"For each $15 000 increase in family income at age 15 years, the risks of the outcomes were reduced by between 9 % in severe mental illness and 23 % in violent crime arrests. However, these associations were fully attenuated in the sibling-comparison models", university researcher Amir Sariaslan from University of Helsinki explains.
A research team from University of Helsinki, University of Eastern Finland and Oxford University followed participants from their 15th birthday up to 2018. The outcomes included a diagnosis of a severe mental illness. Substance misuse and violent crime arrests were also examined.
The researchers expected that the siblings who had been exposed to lower family income levels would have higher rates of the studied outcomes.
"This was not the case. We found that the siblings did not differ in their outcome rates regardless of the family income levels that they had been exposed to", professor Heikki Hiilamo from University of Helsinki says.
The results were quite rigorous. The researchers tested for family income effects from ages 1 up to age 15 years in addition to a range of alternative measures of parental socioeconomic status (e.g., social assistance benefits, disability pension and unemployment).
The study, which is published in International Journal of Epidemiology, concluded that interventions that primarily focus on improving parental earnings will unlikely lead to reductions in the rates of psychiatric disorders, substance misuse and violent crime arrests in their offspring. Whereas low family income may potentially be helpful in identifying families at risk, anyspecific interventions should target causal risk factors and tested in high quality trials.
In traditional epidemiological studies, important confounding factors in the family background, such as genetic risks and childhood environmental factors, are not typically accounted for, meaning that many reported associations between low childhood income and adult outcomes are simply correlational. The researchers therefore used a novel approach of comparing risks of the outcomes between biological full-siblings who were raised in the same families. Given that these siblings were born during different years, they were consequently exposed to different family income levels at specific ages.
By using siblings instead of unrelated individuals as comparators, the researchers were able to account for all factors that were shared between the siblings, including approximately half of their co-segregating genes and their early environmental influences.
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