Planum temporale grey matter asymmetries in chimpanzees (Pan troglodytes), vervet (Chlorocebus aethiops sabaeus), rhesus (Macaca mulatta) and bonnet (Macaca radiata) monkeys

► We used a novel technique to quantify the grey matter of the planum temporale region in chimpanzees and monkeys. ► We found species differences in directional biases in asymmetries in the PT with chimpanzees showing a leftward asymmetry. ► Old World monkeys failed to show population-level asymmetries. ► We conclude that population-level asymmetries evolved in primates after apes split from Old World monkeys.