Episcopalians tend to be better educated and tend to reproduce at lower rates than other denominations.
From Mark Steyn:
If you measure the births of the Muslim world against the dearth of Bishop Kate's Episcopalians, you have the perfect snapshot of why there is no "stability": With every passing month, there are more Muslims and fewer Episcopalians, and the Muslims export their manpower to Europe and other depopulating outposts of the West. It's the intersection of demography and Islamism that makes time a luxury we can't afford.
This is but a mere sampling, read it all. Steyn's ability to eviscerate the fatuous is a wonder to behold, nobody does it better. As Waugh said of Wodehouse, "He is the master of us all." (Thanks to Bro. James.)