According to a study in the New England Journal of Medicine, your friendships or “social networks” are more likely than your genes to make you obese (Jennifer Levitz, “Can Your Friends Make You Fat?”

Wall Street Journal, July 26, 2007, D1). If it is true that people who have overweight friends are more likely to be overweight all else the same, is that an example of a negative externality? Why? (Hints: Is this relationship a causal one, or do heavier people choose heavier friends? Also, people with thinner friends may be thinner.)