Complex sentence: The primary difference in brain size across the primates is due to size differences in the neocortex, and investigation into possible biological bases for neocortical development indicates that its evolution is likely very socially influenced and even selected for in highly social species.
Split into simpler sentences: The primary difference in brain size across the primates is due to size differences in the neocortex. Investigation into possible biological bases for neocortical development indicates that its evolution is likely very socially influenced and even selected-for in highly social species.