Specific studies have shown how rare are the features that allow differentiating autism and mental retardation in young children. Signs that influence diagnosis can be observed around 18 months of age. Using assessment scales such as ECAN (échelle de comportement autistique du nourrisson) [Infant Autistic Behavior Scale] clarifies the nature of these signs as well as their progression. Border cases are very common, and in genuine mental retardation, autism-like signs may be observed, which are reduced with proper treatment.