Body weight is itself influenced by economic variables, often in complex ways. For much of human history, poorer people were thinner than richer people. Even today, inhabitants of rich countries tend to be heavier than those of poor countries. But within countries, wealthier people often weigh less than others. What causes these differences?

The economic determinants of weight are investigated in ''The Growth of Obesity and Technological Change,'' a recent paper by Darius Lakdawalla and Tomas Philipson (National Bureau of Economic Research Working Paper No. 8,946).

Mr. Lakdawalla and Mr. Philipson argue that there are two distinct economic drivers of body weight: a supply-side effect caused by higher incomes and lower food prices, and a demand-side effect caused by the increasingly sedentary nature of work.

Rich countries tend to have relatively low food prices and more sedentary jobs than do poor countries, making them fatter on average. But within rich countries, everyone faces the same food prices and similar technology, so differences in weight are influenced by other factors, like education and income.

Doctors perennially tell us to eat less and exercise more. But economic factors are working against their advice: food keeps becoming cheaper and work keeps becoming physically easier. It's not surprising that we keep gaining weight, despite our doctors' advice.

There has certainly been a sharp increase in weight over the last century, equivalent to a 30-pound gain for a 6-foot man. Remarkably, this change occurred while the calories available from the food supply declined over much of this period. Only in the last 10 years have the calories per person per day climbed significantly over the level available in 1909.

This suggests that, in aggregate, most of the increase results from too little exercise, not too many calories. Mr. Lakdawalla and Mr. Philipson set out to try to determine just what fraction of the weight gain can be attributed to each factor.

They start with a data set that tracks individual reports on occupation and weight. The Bureau of Labor Statistics rates occupations by how physically strenuous they are, so this allows determining the average amount of exercise associated with each person.

Mr. Lakdawalla and Mr. Philipson were rightly suspicious of the self-reported weight figures in their sample and found another data set with both self-reported and measured weight, which allowed them to estimate actual weight from the self-reported figures. One interesting finding: virtually all women tend to underreport their weight, while only heavier men do; lighter men tend to overreport it.

The next step was to determine how food prices and income affect weight. Income was relatively easy, since it was reported in the original survey. Prices were harder, but the authors solved this with a clever trick. About half the states levy a sales tax on food, while half do not. Hence the relative price of food varies from state to state, and this variation is enough to estimate the impact of price changes on body weight.

Mr. Lakdawalla and Mr. Philipson report a number of findings. More schooling tends to reduce weight, but by a relatively modest amount. Being black or Hispanic tends to increase weight, by a quite significant amount. Being in a sedentary occupation for a year leads to a small increase in weight; but staying in that occupation for 14 years causes a significant increase.

These are all statements about the partial effect of each economic variable, holding the other variables fixed. An increase in schooling tends to increase income, so weight might go up on both accounts. But economists are generally interested in the partial effect of an increase in schooling, holding income fixed. Determining such partial effects requires a model of the behavioral forces that jointly determine the relevant variables.

After a detailed statistical analysis of the data, the authors conclude that about 40 percent of the growth in weight in the last 20 years has been caused by the increased supply of food and 60 percent by the increase in sedentary employment.

As the authors put it: people used to be paid to exercise, since most work involved heavy physical labor. Now we have to pay to exercise, by taking time away from sitting in front of computers and TV's.

Technological change, the main culprit in the fattening of America, may yet save us. Research in weight-control drugs, food and technology is booming, so perhaps scientists will find a way that we can have our cake and eat it, too (a particularly apt metaphor in this context).

Mr. Lakdawalla and Mr. Philipson point out that while their study focuses on the supply of calories, the quality of calories matters as well. The National Health and Nutrition Examination Survey reports that the portion of calories generated by fat consumption actually fell in the 1980's. But the increase in total calories consumed in that period pushed up the total fat intake. Examining the changing composition of calories is a promising avenue for future research.

Let me suggest that it would also be of considerable interest to extend this study to other cultures. My unscientific observations suggest that Europeans -- particularly the French -- consume a higher proportion of fat in their diets, but they also eat smaller portions and walk much more.

I wonder if the National Bureau of Economic Research would sponsor a trip to Paris to observe at close hand the eating practices of the French. It would be necessary, of course, to spend many hours sitting at cafes observing their daily habits. Yes, this is tough work, but I suspect that some economist would be willing to take up the challenge.