Fig. 2.

Mild ID (lowest possible IQ stanine scale score of 1 equals lowest 3% IQ scores) is heritable. MZ co-twins of mild ID cases have significantly and substantially lower IQs (mean = 1.96, SD = 1.37, n = 79 pairs) than DZ co-twins (mean = 2.78, SD = 1.52, n = 123 pairs), whose IQs are in turn lower than the population (mean = 5.10, SD = 1.95, n = 382,122 non-twin pairs). These results are from Sweden only because information on twin zygosity is not available for the Israeli sample. Note: individuals with severe ID are not included in the stanine IQ score distributions presented.