Manifestations of Sibling Competition and Survival Strategies

Key Insight

Historically, high infant mortality rates, such as the 43-50% chance of living past age five among contemporary hunter-gatherers (e.g., !Kung) and 18th-century Swedes, intensified sibling competition. The birth of a younger sibling before an older one reached four or five years posed a significant threat; for example, a two-year-old !Kung child's likelihood of dying during childhood exceeded 70% if a new sibling was born. These conditions exerted strong selection pressures, shaping human personality to contend with sibling rivalry.

Siblicide, or the killing of a sibling, is common in various animal species. Verreaux's eagles practice 'obligate siblicide,' where the first chick kills its sibling, the second egg serving as insurance. Blue-footed boobies exhibit 'facultative siblicide,' killing siblings only during resource scarcity, such as when an older chick's weight drops to 80% of normal body mass due to lack of food. While human infants are too dependent to commit siblicide, historical instances like the Ottoman Empire's systemic fratricide between the 15th and 17th centuries demonstrate adult sibling killing for scarce political power, with Mehmed III notoriously murdering 19 brothers.

Human infants, lacking the means for direct siblicide, evolved less violent methods to limit competition. Prolonged breastfeeding acts as a natural contraceptive (lactational amenorrhea, 99% effective for six months), delaying the mother's next pregnancy and protecting the older child from life-threatening conditions like kwashiorkor, or 'the disease of the displaced one,' caused by early weaning. Older children (ages two to five) further sabotage parental procreation through 'infantis interruptus,' for example, by crying at night or resisting bedtime, reducing opportunities for parental sexual intercourse. Similar behaviors are observed in chimpanzees, whose young actively interrupt adult mating, often out of 'jealousy' for maternal resources.