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Each of my upvotes probably breaks down to:

10% - A digital thumbs-up to the author.

20% - I feel like I gained something from the article and I want to repay the author by promoting his work.

30% - I want other people to see it because the world would be better if more people knew about the subject of the article.

40% - Fulfilling a narcissistic craving to express my approval of something similar to a Facebook Like or Youtube Thumbs-Up.

It gets interesting when the link is, for example, a raffle. I want to upvote because of the first, second and last reasons but I want to downvote so that less people hear about it thus increasing my own chance of winning. Individualism vs collectivism encapsulated in a single mouse click.