One of the things I really hate are unqualified linear projections. They're so useless most of the time. A science fiction magazine will give you more insight about the future than the United Nations population projection for the year 2100. This is just as much of an issue when it comes to American Census demographic projections. As I've noted before population projections of the coming non-Hispanic white minority 2040 to 2050 are sensitive to the assumptions behind the basic parameters. The logic of the projection is crystal clear and airtight, but just because a certain set of assumptions holds today, does not mean that those assumptions will hold indefinitely (though the Census projections are much more plausible than the United Nations projections because two generations are so much more strongly impacted by by the inertia of current conditions that four generations). In the 18th and 19th century white Americans, and especially the Anglo-Saxon founding stock, were a highly fertile folk. They took over the American Southwest and the Northwest in large part due to their demographic assault. In New England the 30,000 of 1650 became the 700,000 in 1790 in large part due to fertility rates on the order of 7 per woman! Today no one would expect that Anglo-Saxon Americans would be so fertile, let alone the New Englanders who were prominent in the population control movements of the 20th century. In the 17th and 18th century the Jews of Eastern Europe were a highly prolific group, and the gentile majority in places like Poland viewed the waxing of the proportion of this minority with great suspicion. Today no one views the Ashkenazi Jews as demographic engines, though in places like Israel the fecund Haredi have now helped close the "birth gap" with the Arab population, as its fraction of the Jewish population keeps increasing. I can give you other "counter-intuitive" examples from the recent past, but a little history goes a long way in teaching suspicion (e.g., in the Balkans in the late 19th century rural Christian populations had much higher fertility than urban Muslim ones). These sorts of reversals are not inexplicable. Fertility shifts occur, sometimes within a generation or two. This is why Thomas Malthus turned out to be wrong: he didn't predict the demographic transition. But we shouldn't be complacent and assume we've reached the "end of history" when it comes to fertility transitions. In the early 20th century there was great terror in the American elite due to the immigration of what would later be termed "ethnic whites," in particular Jews and Southern Europeans. And yet the Jewish proportion of the American population peaked in the late 1940s at ~5%. What about the other groups? The General Social Survey has large sample sizes for some ethnic groups, so I decided to look there. The CHILDS variable reports the number of children the respondents have. I limited the data set to whites after the year 2000. I used COHORT to track the number of children according to what decade the individuals were born (so older cohorts have more children since they have more time). I focused on cohorts between 1930 and 1980. For ethnic groups I have British, which includes English, Welsh and Scottish. German, which includes Germans and Austrians. Nordic, which includes Scandinavia and Finland. Since Jewish low fertility is non-controversial, I decided to look at Italians as the primary white ethnics. Irish occupy a middle position, having arrived along with the Germans in the middle 20th century, but retaining a more ethnic stamp because of their dominant Roman Catholicism (it seems that history indicates that Catholic Germans retained a strongly ethnic identity than Protestant ones, because the latter could assimilate easily into mainline Protestant denominations and become WASP. And Catholic Irish who became Protestant where very quickly assimilated, especially in the South).
The Italian American concentration in Northeastern urban areas is not conducive to a high fertility regime. In contrast, Germans are distributed across the "Heartland," where family size is less constrained by the cost of living. What's going on with the British? Poking around the data indicates that this group is very heterogeneous. New England British have much lower fertility than those who live in the South, for example. As the proportion of Southern British increases the fertility for this group may bounce back. My point here is that I think you could make educated guesses about this pattern by 1930, by noting that cities tend to lead to smaller family sizes, and some immigrant groups, such as Italians, were remaining in urban areas. Many WASPs do feel that American cities are strange and alien places, and they have felt so for over a century. But the large concentration of rural WASPs will likely always serve as a demographic check to their decline, as immigrants pooling around urban zones shift to a lower fertility regime.
