The claim that expertise is built by deliberate practice — effortful, feedback-rich work at the edge of current ability, rather than mere accumulated hours — was among the more useful ideas psychology produced in the 1990s, and it was popularised into something the research did not support. The original studies described violinists, where practice accounted for a substantial share of the variation between performers. Later meta-analyses across many domains found the share considerably smaller, and smaller still in fields with unpredictable structure such as management or aviation emergencies. Practice remains the largest single factor a learner controls. What it does not do is account for most of the difference between people, which is the version that reached the general reader.