Autism is defined by a set of behaviours. Key symptoms are: an extreme introversion; social indifference; stereotyped repetitive behaviours; language problems; problems with hygiene; temper tantrums; in some cases hyperactivity; irrational fears; decreased but fluctuating insensitivity to pain.
At 2010, the rate of autism at age 8 was 14.7 per 1,000 which is 1 in 68. Boys are 4.5 times more likely to be affected than girls—rate for boys is 1 in 42 compared with 1 in 189 for girls.
Rates of autism have been rising dramatically. The 2010 rate is: 29% higher than the preceding estimate of 1 in 88 children in 2008; 64% higher than the 2006 estimate of 1 in 110 children; 123% higher than the 2002 estimate of 1 in 150 children.
Black children are affected at a rate 14% higher than Hispanic children and white children affected 45% more than Hispanic children.
The association of autism with severe gastrointestinal problems has been documented since the 1990s.