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Introduction
Happiness is a fundamental object of human existence, 1 so much so that the World Health Organization is increasingly emphasising happiness as a component of health. 2 Happiness is determined by a complex set of voluntary and involuntary factors. Researchers in medicine, 3 economics, 1 4 5 psychology, 6 7 neuroscience, 8 and evolutionary biology 9 have identified a broad range of stimuli to happiness (or unhappiness), 1 including lottery wins, 10 elections, 7 income, 1 job loss, 11 socioeconomic inequality, 12 13 divorce, 1 illness, 14 bereavement, 15 and genes. 9 16 These studies, however, have not addressed a possibly key determinant of human happiness: the happiness of others.
Emotional states can be transferred directly from one individual to another by mimicry and "emotional contagion," 17 perhaps by the copying of emotionally relevant bodily actions, particularly facial expressions, seen in others. 18 19 20 People can "catch" emotional states they observe in others over time frames ranging from seconds to weeks. 17 21 22 23 For example, students randomly assigned to a mildly depressed room-mate became increasingly depressed over a three month period, 24 and the possibility of emotional contagion between strangers, even those in ephemeral contact, has been documented by the effects of "service with a smile" on customer satisfaction and tipping. 25 26
Yet, despite the evidence that certain emotions might spread over short periods from person to person, little is known about the role of social networks in happiness or about whether happiness might spread, by a diverse set of mechanisms, over longer periods or more widely in social networks. As diverse phenomena can spread in social networks, 27 28 29 30 31 32 33 34 35 we investigated whether happiness also does so. We were particularly interested in whether the spread of happiness pertains not just to direct relationships (such as friends) but also to indirect relationships (such as friends of friends) and whether there are geographical or temporal constraints on the spread of happiness through a social network.
Methods
Participants
The Framingham Heart Study was initiated in 1948, when 5209 people in Framingham, Massachusetts, were enrolled into the "original cohort." 36 In 1971, the "offspring cohort," composed of most of the children of the original cohort, and their...