It is proffered that stepchildren are more likely than genetic children to be physically abused because they are unable to ensure the genetic survival of their adoptive parents. This abuse is theorized to be more pronounced in communities where social and economic resources are scarce. The salience of this cross-level interaction hinges on the assumption that the limited resources of a family are first allocated to genetic offspring because these children, unlike their nongenetic siblings, carry the genes of their parents. A multilevel analysis of child abuse incidents reported to police in 133 U.S. cities during 2005 shows that in cities with a high level of community disadvantage, stepchildren are much more apt than are genetic children to suffer a physical injury in a child abuse incident. Such a finding buttresses the position articulated by proponents of sociobiology.