"Ex-mental patients are sometimes afraid to engage in shcarp interchanges with spouse or employer because of what a SHOW OF EMOTION might be taken as a sign of. MENTAL DEFECTIVES face a similar contingency."
Here Goffman quotes directly from L. A. Dexter, "A social theory of mental deficiency", American Journal of mental deficiency", vol. LXII, 1958, p. 923 -- while noting that "for another study of the mental defective as a stigmtized person, see S. E. Perry, "Some theoretical problems of mental deficiency and their action implications," Psychiatry, vol. XVII 1954, p. 45-73.
The quote fom Dexter is:
"It also happens that if a person of LOW INTELLECTUAL ABILITY gets into some sort of trouble the difficulty is more or less automatically attributed to "mental defect" whereas if a person of "normal intelligence" gets into a similar difficulty, it is NOT regarded as symptomatic of anything in particular."