The short, imprecise answer is that a novelist is also a psychologist, assuming that s/he is writing about human characters (or characters with human characteristics).
The definition of a psychologist, according to my dictionary is “a person who has (knowledge of) and is qualified in the study of the mind and behaviour.” This seems like a pretty broad definition. In fact, the American Psychological Association has 56 divisions in its membership. The one respect in which most novelists would not be considered psychologists is in the word ‘qualified’. Very few of us have a degree (particularly the required advanced degree) in psychology. But that doesn’t prevent us (and most of our fellow human beings) from acting as psychologists. Hands up! those of us who have never given advice to a friend or family member on how to handle a troublesome acquaintance/colleague/friend/lover/family member.
There are generally considered to be two categories of psychologists: applied and research. Researchers use scientific methods to learn more about the functioning of the human mind. Since a novel is, by definition, fictional, it is not suitable for the scientific method. This would seem to disqualify novelists from being amateur research psychologists. However, there may be some of my colleagues who believe that they have discovered, and have published, an interesting parameter of human behaviour.
If we consider the applied category, it seems that there are two sub-categories here: teachers (those who pass on to others their knowledge of psychology) and practitioners (those who ‘work with patients in a variety of therapeutic contexts’). To me, it seems doubtful that many novelists write with the purpose of giving insight and relief to troubled individuals. However, for example, a troubled reader may recognise himself as Josef K. in Franz Kafka’s The Trial, and he may draw some useful conclusions from that recognition.
So, for me, novelists if they are amateur psychologists at all, fall into the ‘applied’ category, not exactly as teachers but as practitioners who provide us with insights of the human mind and character. Wikipedia points out that psychologists explore such concepts as personality, the workings of the mind, emotion, motivation, interpersonal relationships, cognition, perception and the unconscious. Novelists also work with these concepts to create characters and situations which may enlighten us, but mostly they entertain us.