The text describes research by Caspi et al. (2002) on criminal behavior, child abuse, and the gene that causes low levels of enzyme monoamine oxidase (MAO). Low levels of MAO have been linked to aggressive behavior. The researchers found that among abused boys, those with the gene causing low MAO levels were far more likely to have committed a violent crime by age 26 than those with the gene causing high MAO levels. This study is evidence that: