Science Fact: People can accurately judge your traits within moments of meeting you, and those judgments are lasting, but your attractiveness is something you can influence.
Explanation: You know how snap judgments work. Surely you've met some dude whom you instantly knew to be a big stupid jerk. Then maybe you felt a little bad about your unkind generalization, right up until you watched him bodyslam an old lady into a dumpster for no apparent reason. "Did you bodyslam an old lady into a dumpster?" the police demand of him when they finally arrive. "Ummm... seven," he concludes, question marks shooting out of his big stupid head, justifying your appraisal of his personality once and for all.
Of course, the same kinds of snap judgements are made about you, every day, often by female humans, and those snap judgments are infallible. OR ARE THEY??
The Science: There are lots of studies on this stuff. In one study, researchers gathered a bunch of Professional Trait-Guessers to guess subjects' traits over the course of a twenty-minute one-on-one interview; then they grabbed a bunch of random doofuses off the street, showed them twenty seconds of interview footage, and found the doofuses were just about as good at gauging traits as the pros. Another study had a bunch of students observe pairs of people in conversation (presumably with a giant spyglass, while poking out from behind a single shrub). This one also found that people are pretty good at guessing traits almost immediately, but they're even faster at guessing traits related to intelligence or threatening behavior.
The reason for this, according to researchers, is that our evolutionary ancestors really needed to be able to make snap judgments like "Is that guy going to hit me with a club?" and "Oh no, snakes!" So we are able to notice things like meanness right away, and it takes a few more seconds to gauge things like niceness and extraversion. But we form a complete picture within seconds, and that initial picture influences every single other thing we think about a person, forever and ever, until the heat death of the universe. It's called the primacy effect.
So What Should I Do About It?
Like most things that are covered around here, the answer comes down to confidence pants; or perhaps to their lesser cousin, exuberance shorts. Basically, you should emote a lot.
Well, that's only part of it, but expressiveness really is important to your love life. One study took still photos of people, then recorded video of those same people interacting. Judges would look at a still picture of some schlub and be like "Yeesh, did he escape from a zoo?" But then they'd see the same exact guy on video, being social and emotionally expressive, and they'd be like "..." because they had fainted due to sexiness. The takeaway point here is we all have some traits we wish we didn't, but when you're watching someone of the opposite sex, you're not being a job interviewer or a Professional Trait-Guesser—you're trying to figure out if he or she is fun, and an active, open person is always fun to be around, even if he looks like a frog.
Are you good at faking happiness or extraversion?