In an editorial published in the Journal nearly 15 years ago at the official release of DSM–IV, ¹ Professor John E. Cooper ^{Reference Cooper2} wondered whether future DSM and ICD versions would ‘serve, stimulate or torment the next generation of mental health workers, depending on their viewpoint’. With only a few years before the expected delivery of DSM–V (in 2012) and ICD–11 (2014), this question is as germane as ever.

In this issue, Michael First, one of the architects of DSM–IV, addresses the troublesome issue of ‘harmonising’ ICD–11 and DSM–V, a goal that is as worthy as it is hard to attain, considering the differences in the origins, purposes and scope of the two classifications. ^{Reference First3} Although both DSM–IV and ICD–10 ⁴ are widely regarded as documents purveying evidence-based credibility to the definitions and diagnoses of mental disorders, they have evolved in different contexts and address partially overlapping but different constituencies. The ICD, of which Chapter V, Mental and Behavioural Disorders is a part, is a statutory responsibility of the World Health Organization (WHO) aiming to provide a common language for the reliable reporting of all known diseases and health states across the world's populations. In contrast, the DSM is essentially a national diagnostic classification of mental disorders, developed by a non-governmental professional body – the American Psychiatric Association (APA) – but widely adopted by USA government agencies such as the Food and Drugs Administration and the Social Security Administration, as well as by the health insurance industry and the USA legal system. Since the introduction of explicit diagnostic criteria and a vastly expanded list of diagnostic rubrics, DSM–III ⁵ and its successors have gained international influence. In addition to a strong expert input, the DSM owes much of its success to a sustained financial support and worldwide marketing strategy. ^{Reference First6} Although DSM–III provided an important stimulus, the process involved in the development of ICD–10 was different. Wide consultations were held by WHO with clinicians and researchers representing clinical traditions and schools of thought from diverse cultures and socioeconomic environments. The process also involved collaboration with the designers of DSM–IV, which culminated in a major international conference. ⁷ Although ICD–10 was published 2 years earlier than DSM–IV, this collaboration achieved a convergence on many features of substance between the two classifications. Yet sameness was never intended and differences were inevitable, as highlighted by three further Journal editorials. ^{Reference Andrews, Slade and Peters8–Reference Cooper10}

The current process of preparation of the respective revisions of the two classifications is unique in its synchronicity and offers an opportunity to resolve lingering discordances through the establishment of a formal ICD–DSM Harmonization Coordination Group. First's overview of ‘opportunities and challenges’ should provide a helpful road map for the process.

Similarities and differences

First's analysis distinguishes between three groups of definitions and diagnostic criteria within the two classifications: a minority group of disorders where ICD–10 and DSM–IV essentially concur; a group of 23 disorders (36 sets of criteria) where differences are substantial and result from divergent conceptual perspectives; and a group of some 50 disorders where differences are mainly a matter of wording and therefore relatively trivial. Although the only disorder with a literally identical definition in ICD–10 and DSM–IV is transient tic disorder, the list of conditions with minor definitional differences reassuringly includes dementia, delirium, substance dependence, subtypes of schizophrenia, recurrent depressive disorder, panic disorder, generalised anxiety disorder, post-traumatic stress disorder and many others. However, the disorders where conceptual differences appear to raise important questions merit a comment.

In contrast to DSM–IV, which provides a single set of ‘operational’ diagnostic criteria for all users, ICD–10 was designed as a ‘family’ of interrelated versions addressing different users. Whereas the ICD–10 Clinical Descriptions and Diagnostic Guidelines ⁴ is the conceptual ‘core’ of the system, the Diagnostic Criteria for Research, ¹¹ and the WHO Guide to Mental Health in Primary Care ¹² are derivatives for use in specific contexts. First's analysis compares the DSM–IV criteria with the similarly structured ICD–10 Diagnostic Criteria for Research, rather than with the more flexible ICD–10 Clinical Descriptions and Diagnostic Guidelines. As a result, the diagnostic discordance between the two systems may appear to be greater than it actually is in everyday clinical practice.

One classification or two?

Both DSM and ICD will continue to produce new revisions and in some respects to compete with one another. It is, of course, confusing to have two rival classifications, particularly because many of the differences between them are trivial or accidental, as First points out. On the other hand, having two parallel classifications with some explicitly stated conceptual differences between them does help to highlight the provisional nature of many nosological concepts and their arbitrary definitions. Where irreconcilable conceptual differences are involved, these should be stated in a manner inviting future research to elucidate the advantages and disadvantages of alternative concepts or definitions. Eventually, both DSM–V and ICD–11 will be assessed against a set of benchmarks of validity and utility. ^{Reference Kendell and Jablensky13}

Criteria for assessing the ‘goodness of fit’ of future classifications ^{Reference Jablensky, Kendell, Maj, Gaebel, López-Ibor and Sartorius14}