The Halo Effect

The first impression we have of a situation or a person will often color any subsequent judgements we make.  If we like a person, we will tend to like everything about him, even the things we know nothing about - we'll give him the benefit of the doubt.  This tendency is called the halo effect, and it has been studied for many decades.

From Daniel Kahneman, Thinking, Fast and Slow:

Early in my career as a professor, I graded students' essay exams in the conventional way.  I would pick up one test booklet at a time and read all that student's essays in immediate succession, grading them as I went.  I would then compute the total and go on to the next student.  I eventually noticed that my evaluations of the essays in each booklet were strikingly homogeneous.  I began to suspect that my grading exhibited a halo effect, and that the first question I scored had a disproportionate effect on the overall grade.  The mechanism was simple: if I had given a high score to the first essay, I gave the student the benefit of the doubt whenever I encountered a vague or ambiguous statement later on.  This seemed reasonable.  Surely a student who had done so well on the first essay would not make a foolish mistake in the second one!  But there was a serious problem with my way of doing things.  If a student had written two essays, one strong and one weak, I would end up with different final grades depending on which essay I read first . . .

I adopted a new procedure.  Instead of reading the booklets in sequence, I read and scored all the students' answers to the first question, then went on to the next one.  I made sure to write all the scores on the inside back page of the booklet so that I would not be biased (even unconsciously) when I read the second essay.  Soon after switching to the new method, I made a disconcerting observation: my confidence in my grading was now much lower than it had been.  The reason was that I frequently experienced a discomfort that was new to me.  When I was disappointed with a student's second essay and went to the back page of the booklet to enter a poor score, I occasionally discovered that I had given a top grade to the same student's first essay.  I also noticed that I was tempted to reduce the discrepancy by changing the grade that I had not yet written down, and found it hard to follow the simple rule of never yielding to that temptation . . .

I was now less happy with and less confident in my grades than I had been earlier, but I recognized that this was a good sign, an indication that the new procedure was superior.  The consistency I had enjoyed earlier was spurious; it produced a feeling of cognitive ease, and my [judgement] was happy to lazily accept the final grade.

Related websites:
Daniel Kahneman, Thinking, Fast and Slow
The Halo Effect from Wikipedia