A study of chimp behavior using five human categories – “extroversion, agreeableness, conscientiousness, neuroticism and openness” – produced the need to create a sixth:
[Analysis] showed that human personas have five dimensions. Chimps’ personas, by contrast, seem to have six. These are extroversion, agreeableness and openness, which match human dimensions, and reactivity … dominance and methodicalness, which do not. Reactivity is similar to the human dimension called neuroticism, though not similar enough, in Dr Freeman’s view, to justify the same label. And methodicalness, which measures the way animals approach things like using tools and making the nests they sleep in, resembles conscientiousness—though Dr Freeman’s raters found this difficult to assess because such activities are not much available to the chimps in Texas. But there is, intriguingly, no human dimension that resembles chimpanzee dominance.