I'd like to believe it, but it doesn't explain why the world population is rising. If the theory was true, the world population would be static... unless we managed to pick up a few extra souls from somewhere.
I believe it happens, but not to everyone. People have different souls; no one person is alike. And I believe that a soul is unique and can't be altered.
I've heard this theory, but it's dumb. All theories about "life after death" are dumb.
There is no life after death. It's just a story to help you come to terms with it better. Notice how religious people are happier when they're dying, because they'll "finally get to see their loved ones that have already passed on again?"
The facts are that all things must come to an end, even the life of us mere mortal humans. We die, and our bodies are either cremated or buried in a box to decay over hundreds of years.