I'm not sure: I'd assumed that the word "defective" was being applied only to those considered "mentally deficient", but you may be right. The context, by the way, is a study the ways that different groups of people react to word association tests - which includes not only what words they respond with, but the time elapsed before they respond, and so on. The article has just noted that there is a stronger correlation between the degree of education than there is between sex. Then comes the passage I quoted.