We're accustomed to seeing happiness as something individual and internal, a state of mind that has to do with the circumstances and psychology of a particular person. Not so, say Harvard professors Nicholas Christakis and James Fowler.
According to their findings in "Dynamic Spread of Happiness in a Large Social Network: Longitudinal Analysis Over 20 Years in the Framingham Heart Study," happiness is better understood as "a network phenomenon, clustering in groups of people."
If you're looking for long-term happiness, the study suggests, the best strategy may simply be to surround yourself with happy people.
You can find a complete list of Prof. Christakis's works in DASH here.