It has been long assumed that nations with rising incomes will experience a falling fertility rate and eventually a shrinking population. The standard reason? The wealthier a woman gets, the fewer children she wants.

But a new study suggests such a future may become the past – implying big demographic implications for policymakers.

In some rich countries, the number of children born per woman has dropped far below the 2.1 rate needed to maintain current population. In Italy, for instance, the rate is just above 1.3. To supply enough new workers, Italy would need high levels of immigration – a problem more worrisome to Italians now than a declining fertility rate.

The study, published in the journal Nature, finds that the trend line linking prosperity and smaller family size may be reversing itself. In countries that attain an extremely high Human Development Index (HDI) – a United Nations metric that factors in education and income levels, along with average life spans – the fertility rate is actually going up again.

Countries such as Italy, Spain, the Netherlands, Germany, and Sweden are beginning to swing back toward replacement fertility levels, though a high HDI alone may not be enough to return them to population stability, let alone growth. (The US population continues to grow steadily, but largely because of immigration.) Other studies have shown that immigration plays only a modest, not decisive, role in rising fertility rates.

While 18 of the 26 highest HDI countries show this reversing trend, Japan, South Korea, and Canada stand out as not fitting the pattern. Attitudes toward women, particularly in the workplace, may account for the aberration of Japan and South Korea. Canada remains a mystery. In any case, more research is needed to both confirm this study's conclusion and suggest further reasons why it happens.

The study's authors – two from the University of Pennsylvania and one in Italy – speculate that high HDI countries may be more friendly to females in general, providing women with state-run childcare and giving them more opportunity for education and employment. Perhaps more women are simply affluent enough to afford their own child care (or have husbands more willing to help at home).

Women who are better off and better educated may be more confident that they can take time from work for child-rearing and still be able to reenter the work force later after children enter school with little or no penalty.

Environmentalists need not be alarmed by these modest upticks in population in prosperous countries. The real battle with overpopulation remains in the much more populous developing world.

Poverty and political uncertainty breed larger families, perhaps – in part, at least – as unconscious insurance that at least one or two of the children will survive to adulthood and provide support for the parents. In these countries, raising the income and education standards for women has been shown to be effective in reducing family size and taming runaway population growth.

Under any economic conditions, family life is something to be treasured. The joys of raising children and aspirations for higher living standards ought not to be at odds with each other. This study makes the encouraging claim that they are not.