The brain's "wanting" (dopamine) and "liking" (endorphins) systems are separate. Intense reinforcement of the wanting system can create a powerful craving for a person who no longer brings pleasure, explaining why people stay hung up on those who treat them badly or with whom there is no future.

Evolution designed an economical system where a single, subconscious "kinship estimate" for each person dictates both altruism towards them and sexual aversion. It's one calculation for two different social behaviors, determining how close your heart should be and how far your genitals should be.

The feelings of love and attachment arise from three collaborating neural circuits: the autonomic nervous system (our physiological state), empathy circuits (our ability to match another's state), and, surprisingly, circuits associated with positive delusions—the belief that our partner is uniquely special and irreplaceable.

When asked to imagine incestuous acts, women's disgust is uniformly high. Men's responses show a much wider variance. This reflects the catastrophic evolutionary cost of a single bad reproductive choice for a female (nine months of gestation) versus the far lower opportunity cost for a male.

The widespread and instinctual revulsion toward incest provides a strong case for emotivism. When pressed for a logical reason why it's wrong (beyond pragmatic concerns like birth defects), most people fall back on emotional expressions like 'it's just gross.' This suggests the moral judgment is rooted in a fundamental emotion, not a rational principle.

Emotions are not superfluous but are a critical, hardcoded value function shaped by evolution. The example of a patient losing emotional capacity and becoming unable to make decisions highlights this. This suggests our 'gut feelings' are a robust system for guiding actions, a mechanism current AI lacks.

People remain disgusted by an object even when they intellectually know it's safe, such as a sterilized cockroach dipped in a drink. This demonstrates that disgust operates on a 'magical' or symbolic level, bypassing our rational faculties and making it a powerful, irrational force.

Unlike other emotions, disgust spreads through contamination in one direction. A single cockroach can render an entire platter of food inedible, but pouring a gallon of honey on the cockroach won't make it less disgusting. This principle highlights the powerful, irreversible nature of disgust.

The strong emotional recoil many feel about incest is a developed response, not innate. Only children, who never experienced the necessary childhood cues (like co-residence with a sibling), understand incest is wrong intellectually but lack the deep, gut-level aversion that is programmed in others.