The answer is Developmental psychologists.
Physiological psychologists focus on the biological basis of behavior, studying how the brain and nervous system influence actions, thoughts, and feelings. They do not specialize in lifespan issues.
Research psychologists conduct experiments and studies to understand various aspects of human behavior. While their work can span different stages of life, they do not specifically focus on lifespan issues.
Developmental psychologists specialize in understanding how people grow and change throughout their lives. They study physical, cognitive, social, and emotional development from infancy through old age, making them the specialists in lifespan issues.
Clinical psychologists assess and treat mental, emotional, and behavioral disorders. They may work with individuals across different stages of life, but their primary focus is on diagnosing and treating psychological issues rather than studying lifespan development.