Monkeys represent others' knowledge but not their beliefs.

The capacity to reason about the false beliefs of others is classically considered the benchmark for a fully fledged understanding of the mental lives of others. Although much is known about the developmental origins of our understanding of others' beliefs, we still know much less about the evolutionary origins of this capacity. Here, we examine whether non-human primates - specifically, rhesus macaques (Macaca mulatta) - share this developmental achievement.