If we’re being honest, most of us would admit that we keep an ongoing mental record of who has done what for whom among our relationships. It sounds a little churlish but this note-keeping is a basic aspect of social functioning that means we can avoid being taken advantage of by free riders, and also helps us decide who to turn to when we’re in need.

When does this sense of social fairness emerge? Developmental psychologists have previously demonstrated that pre-schoolers have a keen sense of reciprocity – for example, they will share more toys with other kids who have previously shared more with them. A new study in Developmental Psychology has flipped this around, showing that already by age three years, children also recognise when others are indebted to them.

Markus Paulus at Ludwig-Maximilians-University of Munich recruited around 40 three-year-olds and five-year-olds to play a sharing game with two toy animals. Over three rounds, the children, who were tested alone, had to choose how many stickers to share with each animal. Crucially, Paulus fixed things so that the kids’ choice for one animal was to share either two or three out of six stickers each round, whereas their choice for the other disadvantaged animal was to share either one or no stickers out of two. This obviously ensured that over the three rounds, the child had shared more stickers with one of the animals than the other.

Next, and this was important to rule out other explanations for the results, Paulus showed the children that the animals also had their own sticker collection. In fact the quantity of their own stickers meant that combined with the ones from the child, each animal now had exactly the same number of stickers. This ensured that the children’s later behaviour wasn’t influenced by perceiving one animal to be wealthier than the other in terms of sticker ownership.

The current study’s major contribution is its demonstration that preschool children are able to register and remember to whom they had allocated more resources, and that they strategically asked this recipient in a subsequent phase to share resources with them

The most important stage came next. Over several rounds the same two toy animals were shown in possession of various enticing toys, such as balloons, marbles and colouring pictures (each animal always had the same number). Each round, the children were given the chance to ask one animal if they could share their toys, and the interesting test was which animal they would approach.

The children showed a consistency tendency to ask for toys from the animal to whom they had earlier given more stickers, and this was just as true for the three-year-olds as it was for the five-year-olds.

“The current study’s major contribution is its demonstration that preschool children are able to register and remember to whom they had allocated more resources, and that they strategically asked this recipient in a subsequent phase to share resources with them,” Paulus said.

Further evidence of the sophistication of the children’s thinking came from another similar experiment in which the animals left the room when the children made their sticker sharing decisions. In this context, the animals were ignorant about which of them the child had shown greater generosity. Crucially the children seemed to realise this – when it came to their turn to ask for toys from the animals, they no longer made more requests of the animal to whom they’d earlier given more stickers. It’s as if the children realised that the animals wouldn’t know who was more indebted, so they didn’t call in the favour they were owed.

Regular Digest readers will recognise that these new findings are just the latest to show pre-schoolers’ sophisticated psychological awareness. For example, by age four, they also know when you’re contradicting yourself and five-year-olds can see through your bravado. In short, you might think you can run circles round a pre-schooler, but watch out, they’ve probably got you sussed.

—It’s payback time: Preschoolers selectively request resources from someone they had benefitted.

Post written by Christian Jarrett (@psych_writer) for the BPS Research Digest.