Final answer:
The DSM-III (1980) used a theoretical approach for systematic classification of mental disorders.
Step-by-step explanation:
The DSM-III (1980) used a theoretical approach for systematic classification of mental disorders. Unlike previous versions, which relied heavily on clinical judgment and subjective criteria, the DSM-III introduced a more structured and standardized approach. It aimed to classify disorders based on a theoretical understanding of their underlying causes and symptoms. The DSM-III used a multi-axial system to assess various aspects of an individual's mental health, including clinical symptoms, social and environmental factors, and overall functioning.