Children with the low MAO gene alone weren't at heightened risk (Caspi et al., 2002 Kim-Cohen et al., 2006). Specifically, children with both the low MAO gene and a history of maltreatment (such as physical abuse) were at heightened risk for antisocial behaviours, like stealing, assault, and rape. They discovered that whether this genetic risk factor is associated with violent behaviour depends on whether children were exposed to a specific environmental factor. In 2002, Avshalom Caspi and his colleagues conducted a longitudinal study of children who possessed this gene, some of whom committed violent crimes and some of whom didn't. For example, some genetic research shows that people who possess a gene that results in low production of an enzyme called monoamine oxidase (MAO) are at heightened risk for developing into violent criminals (Moore, Scarpa, & Raine, 2002). Nature and nurture sometimes interact over the course of development, meaning that the effect of one depends on the contribution of the other. More recent studies have found that much of the influence on children's vocabulary is environmental, or at least can be altered by a change in environment (Suskind, 2015). Many studies of human development are subject to the same confound.
Vocabulary is partly influenced by genetic factors (Stromswold, 2001), so these parents may merely be passing on their genetic predisposition for better vocabularies to their children. So there's an alternative explanation for Hart and Risley's findings: Perhaps they reflect the fact that parents who speak a lot to their children have better vocabularies themselves.
To borrow a term we learned in Chapter 2, genes and environment are confounded.
In intact families, parents and children share not only an environment but also genes. Hart and Risley's study provides evidence for a powerful environmental influence on children's vocabulary, right? Well, not so fast. Conducted a six-month longitudinal investigation that showed that parents who speak a lot to their children produce children with larger vocabularies than parents who don't (Greenwood et al., 2011).