The brain is divided into three parts, of which the limbic system governs emotions, the cortices control rational thinking, plus a bunch of stuff in the middle and down below.
When we perceive a threat, the more primitive and quicker processing limbic system assumes executive control of the brain while the more sophisticated and slower processing cortical regions essentially go off-line.
The limbic system is also involved in a range of emotions such as what we feel when we’re in love and lust.
Think of it this way: The limbic system was designed to extricate ourselves from danger long enough to experience the pleasure of a mate and thus ensure our genetic future. But what happens if our limbic system works too well? What if we perceive the entire world around us as a threat? Or, to the contrary, what if we become addicted to pleasure or novelty?
The Hariri-Caspi studies cited in Part I point to a genetic smoking gun regarding a tiny region of the limbic system called the amygdala. The amygdala is involved in fear and arousal. Think of the amygdala as a smoke alarm. For many of us, our smoke alarm works way too well. For no apparent reason, our smoke alarm sends signals to the rest of the brain.
Thus, in the produce aisle reaching for a head of lettuce, suddenly we feel the same panic as a distant ancestor of ours felt over a the presence of a tiger at the water hole.
Or, over tea and scones with people we enjoy, our brain starts acting is if we were backed into a dark alley with Jeffrey Dahmer in a bad mood.
Normally, the cortices and intermediate regions of the brain can override the limbic system and reassert control. But perhaps the signaling is coming on too strong from the limbic system. Or perhaps the signaling is too weak from the cortices. Suddenly our brain can’t cope.
We may react in a variety of ways, depending on how the rest of our brain is wired. We may explode. We may say inappropriate things to people we very much care about for no apparent reason.
We may become avoidant. We freeze, unable to communicate, seeking a hasty exit.