Surprisingly it cheers them up. No kidding. Why? Well.... There is the adrenaline rush from the suspense, there is the built in genetic fears we all share, and after the movie is wasnt real, we are still alive, not maimed or killed or suffering losses. Its like the relief people feel after seeing an accident on the road, you may feel sorry for the victim, but inside youre also glad its not YOU.
It causes you to be paranoid all the time and get freaked out easily. It also has symptoms like nerveousness, fever, dizzinies, fear of dying, etc...
One major effect they can have on people is that they either believe that they are true(the case of the boogie man)or they either makes their imagination expand.
It adds to the imagination of those with know control and for children fears are planted
We seek to find a rush from horror and if we find it to often we become desensitized to them to where they are not scary at all. The best lesson I learned from horror movies is that if I can overcome that fear I can do anything in life.