The first information we learn about someone gives the brain a starting framework.
Once that framework exists, later behavior can be interpreted through it.
That gives first impressions a head start.
The same behavior can look different
Imagine someone is quiet during a conversation.
If your first impression is that the person is thoughtful and kind, the silence may seem reflective.
If your first impression is that the person is cold and unfriendly, the same silence may seem rude.
The behavior did not change.
The frame around it did.
Early information often gets extra weight
Psychologists sometimes connect this with the primacy effect.
Information presented early can influence how later information is understood and remembered.
The first few details become the opening chapter.
The brain starts building the rest of the story around them.
Why does the brain do this?
Quick judgments are efficient.
In everyday life, the brain constantly has to decide whether people and situations seem safe, friendly, useful, or risky.
A fast initial model saves time.
The downside is that a fast model can be wrong.
Can first impressions change?
Absolutely.
Strong new evidence can revise the original judgment.
Repeated experiences can gradually replace the first interpretation.
The point is not that first impressions are permanent.
They simply receive a powerful starting advantage.
Why do we remember evidence that fits the first impression?
Once a framework exists, confirming information is easier to notice and interpret.
Contradictory information may require more thought.
That can reinforce the original impression.
Are first impressions accurate?
Sometimes.
Sometimes not.
Confidence in a first impression is not proof of accuracy.
A short interaction provides limited information.
The brain often fills in gaps with assumptions.
Your brain likes forming a quick story.
Editing the story later takes more work.
A related piece of human behavior appears in Why do we notice evidence that agrees with what we already believe?, where the brain handles another everyday judgment or response.
And that’s the short explanation.
Related explanations: Why do we notice evidence that agrees with what we already believe?, Why can one good quality affect unrelated judgments?, and Why does confidence make someone seem believable?