Signs of autism may appear during infancy and the disorder is usually diagnosed by the age of 3. Sometimes the child's development appears normal until about 2 years old and then regresses rapidly. Symptoms of autism occur in various combinations, from mild to severe. Infants with the disorder often display abnormal reactions to sensory stimuli (i.e., senses may be over- or underactive). Touches may be experienced as painful, smells may be overwhelmingly unpleasant, and ordinary daily noises may be painful. Loud noises (e.g., motorcycle going by, vacuum cleaner) and bright lights may cause inconsolable crying.

Other signs of the disorder in infants include the following:
--Appears indifferent to surroundings
--Appears content to be alone, happier to play alone
--Displays lack of interest in toys
--Displays lack of response to others
--Does not point out objects of interest to others (called protodeclarative pointing)
--Marked reduction or increase in activity level
--Resists cuddling