Mental and Neurodevelopmental Disorders in the Intersex Population

Population-level research on intersex individuals has been impeded by the difficulty of identifying this population in large-scale data. In this paper, we develop a novel strategy for identifying intersex individuals using ICD-10 diagnostic codes in population-wide health registers. Applying this approach to Danish data spanning 1994-2022, we provide the first population-level examination of the risk of mental and neurodevelopmental disorders in the intersex population. We identify intersex traits in 1.6% of the Danish population - rising to 2.5% among the 1994-2022 birth cohorts, for whom diagnostic data cover the entirety of their lives. We disaggregate the intersex population into six subgroups based on the type of intersex-related diagnoses and examine the risk of fourteen clusters of mental and neurodevelopmental disorders. Across all subgroups, the risk of mental and neurodevelopmental disorders is substantially elevated relative to the endosex population, although specific risk profiles vary: some subgroups show broad elevations across disorder categories, whilst others exhibit more selective patterns. These patterns persist after controlling for age and assigned sex, as well as after the inclusion of sibling fixed effects. Our findings highlight the need for mental health support tailored to individuals with intersex traits. Our ICD-based identification strategy addresses longstanding methodological barriers to intersex research and offers a replicable approach for population-level research beyond mental health and across contexts with comparable health data.