WASHINGTON — Almost 70 percent of Americans have been overweight or obese in recent years, and more than 78 million people in the country have been counted as obese.

The problem has many sources, but one of them is obvious: increased portion sizes. We have a lot of evidence that people will eat whatever is put in front of them, even if they aren't hungry. As portion sizes expand, waistlines expand as well.

According to the Centers for Disease Control and Prevention, the average U.S. restaurant meal is more than five times larger than it was in the 1950s. The average hamburger, once less than 4 ounces, is now more than 12 ounces. The average order of French fries, once less than 3 ounces, is now more than 6 ounces. There is a clear correlation between increases in portion sizes and increases in obesity.

That correlation helps explain why obesity has been more prevalent in the United States than in France. The French eat high-calorie food, but their portion sizes are smaller. In supermarkets and restaurants, and in portion sizes recommended in cookbooks, Americans are given significantly bigger servings. Even at McDonald's, where we might expect identical sizes, servings of soda and French fries have been found to be larger in Philadelphia than in Paris.

Brian Wansink, a Cornell University professor of consumer behavior, helps to explain why portion sizes have such a large effect. He finds that much of our eating is mindless or automatic in that we tend to eat whatever is in front of us. If you are given a half-pound bag of M&M's, chances are that you will eat about half as much as you will if you are given a one- pound bag. People who receive large bowls of ice cream eat a lot more than those who get small bowls.

In one of Wansink's fiendish experiments, people were provided with a large bowl of Campbell's tomato soup and told to eat as much as they liked. Unbeknownst to them, the soup bowls were engineered to refill themselves (with empty bottoms connected to machinery beneath the table). No matter how much soup the subjects ate, the bowl never emptied. The result? Soup consumption skyrocketed. Many people just kept eating until the experiment was ended.

The good news is that once we isolate the sources of excessive eating, we will be able to identify potential solutions. Google found that its New York cafeteria, which offered a lot of high-calorie items, was producing a lot of unwanted pounds. In response to employee complaints, it initiated changes to nudge people toward healthier choices. Large plates and takeout containers were exchanged for smaller sizes, and employees were encouraged to eat less with a sign stating, "People who take big plates tend to eat more."

The redesigned cafeteria took a number of smart steps to make healthy choices simpler and more convenient (and to make less healthy choices less so). As a result, it helped to produce big reductions in both calories and fat consumed from candy.

A striking feature of the Google initiative was that employees were grateful for the nudges. There is reason to think that many consumers would respond the same way. In a series of studies, researchers told fast-food servers to ask customers whether they wanted to "downsize" their high-calorie side dishes. A substantial number (from 14 percent to 33 percent of those served) consistently agreed to do so. Strikingly, they accepted the offer whether or not they were offered a nominal 25 cent discount. Their total calorie consumption was reduced, on average, by more than 200.

Evidence is increasing that lower-calorie servings can be good for business. One reason is consumer demand. Many customers like, and reward, restaurants that provide light options; an easy way to provide such options is to cut portion sizes. Another reason is the increasing practice, often undertaken voluntarily and eventually to be required by the Affordable Care Act, of posting calorie counts on menus. Customers can be surprised to see just how many calories come from the standard portions of their favorite meals. They may not want to switch to a meal they enjoy less, but a smaller portion may suit them just fine. (Parents and dieters, please take note.)

The broader lesson is that obesity levels, in the U.S. and elsewhere, are hardly inevitable. They are a product of the social context in which people's choices are made. With careful attention to the subtle social cues that lead to excessive eating, we should be able to make a real dent in a serious public health problem.

Cass R. Sunstein, the Felix Frankfurter professor of law at Harvard University, is a Bloomberg View columnist. He is the former administrator of the White House Office of Information and Regulatory Affairs, the co-author of "Nudge" and author of "Simpler: The Future of Government," to be published in April.