A recent PLoS paper presents evidence that children as young as 15 months old may have a rudimentary sense of fairness. Adults regularly appear to include fairness measures when making decisions. You might not believe it’s true, but there’s proof: Neuroimaging highlights increased activity in the reward regions of our brains when we consider fair offers and increased activity in the amygdala when we’re faced with unfair options. To be able to weigh fairness and unfairness, we have to have a sense of of the psychological and social state of other. Such “other-regarding” behavior is believed to be a human tendency, although it remains to be confirmed whether it is specifically a human trait.
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