Maltreatment of children is rightly condemned as brutal and harmful. However, the study by Caspi et al. [1] of the MAOA gene does not prove a causal connection between abuse in childhood and antisocial behavior in adulthood, because their research design for abuse was correlational.

As Caspi et al. reported, not all abused children become antisocial adults. Nor do all siblings become antisocial in a family in which abuse has occurred [2]. These findings suggest a possible role for other genes -- a gene x gene interaction, rather than the gene x environment interaction proposed by Caspi et al. The other gene or genes, present in the parents as well as the children (because children inherit their genes from their parents), might predispose the parents to behave abusively and, in conjunction with the low-MAOA allele, might increase the risk that the children will become antisocial.

Another possibility is that an environmental correlate of child abuse might be responsible for the interaction reported by Caspi et al. [3]. For example, abused children are often removed from their homes and are sometimes moved around repeatedly. The resulting disruption of their lives, rather than the abuse per se, could be the triggering environmental factor that predisposes children with the low-MAOA allele to become antisocial.

These observations are meant to clarify, rather than detract from, the pioneering contribution of Caspi et al. Their use of measured genotypes will make hypotheses testable that were previously not even on the table.

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