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We also found similar effects for other kinds of alters. Coresident spouses who become happy increase the probability their spouse is happy by 8% (0.2% to 16%), while non-coresident spouses have no significant effect. Nearby siblings who live within a mile (1.6 km) and become happy increase their sibling’s chance of happiness by 14% (1% to 28%), while distant siblings have no significant effect. Next door neighbours who become happy increase ego’s happiness by 34% (7% to 70%), while neighbours who live on the same block (within 25 metres) have no significant effect. All these relations indicate the importance of physical proximity, and the strong influence of neighbours suggests that the spread of happiness might depend more on frequent social contact than deep social connections. On the other hand, we found no effect of the happiness of coworkers on an ego, suggesting that the social context might moderate the flow of happiness from one person to another.

Happiness spreads significantly more through same sex relationships than opposite sex relationships (P=0

Past research on emotional contagion indicates that close physical proximity or coresidence is indeed necessary for emotional states to spread.23 To further explore whether distance affects the spread of happiness, we varied the cut off for nearby friends. Figure 5 (top) shows that the probability that an ego becomes happy in response to an alter varies for friends who live at different physical distances. An ego is 42% (6% to 95%) more likely to be happy if a friend who lives less than half a mile (0.8 km) away becomes happy (net of controls, including ego’s baseline happiness). In contrast, the effect is only 22% (2% to 45%) for friends who live less than two miles (3.2 km) away, and it declines and ceases to be significant at greater distances.

Fig 5 Physical and temporal social network. Figure shows probability that ego is happy given that alter friend is happy, for different subsamples. Top: effect of gradually increasing maximum distance allowed between ego and alter households. Friends who live less than half mile (0.8 km) away have the strongest effect on ego happiness, and effect decreases with distance. Bottom: effect of gradually increasing maximum time allowed between ego and alter exams. Friends who report becoming happy within past half year exert strongest influence on ego happiness, and effect s increases. Effect sizes are based on generalised estimating equation models of happiness in tables S9 and S10 in appendix on bmj

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Past research also suggests that changes in happiness are temporary and that there is “hedonic adaptation” to diverse stimuli4 (in other words, people get used to good or bad fortune after some time). Figure 5 (bottom) shows such an effect for the interpersonal spread of happiness. An ego is 45% (4% to 122%) more likely to be happy if a friend who was examined in the past half year becomes happy. In contrast, the effect is only 35% (6% to 77%) for friends who were examined within the past year, and it declines and ceases to be significant at greater periods of time.

This result also accords with previous evidence on sex effects in the spread of obesity32 35 and suggests that people might be more likely to take emotional cues from members of the same sex

02, see appendix on bmj), possibly helping to explain why friends and next door neighbours might exhibit stronger effects than spouses (who in our sample were all opposite sex).