A question I do have about Europe though... why are the birth rates so low in many countries? They aren't low here.
It would horribly simplifying things to say this is the sole cause, but Europe is much more densely populated than the United States. Even in the United States, those living in denser areas tend to have fewer children, often below replacement level.
There are also cultural factors; northern Europe reproduces at or close to replacement level, but Mediterranean Europe reproduces far, far below replacement level. Some of this is due to differing social support systems. A common argument is that Italian, Greek and Spanish government policies which favor mothers staying home with children have in fact caused a severe reduction in the birth rate in those countries. (Indeed, across Europe, there is an extremely strong correlation between more women in the workplace and higher birth rates, although this runs counter to conventional wisdom.)
