From a news report:
Using population-based, longitudinal cohort study data from Sweden, researchers examined the relationship of infection during childhood with IQ and adult nonaffective psychosis. They also assessed whether shared familial confounding explains the infection-nonaffective psychosis and IQ-nonaffective psychosis relationships, and whether IQ alters the childhood infection-psychosis link. They included 647,515 Swedish men born between 1973 and 1992 in the analysis who were hospitalized with any infection from birth to age 13 years. They measured hospitalization for diagnosed nonaffective psychosis until the end of 2011, and IQ at age 18 years.
What is the meaning of shared familial confounding in this text? I'm trying to understand it "from the context" and can't. I'm now googling for "familial confounding" and trying to understand what that means, but thus far I found no clear definition, only studies that use this term, and it's a bit hard to understand.
Is it that families in which infections happen more often during the first year of the baby's life are somehow more affected by psychosis - but not because of the infection? But what is the meaning of the word shared?
In short, I'd like to read in plain English what "shared familial confounding" means..
(I first asked this question on Health SE but then realized it's probably better to ask it here)