My son cheats on all of his girlfriends. We would like to know if this is a medical condition or just something he does out of stupidity.

It's not medical. It could be psychological.

Has he always been egocentric, self-involved, always putting himself first?

Has his father been a strong role model as he has grown up? If so, what example does the father provide?

What is his place among the children -- only child, oldest, youngest?

It's not a medical condition. It's just something your son does for his own reasons. The only thing I could suggest doing is just sit him down and make him aware of the following things:

- He's hurting a lot of people with his actions
- Girls talk, and if he's cheating on girls all from the same area he'll soon end up getting himself a reputation and will end up having nobody
- If he wants the kind of life where he doesn't have to commit and can sleep with whoever he wants, he needs to stop committing himself to a relationship and then cheating.

At the end of the day, your son is 18. He's an adult and he can do what he wants, and forbidding him from doing it will only make him rebel and want to do it more. You're best bet is to let him know what he's doing, tell him you disapprove, but then the rest is up to him. Don't condone what he's doing, but the best thing to do is allow him to make mistakes by himself and learn from them, because chances are he won't listen to you until what he's doing comes back and bites him on the arse.

Let him make his own mistakes and learn from them, and just be there for him as a supportive parent when he does.