Psychiatry and psychology have proved difficult enough fields with their emphasis on gathering and documenting subjective data about alien and distressing experiences. The skilled clinician has learned to collate the content of reported symptoms with the manner in which they are reported, a few other directly observable behaviours and the reports of other, more consistently present witnesses, such as family members, before making a diagnosis. This will allow predictions about the course of the distress and disorder and about interventions likely to change that course for the better. The psychiatric diagnostic process has, however, remained largely subjective, and vulnerable to political misuse, both at the rather grand level of an occasional national government allegedly highjacking the system to deal with dissidents and at the less well-recognized and more common level of using diagnosis to exclude individuals from services (Taylor and Gunn, 2008). Exclusion is made easier by the fact that so many of the people who present with the greatest distress, the greatest risk of harm to themselves and others and the greatest challenge to treatment have a complex mix of disorders. A long nurtured dream has been that in psychiatry, as in the rest of medicine, diagnostic tests would one day become available and practice more scientific. There have been many false dawns, from ‘pink spots’ in urine, through the rather non-specific dexamethasone suppression test, to a variety of electroencephalogram (EEG) and X-ray techniques which, when measures were averaged, were able to indicate some significant differences between groups of people with and without disorders, but proved disappointing for detection of disorder in individuals or mapping progress or progression.