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Slime moulds explain irrational humans

Humans aren't the only ones that make irrational choices; new research has found single-celled brain-less slime moulds do it too.

The findings suggest irrational decision making - for example, in response to advertising tricks - is a long-evolved trait.

Dr Tanya Latty and Associate Professor Madeleine Beekman, of the University of Sydney, report their study of single-celled slime-moulds today in the Proceedings of the Royal Society B.

"We were interested in finding out whether or not slime moulds met the definition of irrationality," says Latty.

She says most biological models of animal foraging and consumer choice assume individuals make choices based on the absolute value of items and are 'economically rational'.

But biological decision-makers frequently fail to follow such rationality.

For example, says Latty, someone might normally choose a cheap old car over an expensive sports car.

But, when they are presented with a third option - an expensive old car - they are likely to shift their preference to the sports car.

"All of a sudden the [sports car] starts looking really good," says Latty. "But that's irrational because the value of the [sports car] hasn't changed. All that's changed is that you've been given another option."

Brain wiring?

Scientists have supposed such irrationality is wired into the human, which tends to make decisions by comparing the value of different options.

But scientists have shown that birds and bees also decide by comparative valuation, and Latty and Beekman wanted to see if organisms with no brain do this too.

The researchers looked at what happened when a slime mould (Physarum polycephalum) was given the choice of two meal options.

One high quality meal (5L) had a high concentration of oatmeal but was positioned in the light. The other meal (3D) had a lower concentration of oatmeal but was positioned in the dark.

Slime moulds tend to avoid areas that are well lit because light interferes with their biology and dries them out.

Latty and Beekman found that the slime mould weighed up the risk of going out in the light with the quality of the meal and chose 5L half the time and 3D the other half of the time.

But, when they introduced a third 'decoy' meal, labelled ID, which was also in the dark as per 3D but had an even lower concentration of oatmeal, the slime mould was more likely to choose 3D.

"If something as different from us as a slime mould has this irrational behaviour then it suggests it's not just a weird fluke of the way mammalians brains work," says Latty. "It must be something deeper than that."

Evolutionary advantage

Latty says "comparative valuation" might seem irrational, but could have evolved because it is an easy short cut to making decisions.

"If you are in a risky environment, foraging with lions and tigers and such, it's better to be able to make a quick decision that's right most of the time rather than a perfect decision that takes too long and means you get eaten by something," she says.

Psychologist, Dr Daniel Navarro, of the University of Adelaide, agrees comparative valuation has an evolutionary advantage.

But, he says, in modern society, such behaviour can make you vulnerable to subtle manipulations.

Navarro says a "decoy option" could be used by advertisers to make their product look good.

"I imagine the downsides of comparative decision making might actually be more severe for 21st century humans, precisely because other humans know how it works and might try to exploit the effect," says Navarro.

"For instance, a devious political operator might well decide to start a decoy political party to make their party look good and get elected, and a company might introduce a decoy product under a different brand name to make their main product look better than their competitor's."