Who could blame them? If your genes include a tendency towards violence and your parents teach you violence by means of violent punishments such as spanking, you will probably behave violently. That's the conclusion of a new study on the matter:
Brian B. Botwell et al., Physical punishment and childhood aggression: the role of gender and gene–environment interplay. Aggressive Behavior 2012. Pay per view.
However the results appear to show no or very little effect in girls instead:
"It did not affect females," he said. "Males who were spanked and had the highest genetic risks displayed the most aggressive behavior compared to other males."
Source and more details at Science Daily.