Trait ascription bias
We see ourselves as flexible but others as fixed personality types.
What it means
The tendency to view oneself as variable, nuanced, and responsive to circumstance in personality, behavior, and mood, while viewing other people as comparatively predictable and trait-bound. The asymmetry arises from differing information: we observe ourselves across countless situations and feel the full range of our reactions, but observe others in a narrow slice of contexts that makes them look consistent. It leads us to over-predict others' behavior from a single label and to underestimate how much situations move them. The bias is closely tied to the actor–observer asymmetry and feeds stereotyping and rigid first impressions. It hampers fair evaluation and the willingness to give others a second chance.
Examples
I am 'sometimes shy, sometimes outgoing depending on the room,' but my acquaintance is simply 'an introvert.'
You explain your own snappy email as a rough week, but conclude the client who sent a snappy one is 'difficult' — you have seen yourself everywhere and them only here.
Two dates in, you decide he is 'a pessimist', while insisting your own gloom that evening was just the rain and a long week.
First described in Related to Jones & Nisbett (1971); Kammer (1982).