The typical rightwing theory would be that the immigrant population is outbreeding the natives due to much higher birthrates. Is this the story behind the sickle cell data? If we translate the percentages into absolute numbers based on the number of births in each relevant year, we get absolute births:
Sickle cell tested newborns:
Not sickle cell tested newborns:
This is a growth of 4.72% per year and a shrinking of -2.02% per year respectively. Let’s imagine there was a French population and an immigrant population established maybe in the 60ies. And both populations were breeding merrily away with a perfectly steady fertility rate. In this closed system, what kind of fertility rates would account for the growth rates we see?
Well, with a generation length of 30 years (and human generation lengths almost always fall close to 30 years even with very different fertility rates), a fertility rate of 7.97 kids per woman for the immigrant population and 1.08 kids per woman for the French population would lead exactly to the growth rates we calculated.
That’s of course insane. According to the studies I have seen on this topic, immigrant birth rates have never been close to 8 kids per woman and these days they are certainly far lower. The French fertility rate of 1.08 kids per woman is also crazy low, because we made the assumption of a steady rate over many generations. If the rate was higher in the past, today’s birth rate would have to be even lower to account for the decline. Or conversely, if the birthrate today was actually higher, it must have been below 1.0 in the past.
So how do we square the circle? The sickle cell birth rate increase has to be predominantly driven by recent immigration. That fits the numbers. Birthrates among immigrant usually drop relatively quickly towards the birth rate typical of the country. Legal immigration to France has been massive in recent years. And there is also illegal immigration, estimated by Wikipedia to lie between 80,000 and 100,000 per year . The story is probably one of young immigrants coming to France and then having kids, which also implies that a total immigration stop might lead to a reduction of the percentage of sickle cell babies.
But how can the native French birth rate be this low? The answer is probably that there is a certain amount of intermarriage, which get’s counted for the sickle cell numbers, and the overall fertility rate of ethnic French women is pretty close to that in neighbouring European countries, maybe in the vicinity of 1.4.
By the way, the current rates of growth and decline predict parity in sickle cell births and non-sickle cell births in 2022 and a 66% majority for the former in 2032. Of course, for reliable predictions a more sophisticated model is needed than just extrapolating growth rates.
I am not somebody to grieve for the French genepool, but I think this rapid change is dangerous for a variety of reasons.
It seems probably that within a decade or two, most French people will wake up to a reality were France is still 70% white, but the future is very noticeably 70% black. How will they react?
If the relative growth rates hold and at some point the political power changes hands, how will that effect the ethnic French? Even in a very peaceful best case scenario the new government will have been brought into power by a electorate much younger than the opposition, and with wages and pensions much lower than those of the opposition. In this situation a massive cut in pensions is the logical result in a democracy.
Or maybe the percentage stabilizes somewhere and French and Africans just have to live side by side. Well, the Basques, the Northern Irish, the Ukrainians and all inhabitants of Balkan states will tell you that even in Europe, having different ethnic groups in one country is not a recipe for peace. What does the trouble in the banlieues look like if scaled up five-fold?
I am not sure how big the achievement gap between second generation immigrant and the ethnic French is. But if there is a significant gap, the massive influx of lower qualified workers into the labour market will retard economic growth. That’s not gonna work wonders for ethnic relations. All in all a very worrying development, with France not the only European country in which rapid demographic change might lead to major upheaval in the next decades.
 Illegal Immigration to France