Abstract
In youth, ADHD is more commonly diagnosed in males than females, but higher male-to-female ratios are found in clini-
cal versus population-based samples, suggesting a sex bias in the process of receiving a clinical diagnosis of ADHD. This
study investigated sex differences in the severity and presentation of ADHD symptoms, conduct problems, and learning
problems in males and females with and without clinically diagnosed ADHD. We then investigated whether the predictive
associations of these symptom domains on being diagnosed and treated for ADHD differed in males and females. Parents
of 19,804 twins (50.64% male) from the Swedish population completed dimensional assessments of ADHD symptoms and
co-occurring traits (conduct and learning problems) when children were aged 9 years. Children from this population sample
were linked to Patient Register data on clinical ADHD diagnosis and medication prescriptions. At the population level,
males had higher scores for all symptom domains (inattention, hyperactivity/impulsivity, conduct, and learning problems)
compared to females, but similar severity was seen in clinically diagnosed males and females. Symptom severity for all
domains increased the likelihood of receiving an ADHD diagnosis in both males and females. Prediction analyses revealed
significant sex-by-symptom interactions on diagnostic and treatment status for hyperactivity/impulsivity and conduct prob-
lems. In females, these behaviours were stronger predictors of clinical diagnosis (hyperactivity/impulsivity: OR 1.08, 95% CI
1.01, 1.15; conduct: OR 1.43, 95% CI 1.09, 1.87), and prescription of pharmacological treatment (hyperactivity/impulsivity:
OR 1.24, 95% CI 1.02, 1.50; conduct: OR 2.20, 95% CI 1.05, 4.63). Females with ADHD may be more easily missed in the
ADHD diagnostic process and less likely to be prescribed medication unless they have prominent externalising problems.