The evolution of reptilian to mammalian brains suggests that social behavior make a species more fit for the environment.

Interesting take as neanderthal is usually a pejorative for unintelligent brutes. They went extinct because their caloric needs were took high to sustain their population during the cool down. Homo sapiens survived because they were leaner, more intelligent, and more social.

I believe you mean psychopath. And Darwinian advantage implies progeny not just a 'successful career'.