proposition," says Lee Jantz, coordinator of UT's Forensic Anthropology Center (FAC).
"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."
The team examined 1,500 skulls dating from the mid-1800s through the mid-1980s. They found that the average height from the base to the top of the skull in men has increased by eight millimeters, or about a third of an inch. Skull size has grown by 200 cubic centimeters, about the volume of a tennis ball.
In women, the corresponding increases are seven millimeters and 180 cubic centimeters.
And skull shape has changed too. Skull height has increased 6.8 percent since the late 1800s, while body height has increased by just 5.6 percent and femur length by two percent. And while the overall increase in height has recently slowed or stopped, skull-height has continued to change.
The scientists also spotted changes that show white Americans are maturing sooner. A separation in the bone structure of the skull called the spheno-occipital synchondrosis, which in the past was thought to fuse at about age twenty, is now fusing much earlier - 14 for girls and 16 for boys.
One possible reason is America's obesity epidemic.
"This might affect skull shape by changing the hormonal environment, which in turn could affect timing of growth and maturation," says professor emeritus Richard Jantz.
"We know it has an effect on the long bones by increasing muscle attachment areas, increasing arthritis at certain joints, especially the knee, and increasing the weight-bearing capacity."
The team stuck to Americans of European ancestry because they gave the largest sample sizes to work with. Shifts in skull shape have also been found in Europe - although the effect there is much less dramatic.