Is Adaptive Behaviour too Normal to be Normally Distributed?

Purpose: This study attempts to ascertain if adaptive behaviour complies with the characteristics of a normal distribution.

Methods: Adaptive behaviour data collected from two large state samples of 2900 were reviewed to determine the shape of their distributions. A smaller convenience sample of 37 adults without intellectual disability was similarly reviewed.  

Results: Findings suggest that the shape of the distribution of adaptive behaviour increasingly deviates from normal as cognitive abilities increase.   

Conclusions/Implications: It does not appear that adaptive behaviour is normally distributed. This will impact the diagnosis of intellectual disability because while IQ scores two standard deviations below the mean reliably cut off about 2% of the population, a similar cut-off cannot be assumed for adaptive behaviour.