Background: Studies have found negative associations between socioeconomic position and attention deficit/hyperactivity disorder (ADHD), but it remains unclear if this association is causal. The aim of this study was to determine the extent to which the association between family income in early childhood and subsequent ADHD depends on measured and unmeasured selection factors. Methods: A total of 811,803 individuals born in Sweden between 1992 and 2000 were included in this nationwide population-based cohort study. Diagnosis of ADHD was assessed via the Swedish national Patient Register and the Swedish Prescribed Drug Register. Annual family income during offspring's first 5 years in life was collected prospectively from the Swedish Integrated Database for Labour Market Research and divided into quartiles by (lower) family disposable income. We predicted ADHD from family income while controlling for covariates and also comparing differently exposed cousins and siblings to control for unmeasured familial confounding. Results: The crude analyses suggested that children exposed to lower income levels were at increased risk for ADHD (HR[subscript Quartile 1] = 2.52; 95% CI, 2.42--2.63; HR[subscript Quartile 2] = 1.52; 95% CI, 1.45-1.58; HR[subscript Quartile 3] = 1.20; 95% CI, 1.14-1.15). his dose-dependent association decreased after adjustment for measured covariates (HR[subscript Quartile 1] = 2.09; 95% CI, 2.00-2.19; HR[subscript Quartile 2] = 1.36; 95% CI, 1.30-1.42; HR[subscript Quartile 3] = 1.13; 95% CI, 1.08-1.18). Although the association was attenuated in cousin comparisons (HR[subscript Quartile 1] = 1.61; 95% CI, 1.40-1.84; HR[subscript Quartile 2] = 1.28; 95% CI, 1.12-1.45; HR[subscript Quartile 3] = 1.14; 95% CI, 1.01-1.28) and sibling comparison models (HR[subscript Quartile 1] = 1.37; 95% CI, 1.07-1.75; HR[subscript Quartile 2] = 1.37; 95% CI, 1.12-1.68; HR[subscript Quartile 3] = 1.23; 95% CI, 1.04-1.45), it remained statistically significant across all levels of decreased disposable family income. Conclusions: Our results indicated that low family income in early childhood was associated with increased likelihood of ADHD. The link remained even after controlling for unmeasured selection factors, highlighting family income in early childhood as a marker of causal factors for ADHD.