Halo effect happens when we draw a general impression about an someone based on a single characteristic. All else can be ignored because the the one bright, shining, beautiful personality trait. It comes from the cognitive need categorize things. Ever fell in love with someone based on one thing(or the idea of the person) and respectively came crashing down and noticed everything bad all at once? Well, that person's halo is losing its shine. The following study is a perfect demonstration:

Subjects were given a list of traits describing a person and were asked to evaluate based on the following list:

Intelligent, skillful, practical, industrious, and warm. This person was judged to be wise, humerous, popular and imaginative. The second list was this:

Skillful, practical, industrious, intelligent and cold.

With a simple change from warm to cold changed the perceptions completely. The person was considered calculating and unapproachable.

This is even more proof that first impressions are completely unreliable.