1. Much of the inland West is populated by people who culturally value high reproduction.

2. People immigrate to developed countries.

I was curious, so I looked at US fertility rates by state. Fortunately, wikipedia has a nice chart. The birth rates do appear to be higher in Western states than in the east.

http://en.wikipedia.org/wiki/List_of_U.S._states_and_territo...

It may be cultural, but it may just be economics.

I'd like to see that list controlled for cost of living. States away from the East and West Coasts tend to be much more affordable, which makes having extra kids more feasible.

http://www.missourieconomy.org/indicators/cost_of_living/ind...

EDIT: Scroll about halfway down for a ranked list similar to the one on Wikipedia.

I was referring mostly to Mormons, though there's a large Catholic population from the Hispanic population in the southwest.

I didn't have any particularly prolific group in mind. I was thinking that ascribing birth rates to culture and morality might be premature, especially considering the low birth rates in other developed countries.

People don't reproduce in New England apparently

Considering the popularity of traditional anti-birth-control ideas, the hypothesis has some merit.

In both American political parties, hardly anyone is anti-birth-control. Even if you poll only Catholics, anti-birth-control people are a tiny minority. The war on birth control is about as real as the War on Christmas.

http://www.gallup.com/poll/154799/americans-including-cathol...

Some people want to defend others' rights to conscientiously object to birth control (such as opting out of certain health insurance coverage), but that's more in the vein of Voltaire's famous sentiment. More like, "I do not agree with your morality but I'll defend to the death your right to have it."