Attention deficit hyperactivity disorder (ADHD) is the most common mental disorder in children worldwide, and can have lifelong health impacts, making it a major public health issue. Rates of ADHD have been increasing in Canada in recent years, particularly in Quebec, and increasing attention has been given to ADHD diagnosis around the world. Previous research suggests there is a significant gender gap in ADHD in children, with boys much more likely to be diagnosed than girls. However, reasons for this difference remain largely unknown. Genetic factors are known to play a key role in the development of ADHD. Yet, genes do not act in isolation. The early environment can also strongly influence brain development and may modify the risk of ADHD. Specifically, maternal stress may alter the prenatal environment to increase the risk of developing ADHD, which may be more likely to impact males due to genetic sex differences. The proposed research aims to analyze the relationship between prenatal maternal stress and offspring sex and genes in relation to the risk of later ADHD diagnosis. This analysis will be carried out in four large independent prenatal cohorts from Canada, the UK (ALSPAC), the Netherlands, and Singapore. Through the use of international, longitudinal data, the proposed project will be able to unpack the complex mechanisms that can result in the development of ADHD across different cohorts and settings. This will lead to a better understanding of the relevant genetic and environmental determinants of ADHD, to allow for better treatment options and identify potential areas of early intervention.