Discontinuity in the genetic and environmental causes of the intellectual disability spectrum
2016
Reichenberg, Abraham | Cederlof, Martin | McMillan, Andrew | Trzaskowski, Maciej | Kapara, Ori | Fruchter, Eyal | Ginat, Karen | Davidson, Michael | Weiser, Mark | Larsson, Henrik | Plomin, Robert | Lichtenstein, Paul
Significance Intellectual disability (ID) is present in almost 3% of children and fundamentally characterized by IQ scores below 70. Genetic research has shown that it is among the most heritable traits, and it has been accepted that ID is the extreme low of the normal IQ 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 IQ 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 extreme of the normal distribution of intelligence.
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