Final answer:
Walter Mischel's major contribution to psychology was resolving the person-situation debate, asserting that both situational factors and personal traits are crucial in predicting behavior.
Step-by-step explanation:
The major contribution of Walter Mischel in the field of psychology is the resolution of the person-situation debate. Mischel challenged the long-standing view that personality traits are consistent across different situations. Instead, his work suggested that behavior could not be predicted solely by traits because situations also play a critical role. His conclusion was based on a comprehensive survey of empirical psychological literature. Eventually, this understanding evolved, and now most psychologists recognize the importance of both situational factors and personal traits in predicting behavior.
For Mischel, individuals are 'situation processors' who interpret each situation uniquely, thus behavior is not determined just by the situation, but also by cognitive processes that interpret the situation. The resolution of the person-situation debate by Mischel has had a lasting impact on how psychologists understand and assess personality.