Your brain judges someone in 100 milliseconds, then anchors to that snap judgment. Everything after gets filtered through it, confirming what you already think and ignoring what contradicts it. So your first impression sticks because your brain wants consistency, not truth.
Chapters
So how important are first impressions? Well, neuroscience shows that in about 100 milliseconds your brain constructs an initial image of someone based on primarily visual cues and this is done completely unconsciously. Here's the problem though. Your brain uses something called the anchoring bias and this is where your brain uses initial information as an anchor, making all future judgments around this reference point. And your initial impressions turn into what's called a halo or horn effect a cognitive bias where your first impression of a person influences your thought about that person's character. So a positive first impression leads to a halo effect where you will see positive qualities about that person but a negative first impression creates what's called the horn effect leading to negative assumptions about that person. First impressions then become really difficult to change because your brain prefers consistency and continuity. That's why it selectively looks for information that se supports your initial judgment of somebody and discards all contradictory data. So if you want to make a lasting impression on someone, make sure.