Researchers studied 1,249 U.S. adults, aged 18 to 96, drawn from a nationally representative online panel. Everyone answered self-report rating scales about their symptoms. The adults fell into four groups: 33 with sluggish cognitive tempo only, 46 with ADHD only, 39 with both, and 1,131 with neither.
The study found that sluggish cognitive tempo, or SCT, showed up as its own pattern of symptoms, separate from ADHD. This pattern was not tied to a person's age, sex, or ethnicity. About 6 in 100 adults met the mark for SCT (5.8%), and a stricter rule that also required at least 5 of 9 symptoms plus real impairment set the figure at 5.1%. The two conditions overlapped a lot. Among adults who qualified for SCT, about half qualified for ADHD as well (39 of 72, or 54%).
SCT also explained things ADHD did not. On a measure of organizing and solving problems, SCT accounted for most of the difference (65.9%). ADHD inattention accounted for very little of it (3.8%).
All of this came from self-reports, not clinical diagnoses, and the ADHD group was defined by high symptom counts rather than a doctor's assessment. There is no agreed way to diagnose SCT, and the study looked at one point in time only.
Source: Barkley R. Distinguishing sluggish cognitive tempo from attention-deficit/hyperactivity disorder in adults.. Journal of Abnormal Psychology (2012). doi:10.1037/a0023961
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