That means that curbing birthrates tends to lead to stability, and that's where educating girls comes in. You educate a boy, and he'll have fewer children, but it's a small effect. You educate a girl, and, on average, she will have a significantly smaller family. One robust Nigeria study managed to tease out correlation from causation and found that for each additional year of primary school, a girl has 0.26 fewer children. So if we want to reduce the youth bulge a decade from now, educate girls today.