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Before crowds were considered to show wisdom, they were feared to exhibit madness. Naturally, it was a journalist, Charles Mackay, who first seeded popular concern about the frenzied and irrational actions and beliefs of the mob in his 1841 book Extraordinary Popular Delusions and the Madness of Crowds. Alchemy, ghosts and — most enduringly — economic bubbles, were among the topics that Mackay sets up as crowd-sourced bad science, which he then skewers with glee and wit.

Further removed from the madding crowd was the French polymath Gustave Le Bon, who tried to place the roots of collective behaviour not in delusion but in infection. He had a serious motive — the French political and intellectual elite wanted to understand crowds so that they could control them better to preserve social norms — and in his 1895 work Psychologie des Foules (The Psychology of Crowds), Le Bon suggested a serious cause: emotions could propagate and spread between people in the same way as germs. The result of this crowd psychology, Le Bon concluded, still fell some way short of wisdom. Instead, the likely result of all this anger and fear passed around among vulnerable human hosts, he said, was the surrender of the capacity of the individual within a large group to act rationally. The psychology of crowds that Le Bon wanted society to focus on was mass panic.

More than a century on, Le Bon might be surprised that a popular modern interpretation of his idea aims not to avoid such emotional contagion, but to harness it. It seeks to do so to build links between people and to cement in place the kinds of social structure the French were so eager to preserve by keeping the infection at bay. Today, books and articles on management, teams and leadership typically draw heavily on Le Bon’s theory of how human emotion and behaviour can be passed on in this way. And, they claim, this (largely subconscious) process can be understood and exploited to build relationships, foster team spirit and increase sales and profits.

Mackay would probably enjoy skewering those ideas, too. Emotional contagion — like much of social psychology — is an idea so simple and appealing that it’s too good for many people to check before they reach for it to explain and try to steer human behaviour. There is a solid core of empirical data and theoretical mechanisms to support, say, the idea of contagious yawning (though even that has been questioned recently: see R. Kapitány and M. Nielsen Adapt. Hum. Behav. Physiol. http://doi.org/b4kf; 2017), but there are obvious problems with trying to project the principle too far. If contagious emotion is a replicative process, for example, then why is fear a common response to anger, and why do I fail to copy your envy, jealousy or grief? (see G. Dezecache et al. Trends Cogn. Sci. 19, 297–299; 2015).

As counter-intuitive as it sounds, one way to analyse the reality and limits of emotional contagion is to look for it in animals. Some studies suggest that rats at play make noises that encourage others to join the fun, and that budgerigars copy each others’ yawns and stretches.

This week, scientists report that New Zealand parrots can spread positive emotion, too — or at least behaviour that could indicate their state of mind. The researchers recorded the play calls of keas (Nestor notabilis) and played them back to groups of wild keas. When the birds heard the sounds, they played more vigorously and longer — certainly more than when they heard the calls of a South Island robin (Petroica australis).

The calls did not, however, seem to act as an invitation to join existing birds at play. Some keas that heard them preferred to start their own play — typically embarking on feats of aerial acrobatics. With self-confessed anthropomorphism, the scientists suggest that the play calls of these birds act in the same way as infectious laughter in people (R. Schwing et al. Curr. Biol. 27, R213–R214; 2017). In its homeland, the playful kea is called the clown of the mountains. And as every good clown knows: cry and you cry alone. But laugh and the world laughs with you.