Classicly

The men who had gone in the other rockets had been scientists, greatly skilled men, men of high I.Q.'s. So the brass and the brains had gotten together and reasoned, and pooled data, and considered statistics, and finally decided that the strain of being completely out of one's natural element, exposed to the terrible, thought-twisting blankness of space, might be greater than had been supposed. And the high-strung, sensitive, sometimes slightly neurotic minds of the highly intelligent might well be expected to crack under the strain.