We accept vague personality descriptions as accurate, if we feel they're talking about us.
Horoscopes and many personality tests describe millions of different people with the same text, and yet each person feels it "describes them perfectly." The Barnum effect is the tendency to accept vague, general personality descriptions as accurate, when we believe they were crafted specifically for us.
In a now-classic classroom experiment, a group of students took a supposed personality test. Days later, each student received what they believed was their individual result and was asked to rate how accurate it was, on a scale from 0 to 5. In reality, everyone had received exactly the same generic text, made up of ambiguous statements like "you have a need for others to like and admire you" or "at times you have serious doubts as to whether you have made the right decision." The average perceived accuracy was 4.26 out of 5.
The statements are general enough to be true of almost anyone, but they're presented within a frame (a test, an expert, an algorithm) that makes us believe they were generated specifically for us, which triggers a confirmation bias toward validating them.
Source: Forer, B. R. (1949). The Fallacy of Personal Validation: A Classroom Demonstration of Gullibility. Journal of Abnormal and Social Psychology, 44(1), 118-123.
Any product that delivers "personalized results" (vocational tests, profile recommendations) should be able to show that its result actually varies between different users, not just that it feels personal.
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