Step-parents and infanticide: new data contradict evolutionary predictions.

Evolutionary psychologists have hypothesized, inspired by evolutionary biology, that parents should care less for children with whom they are not genetically related since these young do not contribute to the genetic fitness of the parents. Based on this, evolutionary psychologists have predicted that there will be an overrepresentation of step-parents as offenders in family-related killings of children. Data on child homicide, particularly from Canada, have supported this prediction in that the frequency of children killed was relatively high in families where one of the two parents was a step-parent. Here we present a survey of all child homicide that occurred in Sweden between 1975 and 1995. In contrast to the Canadian data, children in Sweden living in families with a step-parent were not at an increased risk compared with children living together with two parents to whom they were genetically related. In addition, there were no other indications that step-parents are overrepresented as offenders.