Clusters of happy and unhappy people are visible in the network, and the relationship between people’s happiness extends up to three degrees of separation (for example, to the friends of one’s friends’ friends). People who are surrounded by many happy people and those who are central in the network are more likely to become happy in the future.
- Fowler and Christakis (2008) Dynamic spread of happiness in a large social network
This study analyzed a social network of 4739 indivisuals from 1983 to 2003. It’s a pretty solid research. A direct friend that is happy increases the probability of happyness by 25%.
It’s also interesting a following study about lonelyness (Cacioppo, Fowler and Christakis 2009 Alone in the Crowd PDF), showing the same three-degrees pattern. They noticed that one more friend in the network decreases the chances of lonelyness by 10%.