It is in the reptilian brain where instructions for survival appear. As individuals we must feed and defend ourselves, and as a species, protecting the clan and reproduction is automatic.

 In a moment of unbalance between the three brains.... the selfish reptilian brain will always win.

1) the cortex (cerebral hemispheres) manages learning, abstract thought and imagination, it is the rational part.

2) the limbic system (the hippocampus, amygdala and hypothalamus) manages emotions; and 

3) the reptilian brain (the cerebral cortex and the cerebellum). The deepest and probably best measure of our behavior:  

We want to "ascend" because we unconsciously sense that being above will give us more opportunities of survival and reproduction. Like Robert H. Frank has said, “we come to the world equipped with a nervous system that is worried about status. Our reptilian instincts are constantly asking: Where am I?, How appreciated am I? What’s my worth compared with everyone else?”

The reptilian instinct has an implicit code of superiority: it looks to elevate itself above the rest. But it is hard not to be superior without being competitive, ambitious, and envious every once in a while. We fight on two fronts: one is against ourselves. Our limit is the before and after. The conquest is trying to be better than we were. The fight to convince ourselves that our life has a purpose and that we are leaving an imprint. 

Which is why, a lacerating question will always persists: "am I taking full advantage of my life?" The answer demands we define a reference and a new field of competition: compared to whom? This forces us to turn to our “dear peers” to see what they have done in our place, where they travel, what they own, what they think, where they live, what they do, what they’ve achieved and what their place is, in the social scale.

Unfortunately, the R-impulse leaves us with little room for care, compassion, tolerance, non-bias or any other characteristic akin to the "other" parts of our brain.