@christiefan915;
Can obesity be causally linked to a post-1977 growth of fast food restaurants?
No. You can associate the two. You cannot treat post-1977 fast-food growth as a proven cause of the obesity epidemic.
Timing lines up, which is not the same as cause.
Fast-food chains existed well before 1977 (McDonald’s, KFC, Burger King). What accelerated after the late 1960s–80s was franchise density, highways, two-earner households, and eating away from home. Restaurant count roughly doubled from the late 1960s to the mid-1980s; by 1983 fast food was about 40% of U.S. restaurants. Obesity, which had been nearly flat through 1976–80, then rose. Same window, many other changes in that window.
Eating fast food often is linked to weight gain. Putting a restaurant next door is a weaker story.
- Prospective cohorts (for example CARDIA) find that people who eat fast food more often gain more weight. The 2015 Dietary Guidelines advisory review called that association moderate for younger adults. That is about behavior, not store count.
- Studies that try to isolate access—new outlets near homes or schools, highway-driven placement as an instrument—are mixed. Currie et al. found a fast-food restaurant within 0.1 miles of a school raised ninth-grade obesity by about 5%, but calibrated that proximity explained only a sliver of the 30-year rise. Anderson and Matsa, using rural interstates, found no causal effect of restaurants on adult obesity and evidence that people offset restaurant calories later. Other papers find small effects, effects only in some groups, or none.
So: frequent customers get heavier. The map of restaurants does not cleanly explain the national curve.
Why a single causal link fails
Obesity rose across ages, sexes, and regions at once. Fast-food density did not. Calories also rose from grocery-store ultra-processed food, larger portions, sugar-sweetened drinks, and cheaper refined carbs—the same era as the low-fat/high-carb shift discussed earlier. Sitting at work increased. Women entered the labor force and home cooking fell. Those are parallel supply-and-demand changes, not one industry flipping a switch in 1977.
Fair summary
Fast food is energy-dense, easy, and heavily marketed. More of it made overeating cheaper and more convenient. That is a real piece of the environment. It is not a demonstrated cause that can carry the epidemic by itself, and “restaurants opened after the McGovern report, therefore they caused obesity” would not survive a causal test.