About 2.5% of the population has an intellectual impairment (if mean IQ is 100, and SD 15, 2.5% have IQ less than 70 the WHO cut off). Of this 2.5% some will have social/adaptive impaitment similarly 2SD below the mean and have an "intellectual/learning disability".
Move that mean down one SD (15 IQ points). And now OVER 15% of the people meet the intellectual impairment criteria.
Put it another way, due to the shape of the normal distribition moving IQ down 10-15 points amounts to 2-3X increase in the number with abnormally low IQ.
Cut to the modern era, where fewer and fewer jobs are available for those with intellectual impairments, and the world requires more and more technical prowess and the situation is far worse than the figures appear.
Move that mean down one SD (15 IQ points). And now OVER 15% of the people meet the intellectual impairment criteria.
Put it another way, due to the shape of the normal distribition moving IQ down 10-15 points amounts to 2-3X increase in the number with abnormally low IQ.
Cut to the modern era, where fewer and fewer jobs are available for those with intellectual impairments, and the world requires more and more technical prowess and the situation is far worse than the figures appear.