Homicide

Daly and Wilson argue that homicide is not a pathology or a product of social breakdown but an extreme manifestation of normal, evolved psychological mechanisms. They reject the prevailing criminological view that murder is primarily committed by strangers during felonies; their data show that killings disproportionately occur between people in close relationships—spouses, sexual rivals, and step-relations. The authors apply "selection thinking" to demonstrate that lethal violence emerges from conflicts of interest rooted in mating competition, parental investment, and status striving. As they note in the book's abstract, the human race spends disproportionate attention on homicide yet possesses only rudimentary scientific understanding of who kills whom and why. Their framework supplies that understanding by treating murder as continuous with ordinary social motives.

The book pioneered a comparative approach: comparing the observed distribution of victim-killer relationships to an expected distribution derived from evolutionary predictions. Before this study, no one had systematically asked about patterns of killer-victim age disparities in familial killings or compared homicide distributions to baseline expectations. Daly and Wilson analyzed data spanning thirteen centuries and many cultures—from medieval England to contemporary Detroit—testing hypotheses about kinship, stepparental relationships, and sexual jealousy. This methodology transformed homicide research by treating statistical patterns as tests of evolutionary hypotheses rather than merely descriptive facts. Donald Symons praised the book in The Quarterly Review of Biology as "a stunning example of the unique power of selection thinking to illuminate human affairs."

Daly and Wilson argue that infanticide, while appearing to contradict evolutionary logic, can be understood through parental investment theory. For women giving birth under unfavorable conditions—young, unmarried, or lacking partner support—the costs of continuing to invest in an infant may outweigh the benefits of terminating investment and reproducing later under better circumstances. Their analysis of Canadian infanticide data identified demographic predictors: maternal age, marital status, and child age. They also documented the "Cinderella effect": stepchildren face dramatically elevated risk of lethal violence compared to genetic offspring, a finding that challenged purely environmental explanations for child abuse. The chapter on parental homicide cites cross-cultural ethnographic evidence showing infanticide correlates with circumstances of uncertain paternity, poor child quality, or low survival prospects.

The core findings have proven robust. The stepparental risk effect—now widely known as the Cinderella effect—has been replicated in multiple countries using diverse methodologies, including official homicide records, child abuse registries, and survey data. Research on intimate partner violence has confirmed Daly and Wilson's predictions: men's violence against partners correlates with infidelity suspicion, and younger women face elevated risk of partner-killing. The book's evolutionary framework became foundational for subsequent research by scholars including David Buss and Todd Shackelford. However, some critiques persist: scholars have debated whether stepparental violence reflects evolved discriminative parental solicitude or is better explained by demographic confounds. The broader methodology—comparing observed patterns to evolutionary predictions—is now standard in evolutionary forensic psychology.

Homicide consolidated and expanded arguments Daly and Wilson had developed in earlier papers and their 1983 book Sex, Evolution, and Behavior. It also set the research agenda for their subsequent career. Their follow-up book The Truth About Cinderella (1999) elaborated the stepparental investment findings for a general audience. Across their career, Daly and Wilson expanded the analysis to intimate partner violence, sexual jealousy, and the evolutionary psychology of conflict, consistently applying the framework introduced in Homicide. Their work directly influenced David Buss's research on jealousy and mate retention, and the two research programs—Daly/Wilson on violence and Buss on mating strategies—became complementary pillars of evolutionary psychology's approach to social behavior.