Biology and Brain Behaviour

In Robert Sapolsky's brain discussion, he dissects the human brain into three layers. Of course, he does so not literally but schematically.

At the deepest level, there's what's commonly referred to as the "reptilian brain." This spooky, fantastic term simply refers to the basic structure of the brain, whereby the neural connection are effectively the same as a reptile or any other sort of ancient creature. That reptilian region is responsible for all the unconscious regulatory functions that the brain constantly carries out. The middle section is known as the limbic system, and it's a mammalian feature that controls emotional behavior. Finally there's the cortex, a trait best-developed in primates and, specifically, in humans.

The cortex can control things like logic and empathic thinking, and so it's easy to imagine that top level influencing the two below it (e.g. seeing a movie where a character dies, getting sad, and crying.) Though perhaps less intuitive, the reptile brain can also impact the cortex: for instance, studies have shown that people judge moral transgressions more harshly when they're hungry.