Is there a phycological explanation for why people have phobias for specific things?

Question

things like heights and stuff i can understand because of self preservation and what not, but why do people have such a fear of say for instance rats.

Answer 1

The cause of a phobia is a stimulus provoked a fear response out of proportion to it's actual threat (reasonable fear response). How they come about: mostly it's conditioning - see the Little Albert case study by Watson, he induced a phobia by showing a little boy a white rabbit then giving him a fright. Association of stimuli created a fear of anything small, white and furry.

Some phobias are more common - and easier to induce, even - in people (and monkeys. We're not allowed to induce phobias in kids anymore). And these are things that might have posed a threat to our ancestors. So hights. Rats are another of those - (bites can be poisonous, threat to small babies particularly), snakes and spiders (poisenous). Clowns because their make up obscures and fakes facial signals of emotion - we can't "read" them, which disconcerts us. Even bugs that to us indicate "unclean" areas are routinely treated with unproportional fear (most people can't drink water that has an utterly clean, disenfected dead fly/bug in, our aversion is too strong).

So although you can have a phobia of anything through conditioning/something you percieved as a traumatic event (what children see as traumatic can be very different to what adults would expect). However, some stimuli have a certain level of predisposition.