Abstract
<title>Abstract</title> <p>This study estimated age, period and cohort (APC) effects on average levels of prosocial behaviour and mental health problems using repeated cross-sectional data on 9620 children and adolescents (50.7% female) aged 2–18 years living in Siberian regions of Russia. Mental health measures were collected using the Strengths and Difficulties Questionnaire during 23 time periods from 1999 to 2021 in cohorts born between 1983 and 2020. The results showed curvilinear trends, with an overall increase in emotional symptoms and peer problems, and decrease in hyperactivity and prosocial behaviour from early childhood to late adolescence. Across time period and birth cohort, there was an overall decrease in emotional symptoms, peer and conduct problems and total difficulties; hyperactivity and prosocial behaviour were higher in later-born cohorts. There were significant gender differences and APC interactions. Parents whose children were born during the 1992-99 economic crisis reported higher levels of all mental health problems, except emotional symptoms, over and above age and period effects, but also steeper decreases in peer problems and increases in prosocial behaviour over time. The cohort born during the 1914-15 economic crisis had lower levels of internalising problems and total difficulties. During the pandemic, parents reported inclines in all mental health problems and declines in prosocial behaviour. The findings provide the basis for further research into the drivers of time changes in child and adolescent mental health.</p>