Previous studies report that the COVID-19 lockdown had an impact on the mental health of the pediatric population. In this study, we harness the deep neuropsychiatric phenotyping of the population-based COPSAC2010 (n
= 700) cohort at age 10 to study the impact of the COVID-19 lockdown on mental health outcomes with focus on
the role of the genetic vulnerability to attention-deficit/hyperactivity disorder (ADHD), in the form of polygenic
risk scores (PRS). A total of 593 children were examined between 2019 and 2021, resulting in two groups of
different children, those evaluated before the lockdown (n = 230) and those evaluated after (n = 363). Children
assessed after the lockdown presented higher odds of being diagnosed with ADHD and had significantly higher
scores in most neuropsychiatric scales, particularly in subscales pertaining to behavior and attention problems.
We observed a significant interaction between the lockdown and ADHD PRS on several neuropsychiatric dimensions, with a large post-lockdown increase in children with a high PRS, while there was little to no pre-post
difference in children with low PRS. These results indicate mental health consequences of the lockdown in
children and suggest that genetically susceptible individuals are more affected by such stressors in childhood.