“Wales was the primary Celtic holdout in Western Britannia during the ascendance of the Anglo-Saxons. Using computer analysis, the researchers explored how such a pattern could have arisen and concluded that a massive replacement of the native fourth-century male Britons had taken place.
http://harvardmagazine.com/2009/07/who-killed-the-men-england
If you maake terms, you get a new boss.
The Anglo-Saxons did not speak Welsh.
Yet as recently as 100 years ago, 40% of Wales did.
Welsh is still a living language today.
Another factor: when you are conquered and you are a low-status native, it will take you longer to find a wife if one is available.
The conquerors will have taken all the youngest and most attractive.
And the one you eventually find will not have as many children.