Although poor mental health in childhood is widely believed to have a negative effect on social and cognitive development, very little is actually known about the size of the impact of mental disorders on child outcomes. Studies of mental health effects are, by necessity, observational rather than experimental, which makes it extremely difficult to conclusively demonstrate the significance of the effect of childhood mental health on developmental outcomes.
A further difficulty in discerning the true effects of poor mental health on development is that most mental health “disorders” exist on a spectrum, and a simple binary division between those who have been diagnosed with a particular illness and those who have not may not be useful for research. In an important recent paper on this topic, Janet Currie and Mark Stabile attempted to deal with this problem by studying the relationship between exhibiting symptoms of common childhood mental health disorders, and development outcomes.
Despite the fact that little is known about the precise size of the impact of negative mental health effects, almost all studies of the topic have shown that mental health problems, even after controlling for potential confounding variables, do seem to cause children to suffer large, statistically significant negative long-term consequences. In the aforementioned study performed by Currie and Stabile, the researchers found significant negative effects even after using sibling-fixed effects, which controlled for unobservable household characteristics that could not be captured in a normal multivariate regression. Even after controlling for potentially confounding variables, almost all studies have found that the likelihood of grade repetition, criminality, and low adult earnings are all increased by the experience of poor mental health in childhood.
It is also important to note that different types of mental health effects appear to have different importance as predictors of future outcomes. For example, Currie and Stabile found that exhibiting symptoms of ADHD was very strongly associated with poor future development outcomes, whereas long-term developmental effects associated with symptoms of anxiety and depression appears to be much less important.