It is true. It is written about extensively. What do you think happens in a nation with way more women than men for breeding pairs and marriage? In a Christian culture. With a destroyed society. Some go without. Some go abroad. Years of war and decline also flattening fertility rates. Not to mention all the women that died in the war.

You realize that human fertility rates have always been low when male death rates are high? From the stone age to now? Because they are the production and provisioning of a society. Even today all the jobs women work are largely propped up against the wealth men produce to get redistributed.

Its a researched fact. Its called a demographic echo.