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.

Children need stability to develop their sense of self. Rituals provide this essential anchor. If you lack established traditions, invent new ones—like Sunday pancakes or a seasonal task. An action repeated three times becomes a ritual, creating a reliable foundation and a strong family identity.

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.

To counteract historical male parental uncertainty, human babies have evolved to physically resemble their fathers for roughly the first year of life. This visual confirmation—a biological signal saying "I'm yours"—encourages the father's protection and resource investment during a child's most vulnerable period.

Countering the idea that parenting has little effect on outcomes, a twin study found that the twin receiving slightly more maternal affection between ages 5-10 grew up to be more open, conscientious, and agreeable. This suggests that small, differential parenting choices have measurable long-term consequences for personality.

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.

Economist Joseph Hotz theorizes that parents subconsciously enforce stricter rules on their firstborn as an efficiency play. By maximizing the oldest child's success, they create a role model whose achievements and behaviors will 'spill over' to younger siblings, maximizing the return on total parental investment.

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.

A study by sociologist Emma Zhang found an older sibling's arbitrary academic advantage (from being old for their grade) boosts the younger sibling's performance. This demonstrates a powerful non-genetic, non-parental mechanism through which family-level advantages compound and perpetuate broader societal inequality.