Mental retardation (MR) occurs in approximately 2-3% of the population in developed countries [76]. For the diagnosis MR a number of criteria have to be fulfilled, including an IQ lower than 70 and behavioural disabilities that are already evident in childhood. The underlying causes of MR can be diverse, varying from inborn causes such as Down’s syndrome, Fragile X syndrome and fetal alcohol syndrome (these three causes are responsible for 30% of the MR cases [77]), but also malnutrition and problems during pregnancy or birth can increase the risk for MR [78].
