LONDON • China's decision to allow more families to have a second child is an effort to confront a problem that much of Europe is facing too - ageing populations and not enough babies.

But reversing a demographic slide involves a complicated set of incentives that has 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 are those with greater gender equality, trust within society and immigration.

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

Examples of countries that recover from low fertility rates are rare, scholars and experts say.

Immigrants can play a positive role because they tend to be of childbearing age and have 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. But Chancellor Angela Merkel's decision to take in a large number of migrants and refugees of childbearing age is almost certain to have a positive effect on the birth rate.

In the 1960s and 1970s, birth rates in many European countries were low.

In later decades, birth rates in many, such as Germany and Austria, remained low, "but the Nordic countries and France reversed", said Stockholm University's professor of demography Gunnar Andersson. That was partly because of social policies and attitudes in those countries, which promote gender equality and are more friendly to women in the workplace, with leaves and day care.

But the main explanation, demographers said, was that because women were working and had more opportunities, they were having children later, not that they were having a lot more children.

Oxford University's emeritus professor of demography David Coleman said: "In Western Europe since the late 1960s and 1970s, there has been a major trend towards postponement of fertility going hand-in-hand with the emancipation of women in education and the workplace ."

As women grew older and decided that if they wanted children they had better get to it, he said, "the birth rate of those in their 30s and 40s has been going up".

War and economic change matter, too, said Associate Professor Stuart Gietel-Basten of Oxford University's social policy department. Women tend to postpone having children in chaotic periods and catch up later , he said.

Social policy can also help but is not a complete answer, said Professor Francesco Billari, who specialises in sociology and demography.

"A basic element of fertility rebound is a society with increasing gender equality," he said. "When women are in the labour market and social policy helps them and men do more childcare in the household, fertility bounces up."

Prof Billari cited Italy, where the population has continued growing modestly despite traditional worries about abortion, divorce and decline. But while historically richer northern Italy once had fewer children and poorer southern Italy had large families, "now that has completely reversed", he said.

Women in the richer north which has more gender equality and job opportunities are having more children than before, he said.

Women in the south are in a society with high unemployment and a "more traditional gender division of labour and lack of female participation in the workforce", he said, adding that "it's more like China".

The lack of gender equality, small numbers of working women and few social policies to support them help explain why Russia, Central Europe and East Asia generally have not been able to bounce back from low fertility levels, Prof Billari said.

China will be watched carefully, but those experts believe that the new policy will have only limited effect.

They said that is partly because generations of Chinese have grown up with the experience of small families and partly because China's social and housing structure does not favour large families.

"We've seen reforms in the past where some couples are eligible to have a second child and often they don't," Prof Gietel-Basten said.

"There will be a small baby boom, especially in poorer, more rural areas like Sichuan where the policies have been strict," he said.

But in China, as in most of Asia, fertility will not be aided by immigration, since there is little immigration in East Asia, Prof Gietel-Basten added.

NEW YORK TIMES