Brain activity has been identified that may have contributed to the rise of the Nazis and Soviet communism. The mechanism underlies the human tendency to conform and "follow the crowd".
Neither Hitler nor Stalin are likely to have risen to power without millions of their subjects falling into line. The same activity in the brain might explain why people follow fashion trends or join religious groups.
Many studies have demonstrated the powerful effect of group opinion on individual judgements. This can influence everything from choice of clothes and hairstyles to views on abortion or the death penalty.
"We often change our decisions and judgements to conform with normative group behaviour," said study leader Dr Vasily Klucharev, from the FC Donders Centre for Cognitive Neuroimaging in the Netherlands. "However, the neural mechanisms of social conformity remain unclear."
Dr Klucharev believed social conformity might be the result of conflict with group opinion triggering a "prediction error" signal in the brain. A prediction error arises from the difference between expected and actual outcomes. Dr Klucharev, whose research appears in the journal Neuron, said: "The present study explains why we often automatically adjust our opinion in line with the majority opinion.
"Our results also show that social conformity is based on mechanisms that comply with reinforcement learning and is reinforced by the neural error-monitoring activity which signals what is probably the most fundamental social mistake - that of being too different from others."