NEW YORK (CNN/Money) - The recent PBS series "Frontier House" showed how a 19th-century pioneer lifestyle -- churning butter, running after farm animals, rationing food, etc. -- dramatically drained the fat from modern American families.

One participant, horrified by the sight of his pelvic bones, was told by a modern doctor that his "wasting away" was, in fact, fitness -- a state apparently unfamiliar to the man, the wealthy CEO of a private company.

A new study by economists at the National Bureau of Economic Research has rooted out the cause of our modern chubbiness -- technology.

"Technological change ... has raised the cost of physical activity by making household and market work more sedentary and has lowered the cost of calories by making agricultural production more efficient," RAND economist Darius Lakdawalla and University of Chicago economist Tomas Philipson wrote in a working paper published this week on the NBER Web site.

Close to half the U.S. population is estimated to be overweight, and more Americans are obese than smoke, use illegal drugs or suffer from ailments unrelated to obesity. Darius Lakdawalla and Tomas Philipson, Economists

In the old days -- and in developing countries where the economy is still based largely on agriculture -- people were actually paid to exercise; the work you did to support yourself involved far more strenuous activities than pointing and clicking.

And exercise has become far more costly -- anybody who's tried to get a gym membership recently knows how pricey that can be, and going outside and running for a mile or two is far less appealing to many people than hunkering down in an air-conditioned living room with a nice game of Grand Theft Auto III.

At the same time, inefficiencies in food production made food more expensive in the old days, making it more difficult or costly for people to eat. The researchers pointed out that the number of calories available in the world's food supply has increased only slightly in the past century, while the price of food has steadily declined since the end of World War II.

As a result, the Body Mass Index -- a measure of weight based on height -- for Americans in their 40s rose to 27 in the year 1991 from 23 in 1863. The researchers pointed out that such a gain is about the same as a six-foot-tall man gaining 30 pounds. The National Institutes of Health says people having BMIs between 25 and 29.9 are "overweight."

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"Close to half the U.S. population is estimated to be overweight, and more Americans are obese than smoke, use illegal drugs or suffer from ailments unrelated to obesity," the researchers pointed out.

Far from being just a matter of cosmetic pride, obesity is a serious economic issue, Lakdawalla and Philipson noted, since it is a high-risk factor for many of the nastiest ailments, including heart disease, cancer and diabetes. Failing health, in turn, has an impact on Medicare, Medicaid and Social Security benefits.

On the bright side, it's a boon to the private health care sector, which makes up one-sixth of the U.S. economy.