One positive quality makes us assume others we never saw.
We assume an attractive person is also smarter or more trustworthy, without any real evidence of that. The halo effect happens when a general impression about something or someone, formed from a single quality, contaminates how we evaluate the rest of their qualities.
Psychologist Edward Thorndike first documented it while evaluating army officers. Superiors who rated an officer as "good-looking" also tended to rate him higher on traits with no real connection, like intelligence or leadership, simply because one positive quality dragged the evaluation of the others along with it.
Evaluating every quality of a person or product separately takes a lot of effort. It's simpler to form a general impression from a single salient trait, and use that impression to judge everything else.
Source: Thorndike, E. L. (1920). A Constant Error in Psychological Ratings. Journal of Applied Psychology, 4(1), 25-29.
A single strong, visible signal (a polished design, a standout review, a credible spokesperson) can improve the perception of everything else about a product or service, even aspects the user never evaluated directly.
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