I can't find a decent answer for this anywhere. Basically up to and including the early parts of the 19th century France had a very large population compared to other European nations, part of the reason why it did so well militarily. Wikipedia even states that France was more populous than Russia before 1795, and if you look at France from around the 1600s during the reign of Louis XIV up to Napoleon coming to power in the 1790s, this demographic advantage allowed it to expand its borders frequently, and become the leading power in Europe.

However, starting in the early 19th century, the growth of French population stagnated in comparison to the other powers. This occurred despite the fact that the UK, Germany and others suffered population loss due to emigration to North America, Australia and New Zealand, or other places around the world.

This correspondingly had an impact on France's ability to project power. In my reading concerning the World Wars, I remember coming across the statistic that starting out from rough parity during the Franco-Prussian war in 1870-71, by WW1 there were 1.6 German men of military age to every 1 Frenchman. This resulted in France becoming a highly militarized society including a 3-year national service period starting in 1913, as well as 80% of Frenchmen being retained for national service compared to about 50% in Germany. I believe this was basically the only way for France to match the size of the German army. With the losses incurred in the First World War, this disparity grew to an imbalance of 2 to 1 men of military age by the time of the Second World War.

So what caused this demographic decline? What are the leading theories and what are the most plausible reasons for it? I've heard of people saying that the revolutionary ideals having an effect but I've never really found a coherent answer that makes sense.

Thankyou.