["Journal of Child Psychology and Psychiatry, Volume 67, Issue 5, Page 686-695, May 2026. ", "\n\nBackground\nSubclinical hypomanic symptoms are fairly common in the general population but are linked to psychiatric and neurodevelopmental conditions. However, the genetic and environmental origins of these associations are unclear. This twin study examined the phenotypic and aetiological associations between subclinical hypomania and psychiatric and neurodevelopmental diagnoses.\n\n\nMethods\nParticipants were 4,932 twin pairs from the Child and Adolescent Twin Study in Sweden. Hypomanic symptoms were assessed using the parent‐rated Mood Disorders Questionnaire when the twins were aged 18. Specialist diagnoses of 14 conditions and symptoms were ascertained from Swedish population registries. Phenotypic associations between hypomania and these conditions/symptoms were investigated, and their aetiological overlap was examined using the twin method.\n\n\nResults\nSubclinical hypomania was significantly associated with all 14 diagnoses. The highest odds were for psychotic disorders (odds ratio [OR] = 1.48, 95% confidence intervals [CI] = 1.33–1.64, p < .001). The genetic correlations between subclinical hypomania and these diagnoses ranged from 0.12 (95% CI: 0.04–0.33) for eating disorders (other than anorexia) to 0.58 (95% CI: 0.28–1.00) for drug misuse disorders. The nonshared environmental correlations were highest for psychotic disorders (0.52, 95% CI: −0.02 to 0.92) and lowest for body dissatisfaction (0.04, 95% CI: −0.01 to 0.08). For bipolar disorder, psychotic disorders, and attention deficit hyperactivity disorder, genetic, and nonshared environmental correlations with subclinical hypomania were of a similar magnitude.\n\n\nConclusions\nThe association between subclinical hypomania and the diagnosis of multiple psychiatric phenotypes highlights its important role in the developmental pathway to clinical disorders, its complex origins, and that it may represent a quantitative trait for various psychiatric phenotypes.\n\n"]