What is the horn effect?
The horn effect is the halo effect in reverse. While the halo effect makes us more likely to make positive judgments about someone or something based on a single positive characteristic, the horn effect makes us more likely to make negative judgments based on a negative characteristic.
For instance, the horn effect might lead you to unconsciously decide against asking a new colleague for help because you formed a negative first impression of them based on the way they were dressed when you were introduced.