It is a profound mystery how evolution hardcodes abstract social desires (e.g., reputation) into our genome. Unlike simple sensory rewards, these require complex cognitive processing to even identify. Solving this could unlock powerful new methods for instilling robust, high-level values in AI systems.

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.

The "rich gay uncle" hypothesis suggests homosexuality persists by shifting reproductive effort. Instead of having their own children, gay men may invest heavily in their siblings' offspring, promoting the survival of shared genes through kin selection. This is supported in some, but not all, cultures.

In scenarios like Jonathan Haidt's "Mark and Julie" experiment, where incest is harmless and consensual, people still condemn it. This reaction may be less about a moral calculation of harm and more about an individual's fear of being seen publicly opposing a powerful social norm.

When sperm donor half-siblings meet as adults, they may feel attraction. This isn't an innate desire for kin, but a consequence of shared genes creating highly similar preferences. They seem like a "perfect match" because the usual childhood-developed sexual aversion is absent.

Kin detection isn't one-size-fits-all. Older siblings identify kin by seeing their mother care for a newborn. Younger siblings, lacking this cue, instead rely on the duration of co-residence—how long they lived under the same roof with shared parental investment—to develop their sense of kinship.

People's conscious, stated reasons for their actions (proximate explanations) often obscure deeper, unconscious evolutionary drivers (ultimate explanations), such as the drive to reduce mating competition while appearing compassionate.