Lane Kenworthy, The Good Society
March 2021

Obesity is one of America’s chief social problems. People with obesity tend to earn less and are more likely to be depressed. They are at greater risk of diabetes, heart disease, stroke, and some types of cancer, and they tend to die younger. The monetary cost of obesity is estimated to be 1% to 5% of our GDP.¹

The incidence of obesity has risen dramatically in the past generation. Today, 42% of American adults have obesity. Why has this happened? What can we do to reverse the trend?

Skip to:

• Obesity is part of a continuum
• America’s obesity rate is high
• Is less exercise the culprit?
• Is reduced smoking to blame?
• The cause is increased eating
• The food supply hypothesis
• The income inequality hypothesis
• The economic insecurity hypothesis
• What should we do?

OBESITY IS PART OF A CONTINUUM

Obesity is defined as having a body mass index (BMI) of 30 or more.² For a person 6 feet tall, that means a weight of more than 220 pounds. For someone 5’6″, the threshold is 185 pounds.

Reference to “obesity” or “the obese” inclines us to think of persons with obesity as a distinct group, different from others not just in degree but in kind. Actually, obesity is part of a continuum. Figure 1, which shows the distribution of BMI among American adults, makes this clear.

AMERICA’S OBESITY RATE IS HIGH

Figure 2 shows the trend in obesity among American adults since 1960, the first year for which we have good data. After holding constant at about 15% in the 1960s and 1970s, the obesity rate shot up beginning in the 1980s, reaching 42% in 2018.

Obesity is more common in the United States than in any other rich nation, as shown in figure 3.

Figure 4 shows the over-time patterns in these countries. The increase was most rapid in the United States, though nearly all of these nations have experienced a large rise.

What caused the surge in obesity in America?

IS LESS EXERCISE THE CULPRIT?

One prominent notion is that Americans are heavier because we get less exercise than we used to. But this is probably wrong. While it’s true that we’re less active now than we were half a century ago, the timing of the decline in activity doesn’t match up with the rise in obesity.

We don’t have good historical data for a comprehensive measure of activity, such as calories expended, so we have to look instead at individual components. Begin with the most-often-cited culprit: television. Figure 5 shows the trend in average time spent watching TV since 1950. The amount of time has increased, but the increase has been steady. There was no sudden rise that would account for the jump in obesity beginning in the 1980s.

What about video games, the internet, and smartphones? Video games became popular in the 1980s, but mostly among the young, while obesity rates rose among adults of all ages, even the elderly.³ The internet and smartphones arrived on the scene too late to account for the rise in obesity in the 1980s and most of the 1990s.

More Americans now have sedentary jobs and drive to work. Yet as David Cutler, Edward Glaeser, and Jesse Shapiro point out, these shifts have been going on for a long time, with no acceleration in the 1980s.⁴

“Between 1910 and 1970, the share of people employed in jobs that are highly active like farm workers and laborers fell from 68 to 49 percent. Since then, the change has been more modest. Between 1980 and 1990, the share of the population in highly active jobs declined by a mere 3 percentage points, from 45 to 42 percent. Occupation changes are not a major cause of the recent increase in obesity.

“Changes in transportation to work are another possible source of reduced energy expenditure — driving a car instead of walking or using public transportation. Over the longer time period, cars have replaced walking and public transportation as a means of commuting. But this change had largely run its course by 1980. In 1980, 84 percent of people drove to work, 6 percent walked, and 6 percent used public transportation. In 2000, 87 percent drove to work, 3 percent walked, and 5 percent used public transportation. Changes of this minor magnitude are much too small to explain the trend in obesity.”

Another reason to doubt the importance of declining physical activity is that the elderly probably have become more active over time, not less, and yet we observe a rise in obesity among the elderly too, similar in timing and magnitude to that of younger adults.⁵

In short, the evidence suggests that reduced physical activity hasn’t been a key cause of the surge in obesity in America.⁶ This doesn’t mean physical activity plays no role in determining which persons have obesity. And it doesn’t mean an increase in activity can’t help to reduce obesity’s prevalence. But it does suggest that a strategy focused on increasing activity may not get us very far.

IS REDUCED SMOKING TO BLAME?

Obesity is less common among people who smoke regularly, and Americans have been smoking less over time. Is this why obesity has increased? Probably not. As figure 6 shows, smoking has decreased steadily since the mid-1960s, which is long before obesity shot up.

THE CAUSE IS INCREASED EATING

The most likely cause of rising obesity is that Americans began consuming more calories.⁷ Figure 7 shows the trend in eating, measured as average calories in the food supply (adjusted for loss and spoilage) according to data from the Department of Agriculture. These data are available beginning in 1970. The timing of change matches that for obesity; the level is flat in the 1970s and then rises sharply beginning in the 1980s. A data series that goes back to 1950, measuring energy consumption per capita, also shows little or no change up to 1980 and then a sharp jump.⁸

Some analysts have suggested that the rise in obesity is caused not by increased calorie consumption per se, but specifically by a rise in consumption of sugar — or of sugary foods, such as soft drinks.⁹ While the jury is still out on this hypothesis, the over-time pattern correlates only loosely with the trend in obesity. Figure 8 shows that Americans’ consumption of soft drinks began rising sharply in the 1960s, long before the increase in obesity. Figure 9 shows that overall consumption of sugar and other sweeteners (such as high-fructose corn syrup) also started its rise in the 1960s.

In addition, consumption of sugar and other sweeteners has decreased significantly since 2000, yet we see no indication of a reversal in obesity’s rise (figure 2 above).

The trend since 2000 is a question mark for the food consumption explanation, too. As we see in figure 7, the rise in average calories appears to have stopped around 2000. These data aren’t available for years after 2010, but it’s possible that we’ve gone two decades with no additional rise in the amount of calories Americans consume on average. Other sources of information, such as food diaries and family logs of food purchases, suggest that there has indeed been a flattening of food consumption.¹⁰ It’s therefore puzzling that the obesity rise hasn’t stopped or even slowed.

One possibility is that an increase in the share of calories from ultra-processed foods — which includes soft drinks but also fruit drinks, salty snacks, store-bought breads, breakfast cereals, frozen meals, reconstituted meats, cookies and cakes, and more — is behind the continued rise in obesity. By the early 2010s, ultra-processed foods accounted for approximately 60% of all calories consumed by Americans. We don’t have good long-term data on such consumption, but it appears to have risen during at least part of the period since 2000.¹¹ In this story, the obesity rise from 1980 to 2000 owes to an increase in the number of calories consumed and the rise in obesity since 2000 owes to a shift toward consumption of more ultra-processed foods.

THE FOOD SUPPLY HYPOTHESIS

What caused the rise in eating beginning in the 1980s? The most compelling story points to changes in the supply of food. Tasty, high-calorie food became cheaper and more easily accessible in larger quantities, so we began eating more of it.

There is no simple measure that can capture this change. One partial measure, the number of restaurants per capita, correlates strongly with the prevalence of obesity over time.¹²

In this account, the comparatively rapid increase in obesity in the United States owes to its weak regulation of food and restaurants and to its lack of a well-entrenched healthy eating culture. Large-portion restaurants, particularly fast-food ones, proliferated more rapidly in the US. Junk food became available in grocery and convenience stores sooner and in larger quantities. And the shift away from home cooking and limited snacking occurred more quickly and decisively.¹³

THE INCOME INEQUALITY HYPOTHESIS

In recent years some researchers have advanced an alternative hypothesis that blames rising income inequality. Inequality is said to increase status competition, which increases stress, which in turn prompts overeating.¹⁴

In the United States, income inequality has risen sharply since the late 1970s, as has obesity.¹⁵ For the timing to work, though, we need to assume little or no lag. If it takes a long while for inequality to have an impact on obesity, as we might expect, then it can’t have caused the 1980s obesity increase.

What about the trend in stress? None of the research I’ve seen looks into this, but some cite a paper published in 2000 that finds evidence of a rise in anxiety in America.¹⁶ Yet that study concluded that anxiety rose steadily from the 1950s through the early 1990s, which doesn’t match up well at all with the over-time trends in income inequality or obesity.

One piece of evidence cited in support of the income inequality hypothesis is the pattern across affluent countries.¹⁷ Figure 10 shows the cross-country association between obesity and income inequality. The solid line indicates a positive association, suggesting that nations with higher income inequality tend to have higher obesity rates.

However, the positive association is driven by the position of the United States. If we exclude the US, the association disappears entirely; there is no relationship between income inequality and obesity across the other 20 countries (dashed line). So if obesity is exceptionally high in the US due to something other than income inequality — to the supply of cheap high-calorie food, for example — then there is nothing left in support of the inequality hypothesis.

Since countries differ in a number of ways that might affect obesity — from food preferences to education to affluence to physical activity — our best bet analytically is to compare changes over time.¹⁸ Figure 11 shows change in the obesity rate by change in income inequality. The pattern summarized by the solid regression line again suggests support for the notion that income inequality increases obesity, but the dashed line, which excludes the United States, again suggests no association.

Some researchers have found a positive association between income inequality and obesity across the US states.¹⁹ (Obesity data for the US states are from self-reports, so we should be wary. But we can hope the degree of bias is similar in each state.) Figure 12 shows the cross-sectional pattern as of the late 2000s. Here we see no indication of the hypothesized positive association.²⁰

Figure 13 shows change in obesity by change in income inequality across the states. (The change period ends in 2009 because more recent obesity data aren’t comparable due to a change in measurement.) If income inequality is an important determinant of obesity, we would expect states in which income inequality has increased the most to have experienced the largest rise in obesity. But they haven’t.²¹

The bottom line on the inequality hypothesis: It’s plausible. And at first glance the over-time pattern in the US and the cross-country pattern seem supportive. But when we look deeper, the case for income inequality as a key determinant of obesity is weak.

THE ECONOMIC INSECURITY HYPOTHESIS

A related hypothesis suggests that the problem is rising economic insecurity. Here too the mechanism is said to be stress.²²

In the United States, economic insecurity has increased since the 1970s, which is consistent with the timing of obesity’s rise. I’m not sure, however, that the increase in economic insecurity has been large enough to have produced the massive jump in obesity that occurred.²³

The United States may well have more economic insecurity than other rich nations, and it could be that insecurity has increased more in the US than elsewhere. If so, the insecurity hypothesis might be helpful in accounting for the difference in obesity between the US and other countries. Unfortunately, we don’t have an especially valid and reliable measure of economic insecurity for cross-country analysis.²⁴ That’s true also for the US states.

Another piece of evidence seemingly consistent with the economic insecurity hypothesis is the social gradient in obesity in the United States — the fact that obesity rates have tended to be highest among the poor, those who are least secure economically. Then again, there are reasons other than stress why poor people might be more vulnerable to obesity, such as less education, less ability to afford healthy food, and less access to such food. Here too, then, it’s helpful to examine changes over time rather than just levels.

The mid-1990s welfare reform gives us something of a natural experiment. That reform placed strict time limits on receipt of cash assistance for women with low income. If there is any change in recent decades that ought to have heightened stress among low-income women, welfare reform is it. According to the economic insecurity hypothesis, obesity rates in the period after the mid-1990s should have risen more rapidly among low-income women than among other women. But that isn’t what happened, as figure 14 shows. Obesity rates increased at roughly the same pace among middle-income and high-income women as among those with low income.

Another piece of evidence comes from Canada. A recent study examined the association between economic insecurity, measured as a reduction in household income of 25% or more from one year to the next, and obesity from 1998 to 2008. The data suggest a positive but weak association between insecurity and obesity for working-age males and no association for females. During this period the obesity rate in Canada increased even though economic insecurity decreased.²⁵

Like income inequality, then, economic insecurity is a plausible candidate as a cause of changes in obesity, but at the moment the supportive evidence is thin.

WHAT SHOULD WE DO?

Obesity is a significant social and economic problem, and its incidence has risen sharply in the United States in the past generation. How could we reverse that trend?

The key is to get Americans to eat fewer calories.²⁶

One strategy is dissemination of information. Americans have been subjected to a sea of misinformation in recent decades, not just from advertising but also from nutritionists. We’ve been pushed from one fad to the next, from “eat less fat” to “eat fewer carbohydrates” to “eat more of this or that nutrient.” But evidence increasingly favors a simple approach summed up by Michael Pollan as “Eat food [instead of processed substances]. Mostly plants. Not too much.”²⁷

For an information-based strategy to work, people may want to be able to estimate how many calories they are eating. Food labeling already is mandatory for packaged foods sold at grocery and convenience stores, and since 2016 chain restaurants have been required to post calorie and nutrition information on their menus. So far, however, studies have tended to find that menu labeling has little or no impact on eating behavior.²⁸

But maybe counting calories isn’t necessary. A recent study found that people instructed to eat mainly whole foods — vegetables, fruits, grains, lean meats — and avoid processed and high-sugar foods tended to consume fewer calories without having to keep track of how much they ate.²⁹ Junk food is calorie-dense, so a typical person won’t feel full from junk food unless they’re eating more than a weight-maintaining number of calories. For instance, the following sparse diet totals 2,000 calories, which is the weight-maintaining number for a moderately-active woman in her thirties or forties: a muffin (450) for breakfast, a cheeseburger and medium french fries (650) along with a 16-ounce soda (180) for lunch, two slices of pizza (550) and a 16-ounce soda (180) for dinner. Few people who consume mostly junk food will eat that little during a day, so they will end up consuming more than a weight-maintaining number of calories.³⁰

A second approach is to make vegetables, fruits, legumes, and whole grains more available and attractive. We could, for instance, shift our government food subsidies away from meat, dairy, and corn in favor of healthier foods. Another proposal is to reduce the number of communities that lack nearby grocery stores with fresh vegetables and fruits (“food deserts”), though one recent study suggests this has no impact on actual food consumption by low-income persons.³¹

A third approach holds that to make a real dent in the prevalence of obesity we must directly discourage consumption of unhealthy foods. There are two main options here. One is to tax these foods. Some research suggests that such a tax would need to be in the neighborhood of 20% or more to have an impact on eating behavior.³² We could conceivably go farther and impose a tax similar to that for cigarettes, which is 50% to 100% in most states.

The other option is an outright ban. The best-known proposal in this vein was Mayor Michael Bloomberg’s bid to ban the sale of sugared drinks in containers of 32 ounces or more in New York City. Others recommend limiting or banning fast-food restaurants in particular areas. In a country with a strong tradition and culture of individual liberty, this approach is, not surprisingly, quite controversial.³³