It’s well established that humans have the largest brains of any mammal on the planet, and new research suggests that we may have evolved these oversized control centers in order to be able to judge other people.

According to the popular social brain hypothesis, human intelligence evolved in tandem with complex social structures. Evidence for this can be found in the fact that our cerebral cortex – which contains most of the brain regions that regulate social interactions – is proportionally much bigger than that of other animals, relative to overall brain size.

As such, it has been hypothesized that as primates began to form ever more complicated societies, the evolutionary need for greater social cognition provided the spark for the development of larger brains. However, scientists have until now been unable to determine which particular aspects of our social interactions necessitated this cerebral expansion.

In a new paper appearing in the journal Scientific Reports, researchers investigate the origins of “indirect reciprocity”, whereby people sometimes do favors for non-relatives despite not having any guarantee of direct payback. The study authors propose that such exchanges are made possible by the uniquely human capacity for “social comparison”, which refers to the way in which we decide how to behave towards others by measuring their social status against our own.

^{The development of large, complex societies led to a demand for greater social cognition. blvdone/Shutterstock}

Using a computer modeling program, the team created a hypothetical society consisting of individuals of varying social rankings. They then ran a series of “donation games”, in which one individual was randomly selected to make a donation to another. Depending on the social status of those selected, and how making such a donation would affect these statuses, the researchers were able calculate the most beneficial decision-making tactics for all members of a society when it comes to making or withholding donations.

According to their results, individuals and communities benefit the most when people choose to make donations to people of their own social standing or higher, which goes some way to explaining the phenomenon of indirect reciprocity. It is for this purpose, therefore, that social comparison is so vital to all members of a complex society.

Though there is no biological evidence as to how the human brain grew in response to the demands of increasing social complexity, the researchers suggest that the need to be able to engage in social comparison in order to thrive may have led to the evolution of larger cortices. In other words, being judgemental may actually be a sign of intelligence.