What motivates people to answer questions in Stack Overflow?
Apart from ego / personal satisfaction, what mechanism gives Stack Overflow users the urge to increase their reputation points?
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Read the blog - Jeff explains quite a lot of the social concerns that went into SO in it:
Of course, you will have to go back quite a bit to get the early picture and how it evolved to what you see today.
I think people have a variety of reasons for why they answer questions, it's not the same for everyone. That being said, I can only answer why I answer questions:
It's a great feeling to help another programmer (for example: I think an accepted answer is much better than 10 upvotes). Also when you're answering you're not just helping the one asking the question, but likely dozens more later that find your solution via Google...or whatever those other search engines are.
It's not entirely selfless, your reputation is one thing and it's a fun motivator/indicator of participation, your track record of hopefully well written good answers is another..it's a resume of sorts. Even if it's not used that way directly (in a job capacity, though with careers it certain can be) it's your resume as a programmer for others to see.
As a side benefit - I've met many great programmers on SO, not only by answering but asking as well...many of the high rep users have an excellent answer history, some very valuable info for many others in their thousands of answers. I don't mean to devalue lower-rep users with that statement, some have equally great answers (they're just often harder to find by volume)...as I said earlier reputation is an indicator of participation, not ability.