Psychologists may diagnose and treat mental and emotional disorders, perform research, conduct psychological testing or apply psychological principles to the workplace or school. Bureau of Labor Statistics. The BLS divides psychologists into three major groups: clinical, counseling and school psychologists, industrial-organizational psychologists and all other psychologists. Clinical and counseling psychologists need a doctoral degree and must be licensed. They diagnose and treat mental illness and emotional disorders. They work with students and deal with education-related issues. They use psychological principles in various industries to improve productivity or worker morale. The work setting could have an effect on salaries. Not all states reported data on industrial-organizational psychologists. In, people were employed in the U.