Walter Mischel

Designed the marshmallow test, offering preschoolers one treat now or two if they could wait — a simple experiment whose subjects he tracked for decades, finding that early ability to delay gratification correlated with later academic performance and health. His 1968 book Personality and Assessment challenged the field's core assumption that people have stable traits that predict behavior across situations, arguing instead for consistent "if-then" patterns tied to context — a reframing that reshaped how psychologists study personality.