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One of the most general principles of human decision making is that we use relativity as a way to figure out how much we value things. We see a sale sign and the comparison of the current price to a more expensive past price makes us think that we are standing in front of a good deal.  We see a modestly prices sweater next to a much more expensive one and we reason that is it a better deal for the money. And so on.

Relativity is not always the right strategy for figuring out how much to value things (very often it is not), but it gives us a quick and handy tool for going about the world making decisions.

Over the years the same type of relative decision making has been shown in monkeys, birds, and bees, but now it has been shown even with very simple lifeforms — the slime mould, Physarum polycephalum

Latty and Beekman did one such test using two food sources – one containing 3% oatmeal and covered in darkness (known as 3D), and another with 5% oatmeal that was brightly lit (5L). Bright light easily damages the slime mould, so it had to choose between a heftier but more irritating food source, and a smaller but more pleasant one. With no clear winner, it’s not surprising that the slime mould had no preference – it oozed towards each option just as often as the other.

But things changed when the researchers added a third option into the mix – a food source containing 1% oatmeal and shrouded in shadow (1D). This third alternative is clearly the inferior one, and the slime mould had little time for it. However, its presence changed the mould’s attitude toward the previous two options. Now, 80% of the slime mould headed towards the 3D source, while around 20% chose the brightly-lit 5L one.  Even for slime mould relativity matters, suggesting that it is a very basic form of decision making!

The original paper is here.