TweetaturalNews) The size of the average human skull is gradually increasing over time, say researchers, and nobody really knows why. Researchers from the University of Tennessee (UT) in Knoxville recently released a new study showing that over the past roughly 150-or-so years, the average size of the human skull in both males and females has become progressively larger, taller and narrower.

Based on an examination of 1,500 different human skulls dating from the mid-1800s to the mid-1980s, Lee Jantz, coordinator of UT's Forensic Anthropology Center, and his colleagues found that the average male skull has increased in height by roughly eight millimeters, or 0.3 inches, over the past century. Overall skull size in men has also increased in volume by about 200 cubic centimeters, or roughly a space the size of a tennis ball.

Women's skulls have also been increasing in size, with average height having grown by about seven millimeters, or 0.28 inches. And overall skull size in women has also expanded by about 180 cubic centimeters, just slightly lower than the overall rate for men. Both growth rates appear to be limited to men and women of Caucasian descent, at least as far as this particular study was concerned.

"The varieties of changes that have swept American life make determining an exact cause an endlessly complicated proposition," said Jantz in a university-prepared 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."

Another hypothesis for the skull size increase is that Americans today are eating more calories than ever before, and are not exercising or engaging in various physical activities nearly as much their ancestors did. This excess calorie consumption can contribute to larger skull and brain size, say researchers, which is not necessarily a bad thing.

"As we continue to have excess calories then I am guessing that the brain can continue to increase in size," added Richard Jantz, husband of Lee Jantz and coauthor of the new study. "There obviously has to be some upper limit."

The team also noted that Americans are maturing physical much faster than their ancestors. At one time, young girls had their first period around the age of 16 or 17, while today girls 12 or 13 years of age, or even younger, are reaching puberty. This rapid maturity is likely being caused by various food and environmental chemicals, as well as growth hormones added to processed foods

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