Statistical analysis
The statistical analyses were performed using nested case–cohort
designs. First, we explored the associations between ADHD and
bipolar disorder or schizophrenia, by comparing ADHD probands
with controls. Second, we addressed the familial overlap between
ADHD and bipolar disorder or schizophrenia by comparing
relatives of the ADHD proband group with relatives of the control
group (parents, offspring, full siblings, and maternal and paternal
half-siblings).
Probands v. controls
We initially compared the risk of bipolar disorder and
schizophrenia in the ADHD proband group with that among
matched controls. For each case, we randomly selected 10 control
group members matched by birth year and gender. In line with
well-established procedures for nested case–cohort designs,3,15,16
control group participants were alive and living in Sweden
and not diagnosed with ADHD at the time of the first ADHD
diagnosis of the proband.