What is psychologically normal and what isn't is defined by various psychiatric classifications that health professionals use to catalogue the many forms of mental ill-health.

In the past century that boundary has shifted radically.

Successive classifications have added new disorders and revised old ones. Diagnoses have proliferated as new forms of psychological misery have been identified.

These changes can have profound effects.

The wider the net is cast, the more people qualify for a diagnosis, and the more treatment is considered necessary. It means more human experience and behaviour is seen through a psychiatric lens and the mental health industry expands.

These changes may have mixed blessings. Broadening definitions of mental illness allow us to address mental health problems that were previously neglected. Mental illness may also become less stigmatised because it seems more commonplace.

However, inflating definitions of psychological disorders may also lead to over-diagnosis, over-medication, and bogus epidemics.

Many writers worry that capacious definitions of mental illness lead to ordinary problems of living becoming pathologised and medicalised.

Diagnostic inflation and the DSM

These concerns often target the Diagnostic and Statistical Manual of Mental Disorders (DSM).

The DSM is the American Psychiatric Association's influential classification of mental health problems. Since its revolutionary third edition in 1980 (DSM-III), each major DSM revision (DSM-III-R in 1987, DSM-IV in 1994, DSM-5 in 2013) has been challenged over concerns about diagnostic inflation.

Some writers contend that DSM over diagnoses depression and anxiety disorders, misrepresenting many normal responses to adversity as mental illnesses.

Others suggest that it has diluted what counts as a traumatic event for the purpose of diagnosing Posttraumatic stress disorder (PTSD). Eyebrows have also been raised over proposed new diagnoses like internet addiction and mathematics disorder.

These criticisms reached a fever pitch when DSM-5 was launched in 2013.

Leading the charge was Professor Allen Frances, the distinguished psychiatrist who led the DSM-IV Task Force. Professor Frances pilloried the new edition for creating "diagnostic hyperinflation" that would make mental illness ubiquitous.

For example, DSM-5 removed the previous rule that a recently bereaved person couldn't be diagnosed with depression, in a move that raised concerns that normal grief was being medicalised.

It also listed new disorders representing relatively mild cognitive declines and bodily complaints. It introduced a disorder of binge eating and another for frequent temper outbursts in children.

In response to shifts like these, Professor Frances led a campaign to "save normality" from psychiatry's territorial expansion.

THE EVIDENCE AND Our research

It has now become a truism that the DSM has progressively inflated psychiatric diagnostic criteria.

In our recently published research, we tested this truism with surprising results.

We scoured the research literature for studies in which consecutive DSM editions were used to diagnose the same sample of people on a single occasion. For instance, a study might use DSM-III and DSM-III-R diagnostic criteria to diagnose schizophrenia in a sample of inpatients.

We found more than 100 studies that compared rates of diagnosis of at least one mental disorder across a pair of DSM editions from DSM-III to DSM-5.

In all, 123 disorders could be compared based on 476 study findings. For each comparison, we evaluated diagnostic inflation by dividing the rate of diagnosis in the later edition by the rate in the earlier one. We refer to this as the "relative rate".

For example, if 15 per cent of a sample received a certain diagnosis by DSM-5's criteria and 10 per cent of the sample received it by DSM-IV's criteria, the relative rate would be 1.5, indicating diagnostic inflation.

If the percentages were reversed, the relative rate would be 0.67, indicating deflation. A relative rate of 1.0 would show stability.

Strikingly, we found no consistent evidence of diagnostic inflation.

Relative rates for each new edition were 1.11 (DSM-III-R), 0.95 (DSM-IV) and 1.01 (DSM-5), none of which differed reliably from 1.0 or from one other. The average relative rate overall was exactly 1.0, indicating an absence of diagnostic inflation from DSM-III to DSM-5.

But while there we found no general pattern of inflation, our findings did show that specific disorders have inflated.

Attention-deficit/hyperactivity disorder (ADHD) and autism both inflated significantly from DSM-III to DSM-III-R, as did several eating disorders and Generalised Anxiety Disorder from DSM-IV to DSM-5.

However, a similar number of disorders significantly deflated, including autism from DSM-IV to DSM-5.

BROADER Implications

These findings call into question the widespread view that the DSM has caused runaway diagnostic inflation. No consistent trend toward diagnostic expansion has occurred, nor has any DSM revision been singularly prone to inflation.

Normality may not need saving after all.

This conclusion matters for several reasons. First, it implies that worries about growing over-diagnosis or over-medication should focus on particular disorders for which diagnostic inflation can be demonstrated, like Attention Deficit Hyperactivity Disorder (ADHD) for example, rather than seeing diagnostic inflation as rampant and systemic.

Second, it restores some confidence that the DSM's process of diagnostic revision doesn't invariably make psychiatric diagnosis more expansive.

Third, it suggests that supposed epidemics of depression, anxiety, ADHD or autism, must be evaluated sceptically. If steep increases in diagnoses occur for disorders whose criteria haven't inflated, there may be cause for real alarm.

If such increases occur for inflating disorders, they may simply be due to the diagnostic bar being lowered.

Two kinds of diagnostic expansion

However, our findings don't mean we should be complacent about diagnostic expansion.

Although there has been no wholesale inflation of existing disorders, diagnostic expansion can, and has, occurred through the addition of previously unknown disorders.

New DSM editions have always identified new ways of being mentally ill, and some of the rhetorical heat generated by DSM-5's critics was directed at new diagnoses.

The fact that psychiatric classifications continue to evolve shouldn't surprise us, and nor should the fact that they sometimes expand. Such changes are not unique to the mental health field either.

As Professor Frances has drolly observed, "modern medicine is making such rapid advances, soon none of us will be well."

But our findings suggest that although new ways of being mentally unwell may continue to be discovered, the old ways have tended to stay the same.

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