5.10 Is obesity contagious? A study closely followed a large social network of 12,067 people for 32 years, from 1971 until 2003. The researchers found that when a person gains weight, close friends tend to gain weight, too. The researchers reported that obesity can spread from person to person, much like a virus.

Explain why the fact that, when a person gains weight, close friends also tend to gain weight does not necessarily mean that weight gains in a person cause weight gains in close friends. In particular, identify some lurking variables whose effect on weight gain may be confounded with the effect of weight gains in close friends. Draw a picture like Figure 5.1 to illustrate your explanation.