John Locke developed associationist psychology in "An Essay Concerning Human Understanding" (1689), which was later extended by David Hartley in "Observations on Man" (1749).
NOT BSTOTAL BS
MOSTLY BS — Verdict: Mostly False
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The Short Version
The claim significantly overstates Locke's role and contains a factual dating error. Locke's chapter on "association of ideas" was added in the 1700 fourth edition of the Essay, not the 1689 first edition, and authoritative philosophical references explicitly state he did not develop a formal or systematic associationist psychology — he treated association primarily as a source of cognitive error. While Hartley's 1749 work is correctly identified as a landmark in associationist psychology, crediting Locke with "developing" the field misrepresents the historical record.
Caveats
Locke's 'association of ideas' chapter was added in the 1700 fourth edition of the Essay, not the 1689 first edition cited in the claim — a factual error.
Multiple authoritative sources (Stanford Encyclopedia of Philosophy) explicitly state that Locke did not develop a formal theory of psychological association; systematic associationism is generally credited to later thinkers such as Hume and Hartley.
Describing Hartley as merely 'extending' Locke understates Hartley's original contribution in creating the first comprehensive associationist psychology, which drew on multiple predecessors beyond Locke alone.