http://en.wikipedia.org/wiki/Meme
The basic idea of the meme theory is that thoughts and information have a life of their own. They are born, they evolve, they die. They pass from one person to another, with the people acting as hosts for the memes.
If meme theory is correct, then it can easily be seen how a meme could be self-propogating. For example, some memes could flourish by instructing their hosts to reproduce as quickly and as often as possible (think catholics). Some memes could flourish by instructing their hosts to attack other hosts with conflicting memes (think holy crusades). Memes could also be beneficial to their hosts (think not eating pork at a time when there's no such thing as a refrigerator).
Has anyone else ever given this theory any serious consideration? Personally, I think meme theory "works", and goes a long way toward explaining certain beliefs.