Mental Health Disorder Trends in Denmark According to Age, Calendar Period, and Birth Cohort.

Importance: Research suggests an increase in mental disorder incidence in recent years, but this trend remains unexplained, and there is a lack of large studies based on a representative sample that investigate mental disorders over the full spectrum.

Objective: To explore sex- and age-specific incidence of any mental disorder and 19 specific disorders according to birth cohort and calendar period.

Design, setting, and participants: This was a population-based cohort study among 5 936 202 individuals aged 1 to 80 years living in Denmark at some point between January 1, 2004, and December 31, 2021. Data were derived from the Danish Civil Registration System and analyzed from February to August 2024.

Exposures: Birth cohort (8 categories from 1924 to 2011) and calendar period (six 3-year categories from 2004 to 2021).

Main outcomes and measures: Incidence rates of mental disorders from 2004 to 2021 by sex and age, according to birth cohort and calendar period, including the first years of the COVID-19 pandemic, using Danish health register data.

Results: The population comprised 2 933 857 female individuals and 3 002 345 male individuals, who were followed up for 83.8 million person-years, with median ages at start and end of follow-up of 30.2 and 46.2 years, respectively. There was an overall mental disorder incidence rate of 55.27 every 10 000 person-years. For diagnoses of any mental disorder, higher incidence rates were observed for more recent birth cohorts and calendar periods in the younger ages. Over older ages, incidence rates did not vary so greatly. An increase was observed in rates of most types of mental disorders, especially among young people, and decreases for other types (eg, substance use disorders). Prominent sex differences were also observed. For example, for schizophrenia, a large increase was seen in incidence rates for female individuals in more recent birth cohorts at younger ages, but no change for male individuals, leading to a higher peak incidence for female individuals than for male individuals in the most recent periods. For personality disorders, a large increase was observed in incidence for female individuals over time and a slight decrease for male individuals.

Conclusions and relevance: This comprehensive investigation of mental disorders incidence in Denmark indicates sex- and age-specific patterns according to birth cohorts and calendar periods. While trends may partly be explained by increases in incidence, several other factors may contribute, such as diagnostic practices, health sector capacity, and risk factors for mental disorders.