First, we must realize that any answer to this is only speculative. There is no way to isolate, with any kind of certainty, the impact that reality TV has on individuals or on society. Let us imagine, for example, that we could somehow prove that Americans have come to care less about one another than they used to. It would be very hard to show that reality TV caused this rather than A) the two being unrelated or B) the lack of caring causing reality TV.
That said, we can at least argue that such shows have some negative effects on our society. Let us look at two of them.
First, we can say that they encourage people to aspire to fame rather than to being valuable members of society. In today’s reality TV world, a person who is no one can become “someone” if they manage to get on a reality show. This makes people want to do the sorts of stupid things that allow them to get on those shows. It makes them crave fame and it makes them work towards fame rather than towards anything more productive.
Second, we can argue that they promote bad behaviors because they glamorize such behaviors. These shows are rarely about people acting in any way properly. Instead, young people see teen pregnancy or alcohol and promiscuity being presented as glamorous and appropriate. This, we can argue, influences watchers to have lower moral standards in their lives.