Jung diagnosed hidden psychological 'complexes' by timing a patient's replies to words

At the Burghölzli clinic in the early 1900s, Jung had patients free-associate to a list of stimulus words while he measured hesitations, blushing, and slips. A delayed or odd response to an ordinary word like 'mother' pointed to a buried emotional knot he named a complex, the word that gave psychology the term.

— Carl Jung, Studies in Word-Association — 1906
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