Firstborn children may receive simultaneously less parental resources and more responsibilities if younger siblings are born. This happens during the vulnerable developmental period of ADHD. In addition, due to higher levels of insecurity, parents are assumed to focus more on potential physical or psychological abnormities in their firstborn children. This may result in a diagnostic bias in firstborn children.

Objective: To examine the association of refractive error (myopia, hyperopia, astigmatism) and strabismus with ADHD. Method: Based on data from the large, representative, epidemiological sample of the German Health Interview and Examination Survey for Children and Adolescents (KiGGS) study (N = 13,488), the associations of myopia, hyperopia, astigmatism, and strabismus with ADHD were examined, with and without consideration of other common ADHD risk factors. Results: In single logistic regression models, all examined forms of refractive error and strabismus showed an association with ADHD. After controlling for confounding variables, results remained stable and showed an increased risk for ADHD in children with hyperopia, astigmatism, and strabismus compared with the control group. Only the association between myopia and ADHD in children was not significant. Conclusion: Hyperopia, astigmatism, and strabismus seem to be independently associated with ADHD. Health care professionals in different medical fields should consider this association to adequately diagnose and treat affected children.

Because with the present design no conclusions on causality or directionality of the found associations could be drawn, results are discussed quite broadly in the light of several previously published interpretations, also to serve as a generator for further research. One more innovative speculation is that children and adolescents with ADHD may consume more frequently candy and fruit gums in order i) to compensate for their higher needs of energy resulting from hyperactive behaviour and/or ii) to compensate for the ADHD-typical deficits in the "reward cascade".