Why population-based longitudinal studies are vital for mental health prevention and policy.

There has been an increase in the prevalence of mental health problems in young people worldwide for over a decade. Population-based longitudinal studies can help show why this has occurred and whether it will continue throughout life for the present cohort of young people. This in turn will guide the policy response. We discuss two recent articles published in Australian and New Zealand Journal of Psychiatry which highlight the value of longitudinal studies in understanding the causal structure of mental health problems. Australia and New Zealand have a rich tradition of longitudinal studies dating to the 1970s, with some studies now enrolling offspring of the original cohorts. Advances in methods, computing technology and the availability of linked administrative data will make these studies even more valuable in future. The studies will need to evolve to capture shifting environmental factors, such as the digital environment. To get the most value from these studies, there needs to be secure long-term investment by central funding agencies and support for vibrant collaboration networks between the studies.