Client's are often very well informed. Having vast information at their fingertips, they sometimes seem to know as much as I do about an issue or diagnosis, and I have been practicing psychology for three decades. But there are problems in self-diagnosis. One is that some disorders mimic others. ADHD can look an awful lot like hypomania, or even the type of boredom a visual learner experiences when having to listen to unstimulating lecture. And then, what about PTSD--that can effect attention as well, and, further, ADHD is a risk factor for PTSD, so that both of them may exist. And then if there is bipolarity in the family there may be a hypomanic edge to the whole configuration. And what if the person has a gregarious or sociable personality, which also can be associated with high activity. That an mimic ADHD, even though it exists within the context of personality disorder. Sometimes context helps. Another problem are overused diagnosis. For example, the use of "narcissism." Some people seem narcissistic but really are just expansive because of a manic mood. And finally there is the issue of how personality intersects with clinical disorders. Depression in a person with avoidant personality is going to look different than in a person with compulsive features. A clinician saves time by getting the thing right-- though nowadays people are getting pretty good at it themselves.