When you think about it, it's quite silly, this notion that we "know what type of person" someone is based on a few press conference soundbites, commercial endorsements, or tweets. And that's often all the exposure that we have to celebrities and politicians before we jump to the conclusion that we know about their personality or can predict what they would or would not do.

Sure, Weiner can toss off a witty one-liner with the best of them. Yes, he seems to relish a good argument. But when you get right down to it, should it really surprise us that a few cable talk show appearances aren't sufficient to reveal the full spectrum of what another human being is capable of?

No, it shouldn't. In fact, it's all the less surprising when you realize that we do the very same thing with the people we actually meet each day. We're just as quick to jump to the conclusion that the behaviors we observe in our immediate surroundings tell us something about the internal tendencies and fixed personalities of those who are around us.

So we assume that the driver who cuts us off is an inconsiderate jerk, the waiter who screws up our order is dispositionally incompetent, and the saleswoman who says we look good in that shirt has a thing for us. Sure, we get it right sometimes. But just as often we've overlooked the power of context to shape how other people around us think, decide, and behave.

Because a lot of the time, the driver is a generally mild-mannered family man who's just running late for a meeting. Or the waiter is simply trying to cover for a missing co-worker during an understaffed lunch rush. And for the record, Romeo, the saleswoman works on commission–she compliments every customer.

But time and time again, these people whom we claim to know–whether from daily life or the daily news–do wind up hurting flies.

Or at the very least, unzipping them.

Coda (i.e., shameless book plug): At the risk of leading to you to jump to negative conclusions about my personality, allow me to note that this is exactly what my forthcoming book is about: how we rely on assumptions about personality and overlook the ways in which ordinary situations shape human nature. The video trailer for the book elaborates:

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Sam Sommers is a social psychologist at Tufts University in Medford, MA. His first book, Situations Matter: Understanding How Context Transforms Your World, will be published by Riverhead Books (Penguin) in December 2011. You can follow him on Facebook here and on Twitter here.