LONDON — China’s decision to allow more families to have a second child is an effort to confront a problem that is facing much of Europe, too — aging populations and not enough babies. But reversing a demographic slide involves a complicated set of incentives that have more to do with social mores than with government policies, experts say.

Studies indicate that countries with healthy demographic trends are not those that promote birth, but those with higher levels of gender equality, of trust within society and of immigration.

So even for authoritarian China, raising the fertility rate will not be simple.

Examples of countries that recover from low fertility rates are rare, scholars and experts say. Immigration can play a positive role — not because immigrants have many more children than natives, but because they tend to be of childbearing age and have their children in their new countries.

Germany, for example, has a low fertility rate of about 1.4 or 1.5 children born per woman, well below the replacement rate of 2.1. The problem has been especially acute in the former East Germany, similar to the situation in other post-Communist nations, leading to some dire predictions of a shrinking German population.