Birth rates are falling pretty much around the world.

In the United States, we're averaging about 2.1 kids per woman - right around replacement levels. This fertility rate is significantly lower than it was some fifty years ago; in 1960 the U.S. average was 3.3 births per woman.

In many other Western countries, the rates are even lower. In Canada, the 2008 rate is 1.6, down from 3.6 in 1960. In Europe the rates are lower still. The European Union average in 2008 is 1.5; countries like Italy and Spain are at 1.3, significantly below replacement levels. If you want to experience the robust Italian or Spanish cultures - it would seem wise to go now.

Rates in Eastern Europe are, in general, below those in the West. Russia today is at 1.4 births per woman - despite a heroic offer from the Russian government to give its citizens a free refrigerator for every conception. The Czech Republic is at 1.2.

Fertility rates in Latin America are falling, although still largely at or slightly above replacement levels. Brazil's current rate is 2.2; Mexico is at 2.4.

Unsurprisingly, given the country's one child policy, China has a current fertility rate below replacement levels -- although, at 1.8 children per women, it's higher than many might guess. Families in the countryside often break the law; wealthy families pay the associated fines. As recently as 1960, China's rate was 5.6.

India's birth rate has also plummeted since 1960, from 5.9 to 2.8 today - still above replacement, but the direction is clear.

And some of the lowest birth rates in the world are found in Asia: South Korea at 1.2, Taiwan and Singapore at 1.1, Hong Kong at 1.0, and Macau 0.9.

What's going on?

There are lots of sophisticated answers to that question, but here's the simple way to look at it.

The highest fertility rate in the world in 2008 - 7.3 children per woman - is in Mali, a landlocked nation in Western Africa. Although it is the seventh largest country in Africa, Mali is one of the poorest countries in the world. Mali's key industry is agriculture. Cotton (an extremely labor-intensive crop) is the country's largest export.

Now, what role do kids play in an agrarian economy? They are, above all, labor. More children allow families to produce more crops. Kids are assets.

On the other end of the spectrum, Hong Kong is one of the world's leading financial centers. It has the greatest concentration of corporate headquarters in the Asia-Pacific region and has been ranked the freest economy in the world in the Index of Economic Freedom for 14 consecutive years. Macau's economy is based largely on tourism, much of it geared toward gambling.

In industrial and professional service economies, kids' roles change. Net-net, in these economies, kids cost money. They are liabilities.

Oh, we still love those little liabilities - but for most couples in an industrial or knowledge-based economy, one or two will do. As a result, as economies around the world continue to industrialize, birth rates will almost certainly continue to fall -- with implications for workforce availability in many countries.

Children have moved on the balance sheet of life.