Secretary of Defense Robert McNamara believed that extra training would make these men suitable soldiers and productive members of society. He was mistaken.

From In the Know Paperback: Debunking 35 Myths About Human Intelligence by by Russell T Warne.  Page 329.  On the difficulty high IQ people sometimes have understanding the world as seen and experienced by others at different points on the normal IQ curve.  

On a larger scale, the differences in how highly intelligent people and average or low-IQ people think causes problems because bright people have a disproportionate say in how society is run. This was especially apparent in a US government initiative called Project 100,000. Between 1966 and 1971 (during the height of the Vietnam War), the US Department of Defense increased the number of men eligible for the draft by lowering the minimum IQ needed for military service from 92 to 71 (Gregory, 2015, pp. 100–102).3 Secretary of Defense Robert McNamara believed that extra training would make these men suitable soldiers and – after their service – productive members of society.4 Over the course of Project 100,000’s existence, 354,000 men were inducted under relaxed psychological and medical standards; 91% of these men were inducted due to the lowered minimum IQ (Rand Corporation, n.d., p. 5).
 
Project 100,000 was a spectacular failure. Men in Project 100,000 were harder to train and were less competent soldiers, which placed lives at risk. Over half of the men were dishonorably discharged (Gregory, 2015, p. 196). They experienced psychiatric problems at a rate that was 10 times higher than other soldiers (Crowe & Colbach, 1971), and their death rate was three times higher than average (Gregory, 2015, p. xiv). While some men from Project 100,000 were good soldiers, the extra training and supervision in the military did little for most soldiers to compensate for their low IQ. The cause of Project 100,000’s failure was not the American military’s lack of motivation or resources to bring low-IQ men up to standard levels of performance. Instead, the failure originated in McNamara’s and other decision makers’ lack of understanding that IQ differences lead to fundamental differences in people’s ability to function in their environment. Contrary to McNamara’s – and Collins’s (1979) – beliefs, people are not interchangeable cogs that can be trained to fill nearly any job (Gottfredson, 1986).

I posted a small snippet about Project 100,000 in Behavioral Sink a couple of years ago. 

This insight undergirds the concerns (though these aren't his terms) of Charles Murray in The Bell Curve and in Coming Apart as well many other classical liberal thinkers who are tremendous proponents of meritocracy but highly concerned when efficient meritocracy becomes combined with assortative mating.  And certainly the discussions about social/mental bubbles are traceable to this epistemic mismatch.