Halo effect — Thorndike (1920), "A Constant Error in Psychological Ratings": military officer ratings correlated implausibly highly ("too high and too even"); applied to web by NN/g — appearance bleeds into inferred competence, trustworthiness, usability

Claim: The halo effect: a global positive impression (e.g., attractive design) bleeds into ratings of logically independent attributes — competence, trustworthiness, usability — that the rater has no separate information about.

Thorndike's original finding: when officers rated soldiers on multiple traits, the ratings correlated implausibly highly across traits that should have been independent — "too high and too even" to reflect actual independent assessments. The Nielsen Norman Group has applied the same finding to web design: an attractive site is rated more competent, more trustworthy, and easier to use, separate from any evidence of those qualities.

Source: Thorndike, E. L. (1920). "A Constant Error in Psychological Ratings." Journal of Applied Psychology 4(1):25–29. Applied to web by Nielsen Norman Group.

Confidence: Verified. One of the oldest, most-replicated social-cognition biases; direction is robust.

Caveat: The mechanism (appearance → inferred competence) is robust; the leap to specific conversion numbers is not licensed. The halo effect tells you attractive will be judged competent — not attractive will convert at +X%.