A new study that suggests happiness in social networks (friends and neighbors) is contagious has gotten a good deal of press. You can read the study here. The researchers found that "clusters of happiness result from the spread of happiness and not just a tendency for people to associate with similar individuals."
Today, the Freakonomics folks questioned whether the causation wasn't the "similar influences" and "shared experiences" of friends and neighbors, rather than the contagiousness of happiness.
One of the most interesting results is that this cluster phenomenon is not evident among coworkers. Any theories?