What can siblings and cousins tell us about the role of early life family income in the aetiologies of violent crime, substance misuse and psychiatric morbidity?

Abstract

In our recent paper,1 we examined associations between family income during childhood and adolescence on subsequent risks of developing psychiatric disorders, having substance misuse problems and being arrested for a violent crime. We used nationwide population data on 650 680 individuals born in Finland between 1986 and 1996 and their siblings. Our initial analyses indicated, as expected on a population level, that higher family income was associated with lower risks of all examined outcomes. We then used a sibling-comparison design, where we compared risks of the outcomes between biological full-siblings who, owing to the fact that they were born during different years, grew up in the same households during periods when the parents had varying levels of income. If the reported associations were consistent with a causal inference, we would have expected the siblings who were exposed to lower family income relative to their co-siblings to have higher risks of the outcomes, but we did not find any support for this. In fact, the point estimates for the sibling-comparison estimates were all close to 1, indicating no difference between the siblings regardless of their family income exposure, and the confidence intervals were narrow, indicating a high level of precision of the estimates. Complementary sensitivity analyses indicated no moderation effects by demographic characteristics (e.g. sex, birth year and birth order) and stability of the findings across a wide range of alternative exposure and outcome definitions. In response to the points raised in the letter Rod et al.,2 we have the following comments.