“”A physician who treats himself has a fool for a patient.
|—Dr. William Osler (1849-1919)|
Self-diagnosis, as the name would suggest, is when a person attempts to diagnose themselves when experiencing certain symptoms, typically with the help of symptom checkers found in books and, increasingly, the Internet. Amateur diagnosis of one's children, friends and family falls within the same category.
The problem with this is that even when the symptom checker one is using is from a trustworthy source, the vast majority of people have no medical training and thus may have difficulty noticing things that a professional doctor or nurse may notice, and also may not have access to equipment which may help them determine the cause of the symptoms.
Symptom checkers may be useful for finding what one should do (e.g. whether to go to a pharmacy, book an appointment with their doctor, call an ambulance, etc.), but in and of themselves they are useless and potentially dangerous as they may result in the individual using ineffective treatments whilst snubbing help which is effective. Alternatively, the nocebo effect may make the person's condition feel even worse, resulting in a feeling that they are in mortal danger and thus seek emergency help even though they don't need it. This of course results in people who genuinely are in mortal danger having to wait longer for treatment.
Psychiatric self-diagnosis (often incorrect) is popular, often focusing on mental health conditions which have attracted a certain amount of media attention in recent years, such as Asperger's syndrome, attention-deficit hyperactivity disorder, bipolar disorder, obsessive compulsive disorder, and various personality disorders. Often such self-diagnoses serve as a scapegoat for other problems with people's lives or behaviour. More far-fetched examples include self-diagnosing with multiple personality disorder, "otherkin" and "therian" conditions, and other purported phenomena such as indigo children. This tendency to over "diagnose" disorders tends to diminish the apparent legitimacy of mental health conditions which, of course, is of no help to people who are actually mentally disabled or ill.