Autism is a common and genetically heterogeneous disorder, with an estimated heritability of >90%. Its specific underlying causes are largely unknown. Here, I propose that low levels of autistic vulnerability, reflected in social-cognitive processing differences, do not necessarily manifest in a behavioural phenotype but are usually compensated for during development. They are more likely to lead to a recognizable syndrome among individuals of low intelligence, who are male or have independent neurodevelopmental vulnerability owing to a wide range of gene mutations, chromosomal anomalies or environmental insults. Consequently, the apparent association between mental retardation and autistic syndromes is not because they usually have common causes, but rather because the presence of both features greatly increases the probability of clinical ascertainment.