Final answer:
Jack may have persistent depressive disorder (PDD) or dysthymia, which is characterized by chronic low-intensity depression with symptoms persisting even between major depressive episodes.
Step-by-step explanation:
The question relates to clinical psychology, a branch of psychology focused on diagnosing and treating mental health conditions. If Jack does not return to a normal state of well-being between episodes of major depression, and instead experiences persistent symptoms of a mood disorder, he may be suffering from a form of depression called dysthymia, also known as persistent depressive disorder (PDD).
Unlike major depressive disorder, which typically involves intense episodes of depression, dysthymia is characterized by a chronic, lower-intensity mood disturbance. People with dysthymia may experience symptoms such as a depressed mood for most of the day, more days than not, along with low energy, sleep disturbances, low self-esteem, and an impaired ability to concentrate or make decisions.