Figure 2.

shows four possible models of the developmental emergence of behavioural symptoms of ASD and ADHD. For simplicity, bidirectional interactions between genetic and environmental risk factors, intermediate phenotypes and behavior over developmental time are not shown. A: ASD and ADHD are associated with condition-specific liabilities; in addition, there are some inherited liabilities that specifically lead to comorbid ASD and ADHD. B: Here, ASD and ADHD are caused by a combination of transdiagnostic inherited liabilities, and condition-specific liabilities. C: Here, common inherited liabilities and adaptive processes are activated at condition-specific points in development. Comorbidity is created by a longer period of activation. Condition-specific genetic and environmental factors affect the timing of expression of common inherited liabilities. D: Inherited liabilities for ASD and ADHD are condition-specific, but require the absence of condition-general protective factors to be expressed. Here, comorbidity simply results from the true statistical overlap of the presence of inherited liabilities for ASD and ADHD. Reproduced with permission from Johnson et al., 2015.

Key: RM = Risk Marker; PF = Protective Factor; A = ASD; D = ADHD; AD = Adaptive response. GE = genetic and/or environmental risk factors.