That may be quite true... but perhaps for different reasons...Actually I what I mean is, out of the bulk of purported possession cases, how many possessed were/are believers or religious.
Out the little readings I've done about the subject, demons appear to affect to those who previously believe in that kind of events.
In highly Catholic populations, for example, seems to be accepted, common, I'd even say.
Places with a higher numbers of atheists or non believers in general on the other hand...this what I wonder.
If you are an atheist, you aren't necessarily seeking, pursuing or delving in the spiritual realm. They aren't "inviting" spirits to come into their lives. Not that an atheists can't be but if you aren't seeking and delving into the spirit realm, obviously you are less likely to encounter the situation.
My wife's family, though Catholic... by birth, read tarot cards and mixed religion with witchcraft. So, yes, in many parts of the world Catholicism and demon possession can occur (at least in the Latin Catholicism.)