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 necessarilymeanthat weight gains in a person cause weight gains in close friends. In particular, identify some lurking variables whose effect on weight gain may beconfoundedwith the effect of weight gains in close friends. Draw a picture likeFigure 5.1to illustrate your explanation.