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In every society, people form social coalitions — we draw boundaries between 'us' and 'them'. But how do we decide who is one of 'us' and who is one of 'them'? One way is to use arbitrary categories. For example, we say that those living 49 degrees north of the Earth’s equator are Canadian, whereas those living south of it are American. Another possibility is to use physical characteristics. But what about when neither of these options are available?
By monitoring brain activity in healthy volunteers learning about other people’s political values, Lau et al. obtained insights into how people make these decisions. Participants lying in a brain scanner were asked to report their position on a political issue. They then learned the positions of three other hypothetical participants – A, B and C – on the same issue. After repeating this procedure for eight different issues, the volunteers had to decide whether they would align with A or with B on a 'mystery' political issue.
So how do participants choose between A and B? One possibility is that they simply choose whichever one has views most similar to their own. If this is the case, the views of hypothetical person C should not affect their decision. But in practice, C's views – specifically how much they resemble the volunteer's own – do influence whether the volunteer chooses A or B. This suggests that we choose our allies based on more than just their similarity to ourselves.
Using a mathematical model, Lau et al. show that volunteers also take into account how similar the views of the other ‘participants’ are to each other. In other words, they consider the structure of the social group as a whole. Moreover, the results from brain imaging show that different regions of the brain are active when volunteers track the structure of the entire group, as opposed to their own similarity with each individual.
Notably though, the activity of the group-tracking region explains people's alignment choices better than the activity of the similarity-tracking region. This suggests that we base our judgments of 'us' versus 'them' more on the structure of the group as a whole than on our own similarity with individual group members. Understanding how we determine whether others are on the same ‘team’ as ourselves could ultimately help us find ways to reduce bias and discrimination between groups.
