Significance intellectual disability (ID) is present in almost 3% of children and fundamentally characterized by lQ scores below 70. Genetic re- search has shown that it is among the most heritable traits, and accepted that ID extreme low the normal lQ distribution. However, we show that, while the genetic and environmental factors influencing mild ID (lowest 3% of IQ distribution) are similar to those influencing IQ in the normal range, factors influencing severe ID (lowest 0.5%) differ from those influencing mild ID or lo scores in the normal range Therefore, severe ID is a distinct disorder, qualitatively different from the majority of ID, which in turn represents the low ex treme of the normal distribution of intelligence