Professor McIntosh emphasizes that depression often involves the interactive influences of self-focused thinking, rejection from others, and low serotonin levels. The professor's emphasis best illustrates :

A biopsychosocial approach

Explanation:

This approach was first proposed by George Engel in 1977. It takes a look at a broader and whole view of a patient as it tends to ascertain how biological, psychological and social factors are interconnected in affecting the health of an individual. It is usually employed in Health psychology in dealing with disorders in patients.

Professor McIntosh's emphasis that depression often involves the interactive influences of self-focused thinking (Psychological factor), rejection from others (social factor), and low serotonin levels (biological factor) clearly illustrates a biopsychosocial approach.