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brain. A high neocortex ratio is thought to correlate with a number of social variables such as group size and the complexity of social mating behaviors. Humans have a large neocortex as a percentage of total brain matter when compared with other mammals. For example, there is only a 30:1 ratio of neocortical gray matter to the size of the medulla oblongata in the brainstem of chimpanzees, while the ratio is 60:1 in humans.