Many interesting answers so far! As many of you have pointed out, sigs and usernames are also very important factors as we jump to conclusions, but text is less delusive IMO.
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It seems as if mating is most important to us, since we primarily try to find out the gender of the poster first. Then we try to seek out if it is a friend or an enemy (politics), and after that comes religion and culture.
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I would like to read more answers on this issue. How much to you think your subconsciousness affect you when a person with Bozo the Clown as avatar is seriously stating an historical fact not previously known to you? The example in the OP is not serious of course, but what if it was? Would you be more willing to believe it, if a poster with a professor as avatar stated it?