Morgan's Canon, also known as Lloyd Morgan's Canon, Morgan's Canon of Interpretation or the principle of parsimony, was coined by 19th-century British psychologist C. Lloyd Morgan,^[1] and remains a fundamental precept of comparative (animal) psychology. In its developed form it states that:^[2]

In other words, Morgan believed that anthropomorphic approaches to animal behavior were fallacious, and that people should only consider behaviour as, for example, rational, purposive or affectionate, if there is no other explanation in terms of the behaviours of more primitive life-forms to which we do not attribute those faculties.

Morgan was reacting to interpretations of animal behavior–specifically the anecdotal approach of George Romanes–that he deemed excessively anthropomorphic. The prestige of Lloyd Morgan's canon partly derives from cases he described where behaviour that might at first seem to involve higher mental processes could in fact be explained by simple trial-and-error learning (what we would now call operant conditioning). A famous example is the skilful way in which Morgan's terrier Tony opened the garden gate, easily taken by someone seeing the final behaviour as an insightful act; Lloyd Morgan, however, had watched and recorded the series of approximations by which the dog had gradually learned the response, and could demonstrate that no insight was required to explain it.

As the study of animal cognition has become popular, a disciplined use of Lloyd Morgan's canon has become more and more important. D.A. Dewsbury called Morgan's Canon "perhaps, the most quoted statement in the history of comparative psychology"^[3] and Frans de Waal echoed this in The Ape and the Sushi Master: "perhaps the most quoted statement in all of psychology", though he also points to the rider that Morgan later added: "there is nothing really wrong with complex interpretations if an animal species has provided independent signs of high intelligence".^[4] It has played a critical role in the growth of the prestige of behaviourism in twentieth century academic psychology.