This might come as an unpleasant surprise for the fast-food industry, but when people can read how many calories there are in their fast food, they do cut back. In a study of millions of transactions at several hundred Starbucks outlets, economists from Stanford University found that consumers in New York City responded to required calorie postings by cutting almost 15 calories off their average purchases, a calorie reduction of 6 percent.

Drinks were mostly unaffected. Customers instead bought less food with their drinks and, to a lesser extent, bought lower-calorie items. The policy was particularly effective with heavier eaters. A separate analysis of purchases by Starbucks cardholders found that customers who consumed at least 250 calories per purchase cut their intake by 26 percent after the postings appeared.

This research seems to contradict the findings of an earlier study that found that calorie postings at 14 outlets of McDonald’s, Wendy’s, Burger King and KFC in low-income New York City neighborhoods had no discernible impact on customers’ purchases in subsequent weeks.

The research at Starbucks relied on many more transactions and outlets over more than a year, so it is more precise. And despite the discrepancy, both studies seem to suggest similar conclusions: calorie counts will be most effective in higher-income neighborhoods, where consumers are better educated. And they will work better when consumers have a choice of places  with lower-calorie alternatives  to eat.