Being judgmental led to evolution of diverse personalities

25 March 2009, by Tom Marshall

A new mathematical model of trust and cooperation shows that variability in trustworthiness encourages us to watch and remember the behaviour of others. This so-called 'social awareness' leads to a broader range in personalities, and this in turn reinforces the need for social awareness. In short: variation begets variation.

Prof John McNamara and colleagues, at the Universities of Bristol and Exeter, built the model to try to identify evolutionary processes that might favour the existence of lots of different personality types. They adapted the classic 'Trust Game' (see diagram below) to include the option to be socially aware - to watch the behaviour of others and use this experience to form opinions.

Being socially aware came at a cost in the model. 'These costs could reflect the amount of time it takes to pay attention to what others are doing instead of investing in looking out for predators or doing other things that are important to you,' says Dr Sasha Dall, co-author on the paper.

In the model, people could keep an eye on others and make informed decisions as to whether they could be trusted (i.e. be socially aware), or they could simply decide to trust or not trust everybody indiscriminately. This mixture of tactics caused a mixture of responses.

If people were always trusted, they became dishonest. But if they were being watched, they were only as dishonest as they could get away with. They were honest often enough to make others trust them.

Likewise, if people were always honest, then being socially aware was a waste of time. But if only some people were honest while others were not, it was worth investing effort in becoming socially aware.

This is the first time social awareness has been factored into such behavioural models. The results appear in Proceedings of the Royal Society.

Until now scientists thought distinct personality traits were due either to random variation or to learnt behaviour. But it now seems they might be selected for directly in nature.

McNamara and colleagues wanted to see if personality variation could be explained from an evolutionary perspective. Could a mixture of personality types exist for adaptive reasons?

'You have populations where individuals are doing radically different things in the same population even though they don't appear to be very different - they're the same sex, same age, but they do radically different things' says Dall.

You might expect these distinct personality traits to be less apparent outside of the mammals, but it appears to be further reaching than this. In fact, wherever animal behaviourists have looked, they have found the same scenario: birds, fish, squid, octopuses, and even pond skaters. 'Pretty much anywhere anybody's looked, they've found it!' says Dall.

Researchers are now including social awareness in other models to see how it could shape other aspects of behaviour. 'We have an intuition that this feedback of variation generating selection for variation and generating this social awareness...is something that could, in principle, influence evolutionary outcomes in a wide range of contexts' says Dall.

The Trust Game

The trust game has two players - Player 1 and Player 2 - and can be used to model an interaction where individuals are relying on the trustworthiness of others. The interaction could be between people or animals; see the picture for a diagram explaining how the game works.

Player 1 decides first. If Player 1 doesn't trust Player 2, they both get a small reward. Player 1 has to trust Player 2 for them both to get a bigger reward. But Player 2 will get an even bigger reward if he betrays Player 1's trust, leaving him with nothing. The best thing for Player 2 to do, therefore, is to always take advantage of Player 1 if he knows he will be trusted.

So if Player 1 and Player 2 interact only once, Player 1 should never trust Player 2 because he will always take advantage of this trust. Instead, Player 1 should settle for sharing a small reward with Player 2.

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Copied from Natural Environment Research Council