24 percent of the population has been diagnosed with some form of mental illness.

Judge as of 2018-02-01. The population is the US population, unless the context shows another one was meant. The figure tested is the share of people who have been diagnosed with a mental illness. TRUE: an authoritative source available around that time (e.g., NIMH, SAMHSA/NSDUH, CDC, or peer-reviewed epidemiology) puts the share of that population diagnosed with some mental illness at about 24% (roughly 22–26%). MOSTLY TRUE: a credible figure for diagnosed mental illness is within about 20–28%, or the figure is right but covers a slightly different group, such as adults rather than the whole population. MIXED: about 24% matches a closely related measure, such as annual or lifetime prevalence of mental illness that is not necessarily diagnosed, while diagnosis-based data differ materially. MOSTLY FALSE: the 24% figure has some basis in a prevalence statistic but is presented as the share diagnosed, and credible diagnosis data differ substantially; or the true figure is well outside 20–28% but within about 15–33%. FALSE: credible data put the share diagnosed far from 24% (below about 15% or above about 33%), and no relevant statistic supports the figure.

Set before the research began.

1. E1 Prevalence of Mental Illness in the United States: Data Sources and Estimates Congressional Research Service (via EveryCRSReport) · 19 January 2018

   “One analysis of NCS-R data estimated that 26.2% of adults had a mental illness within a 12-month period (hereinafter called 12-month prevalence).”

2. E2 Prevalence of Mental Illness in the United States: Data Sources and Estimates Congressional Research Service (via EveryCRSReport) · 19 January 2018

   “an analysis of NCS-R data estimated the 12-month prevalence of mental illness excluding substance use disorders to be 24.8% among adults.”

3. E3 Prevalence of Mental Illness in the United States: Data Sources and Estimates Congressional Research Service (via EveryCRSReport) · 19 January 2018

   “This report uses the term "diagnosable" as distinct from "diagnosed" in acknowledgement of the potential for surveys to identify people suffering from mental illnesses that have not been diagnosed by a mental health professional.”

4. E4 Lifetime Prevalence and Age-of-Onset Distributions of DSM-IV Disorders in the National Comorbidity Survey Replication JAMA Network (Archives of General Psychiatry) · 1 June 2005

   “Lifetime prevalence estimates are as follows: anxiety disorders, 28.8%; mood disorders, 20.8%; impulse-control disorders, 24.8%; substance use disorders, 14.6%; any disorder, 46.4%.”

A Congressional Research Service review from January 2018 reports that about a quarter of US adults had a mental illness within a 12-month period. It gives 26.2% from one analysis of national survey data (E1) and 24.8% when substance use disorders are excluded (E2), so 24% matches a closely related prevalence measure. The same review says these figures count "diagnosable" illness, as distinct from people actually "diagnosed" by a professional (E3). A 2005 study puts lifetime prevalence of any disorder at 46.4% (E4). None of the evidence gives a share of people actually diagnosed, so the figure fits annual prevalence but not the "diagnosed" wording, which matches the criteria's definition of mixed.