Dyslexia is characterized by persistent difficulties in becoming an expert reader despite massive practice. Autism is characterized by persistent difficulties in online social interactions. Using simple perceptual and motor tasks, I will show opposing types of difficulties in the two populations: slow update of internal representations in autism versus fast forgetting of recent representations in dyslexia. I propose that fast forgetting leads to poor language and reading acquisition, which rely on accumulation of language related statistics, whereas slow update impairs the acquisition of social understanding, since it slows update of motor plans and predictions based on external stimuli, such as gaze and intonation.