John Wilkes Booth was the son of prominent, wealthy actors. He, too, became an actor and was so popular, he was one of the first to have his clothes ripped off by fans. Hulton Archive/Getty hide caption
Psychology is the systematic investigation of the human mind, including behavior and cognition. It thus encompasses both normal and abnormal behavior. On the other hand, psychiatry is a branch of medicine dealing with mental illness.
In many ways, psychological testing and assessment are similar to medical tests. If a patient has physical symptoms, a primary care provider may order X-rays or blood tests to understand what's causing those symptoms. The results of the tests will help inform develop a treatment plan.