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.

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 predictive power of embryo screening can be validated without controversial longitudinal studies on children. By testing if models can accurately predict trait differences between adult siblings using only their DNA, companies can prove efficacy for embryos, who are essentially unrealized siblings.

Taiwan's historical "minor marriages," where unrelated children were raised as future spouses, show culture can override behavior but not underlying psychology. These unions had more divorces and affairs, demonstrating the persistence of the Westermarck effect's sexual aversion despite societal pressure.

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.

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.

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.

It's a profound mystery how evolution encoded high-level desires like seeking social approval. Unlike simple instincts linked to sensory input (e.g., smell), these social goals require complex brain processing to even define. The mechanism by which our genome instills a preference for such abstract concepts is unknown and represents a major gap in our understanding.