An ASD "comorbidity" can have several fundamentally-distinct causal origins: it can arise due to shared genetic risk between ASD and non-ASD phenotypes (e.g., ASD and microcephaly in the context of the MECP2 mutation), as a "secondary symptom" of ASD when engendered by the same causal influence (e.g., epilepsy in channelopathies associated with ASD), due to chance co-occurrence of ASD with a causally-independent liability (e.g., ASD and diabetes), or as the late manifestation of an independent causal influence on ASD (eg, attention-deficit/hyperactivity disorder).