Whether dementia is becoming more or less common, and whether we can tell

Two things are true at once and easily confused. The number of people living with dementia is rising in every ageing country, because more people reach the ages at which it is common. The risk at any given age has fallen in several high-income cohorts studied over the last three decades — in the United States, England, the Netherlands and Sweden, by something like a tenth to a quarter per decade — plausibly from better education, blood pressure control and less smoking. Whether that fall continues, has stopped, or reverses with obesity and diabetes is the question, and the evidence has stopped agreeing: the newest cohorts show a flattening in some countries and continued decline in others. Measurement makes it harder. Diagnosis rates rose sharply after 2010 in England when doctors were encouraged to record it, coding changes moved deaths into dementia in a single year, and the cohort studies that measure incidence directly are small and slow. So the prevalence trend is certain, the incidence trend is contested, and the two are routinely reported as though they were the same thing.

Status

contested

Direction

mixed

Would change our view

A new wave of any of the long-running cohorts showing incidence clearly falling or clearly rising; or a registry-based incidence series large enough to settle it in one country.

Trackers

Gaps

Incidence data outside North America and Northern Europe; anything comparable from Japan or Korea; the effect of diagnosis-rate campaigns separated from true change.