AHC: One Billion Europeans

Avoiding WWI and WWII should already do it IMHO.

Actually avoiding WWI should already do much. Prior to WWI, many European countries showed significant population growth and prosperity. If that had continued, you'd likely have a more conservative setting with higher population growth than IOTL plus millions of dead avoided plus the possible kids of those who died IOTL.

I'd also assume that a continuing "belle epoque" should preserve the traditional conservative elites much more than IOTL, leading to a more conservative Europe with higher population growth.

Then you should also avoid the Russian revolution. Without WWI, WWII, Stalin and the Soviet Union Russia alone could contribute 100 million additional Europeans.

Additionally, in a world where WWI and WWII is avoided you'd likely have enough population surplus during the first decades so that Algeria, Tunisia and Libya today would be considered part of their respective homelands with a European majority population.

Another point to consider is that without WWI, colonialism would likely drag on. Europe could afford staying in its colonies and investing there much longer. That should lead to more immigration from there into Europe during and after the colonial era - Germany today could easily have 1 million descendants of former colonials that live in the country for generations.
 
Wasn't the French population already stagnated by 1914? Declining birth rates also seem to be a function of wealth than damage by war--more affluent demographics of the United States (whites, Asians) generally have less kids than poorer portions of the population.

It's also worth noting that kids were a form of insurance Back In The Day. The average Chinese saved about 5% of his/her income before the One-Child Policy, afterwards it spiked to 33%. In Europe, a generous welfare state replaces much of this saving. Maybe doing away with old-age pensions and the welfare state could elevate European population.
 
Wasn't the French population already stagnated by 1914? Declining birth rates also seem to be a function of wealth than damage by war--more affluent demographics of the United States (whites, Asians) generally have less kids than poorer portions of the population.

France had sluggish growth for centuries compared to the rest of Europe, but in the post-WW2 they've grown the most steadily out of anyone.

So it's not guaranteed that no-WW1 would make no difference.

Of course people are also sort of forgetting how rural a lot of Eastern Europe was and how much potential that has for an agrarian-driven boom in the era of fertilizers. Plus WW1 through WW2 was exceptionally brutal in the Balkans, Poland and Russia, so basically Eastern Europe alone could make up the difference you need provided they remain somewhat more rural and poor on average.
 
Wasn't the French population already stagnated by 1914? Declining birth rates also seem to be a function of wealth than damage by war--more affluent demographics of the United States (whites, Asians) generally have less kids than poorer portions of the population.

Oh, population growth will slow down eventually. But in the better developped regions it's still some decades for that to happen, and the majority of Europe has still some decades to go until reaching the level of development in, say, Britain or Germany. Remember that even the 50s saw considerable population growth, and the subsequent reduction is probably more due to the invention of contraceptives. Although with 50 years of prosperous Europe you'd likely see contraceptives invented earlier...

And then without the wars I would assume that Europe as a whole is more conservative and more likely to have a higher birth rate, limiting the reduction of birth rates due to growing wealth somewhat.
 
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