Do they just look at the stars and make up signs and give people readings or is there more to it?

Some people believe in palm reading/tarot/astrology, but why? I understand there may be a link between the visualisation of symbols/signs on hands, cards etc and convincing belief but how else does it affect the person being assessed? Have there been any scientific testing (not biased!) of these fortune telling systems, and where is the source of these?