OBJECTIVE: Serious defects in social skills acquired during childhood may be associated with aggressive behavior in later life. The authors studied whether being an only child was associated with criminality in adulthood and, secondly, if parental factors increased the putative risk. METHOD: The authors used an unselected, prospectively collected large birth cohort. Data on crimes were linked with being an only child as well as with perinatal risk and maternal and paternal psychological risk factors among male subjects. RESULTS: The risk for violent crimes later in life was elevated among the only children. If perinatal or parental risks were combined with being an only child, the odds ratios for violent offending increased four-fold to eight-fold. A corresponding risk increase between being an only child and nonviolent offending was not detected. CONCLUSIONS: These results support the hypothesis that growing up as an only child is associated with violent criminality among male subjects.