As important, pay attention as to how your first impression may prejudice you against someone else. They are fine when you like someone on first meeting; they are not so fine when the first meeting is negative.
Positive first impressions lead to social cohesion; negative first impressions lead to biases and social prejudice. It creates false impressions and can, for example, lead researchers to dismiss disconfirming information; businesses to become complacent; teachers to over- or underrate a student’s real performance; the police to wrongly identify suspects; banks to make careless loans.
If one takes on a dangerous and menacing demeanor I will assume that in that person's heart they choose to be dangerous and menacing.
If one looks pleasant and pleasing I will accept that they will make an effort to be pleasant and pleasing.
The sequence that we encounter matters in how we judge subsequent information.
The exaggerated impact of first impressions is related to the halo effect, that phenomenon whereby the perception of positive qualities in one thing or part gives rise to the perception of similar qualities in related things or in the whole.
Most conclude that if she was good in one category (sociable), she will also be positive in another (generous).
The halo effect is powerful, but it questionable whether it matters much in long-term relationships, such as that between teacher and student.
If you had never seen or heard of Einstein, the first time you saw him your impression would most likely be negative.
Now his face is associated with genius, not madness because he is the person who has come to define what genius is.