In a 20-year study of almost 5,000 peo­ple it was found that hap­pi­ness is more con­ta­gious than pre­vi­ously thought.

While there are many deter­mi­nants of hap­pi­ness, whether an indi­vid­ual is happy also depends on whether oth­ers in the individual’s social net­work are happy. Happy peo­ple tend to be located in the cen­tre of their local social net­works and in large clus­ters of other happy peo­ple. The hap­pi­ness of an indi­vid­ual is asso­ci­ated with the hap­pi­ness of peo­ple up to three degrees removed in the social net­work. Hap­pi­ness, in other words, is not merely a func­tion of indi­vid­ual expe­ri­ence or indi­vid­ual choice but is also a prop­erty of groups of peo­ple. Indeed, changes in indi­vid­ual hap­pi­ness can rip­ple through social net­works and gen­er­ate large scale struc­ture in the net­work, giv­ing rise to clus­ters of happy and unhappy indi­vid­u­als. These results are even more remark­able con­sid­er­ing that hap­pi­ness requires close phys­i­cal prox­im­ity to spread and that the effect decays over time.

The New York Times cov­ers the find­ings, pro­duc­ing two typ­i­cally ele­gant graph­ics to dis­play some of the fas­ci­nat­ing results (for exam­ple, it was found that a neighbour’s mood effects your hap­pi­ness dras­ti­cally more than a cohab­it­ing spouse’s).

The authors of the study—who evi­dently spe­cialise in social net­work analysis—have pre­vi­ously found that both obe­sity and quit­ting smok­ing are socially con­ta­gious; but should we really be infer­ring these “net­work effect” conclusions?

Another arti­cle, printed in the same issue of the British Med­ical Jour­nal, uses the same research design to show how it can lead to con­clu­sions that dis­ap­pear once envi­ron­men­tal con­founders are controlled—in this case find­ing that height, headaches, and acne are con­ta­gious, when this con­clu­sion is evi­dently a con­fu­sion between cor­re­la­tion and cau­sa­tion.

via Mind Hacks