Temperament

Any parent of more than one child will tell you kids arrive with a personality already installed. Alexander Thomas and Stella Chess put a science to that hunch in their landmark New York Longitudinal Study, and gave the NCE two must-know terms: the three temperament types and goodness of fit.

Temperament is the biologically based, early-appearing style of behavior — the how of behaving, not the what. Thomas and Chess sorted infants into three broad types based on nine underlying dimensions (activity level, rhythmicity, approach/withdrawal, adaptability, intensity, mood, and so on):

• Easy (~40%): regular rhythms, positive mood, adapts readily, approaches new situations.
• Difficult (~10%): irregular, intense negative reactions, slow to adapt, withdraws from novelty.
• Slow-to-warm-up (~15%): mild, low-activity, initially withdraws from the new but gradually adapts with gentle, repeated exposure.

(Roughly 35% don't fit neatly into one category — the percentages don't sum to 100.)

The big clinical idea is goodness of fit: outcomes depend less on the temperament itself than on how well the environment and caregiving demands MATCH the child's temperament. A difficult child with patient, flexible parenting can thrive; an easy child in a mismatched, high-friction environment can struggle. Fit — not blame — drives adjustment.

Clinical Pearl: Know the three types and their signature percentages — Easy ~40%, Difficult ~10%, Slow-to-warm-up ~15% — but the concept examiners prize is goodness of fit: healthy development comes from matching caregiving to the child's temperament, not from the temperament being "good" or "bad."