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House sparrows are healthier when they live in groups in which different individuals have different personality types, rather than if all individuals share the same personality type.

The “surprising” findings suggest that personality diversity promotes not only a healthier society, but also better physical and mental health for each individual within that society, says Zoltan Barta at the University of Debrecen in Hungary.

While his team’s study focused on birds, its results may be applicable to other social species as well. “You can’t ignore the analogy with humans,” he says.

Barta and his fellow researchers trapped 240 wild house sparrows (Passer domesticus) and ran a “personality” test on each bird by placing it alone in a cage for up to 10 minutes and observing its behaviour.

“We don’t know bird emotions well enough to classify them as ‘funny’ or ‘shy’ or ‘extroverted’ like we would for describing human personalities,” says Barta. “But we can get standardised, measurable information about their personality differences by seeing what they do when they’re isolated.”

The birds’ responses are akin to what humans might do if placed alone in a prison cell, he says. “Some are going to try frantically to get out; others are going to just sit in a corner and try to understand what’s going on; others might go wandering about exploring to see where they can get a glass of water. And this, in many ways, represents personality.”

The fact that such personality testing in animals yields consistent results – the same animal gets the same personality score every time it is tested – indicates that it is a “robust” measurement of personality, says Attila Fülöp, also at the University of Debrecen, who collaborated on the study.

After giving a personality score to each bird, the researchers placed the birds in aviaries in groups of 10 birds according to personality. In some groups, they placed only birds of similar personality scores. In other groups, they placed birds with a variety of personality scores. They weighed and took blood samples from each bird before and during the experiment.

Over the next nine days, they noted that birds in diverse groups had better body weight, fewer signs of physiological stress and less tissue oxidative damage than birds living in homogenous groups. And this was true of all the birds in the diverse group, regardless of individual personality type, says Barta.

“It’s quite a difficult thing to relate social behaviour to physiology and health, [but] in this case, we saw effects within a matter of days,” he says.

The results are somewhat counterintuitive, says Barta, adding that he and his colleagues wanted to find the “evolutionary explanation” for the personality differences they had observed in their decade of work with birds. “You’d think that if you live in the same environment, you should all behave in the same way, but we didn’t find that in our study,” he says.