I knew something was wrong with the U.S. when my closest college friends a year ago suggested only half jokingly that permits should be required to give birth. They were sick of subsidizing people to have babies who will only be a drain on resources, they said.

One said she supported a one child per family rule. She and her husband chose not to have children to lighten the burden on a crowded earth, she said.

I was pregnant with my third child at the time and another friend had her third sleeping in another room, so the conversation was uncomfortable.

I felt, in fact, as if I was representing a mining company exploiting the Amazon at a Natural Resources Defense Council conference. But I pressed the matter because I could not comprehend how a group of educated women would consider adopting the policy of China, where millions of girls have been aborted, abandoned, and murdered because of the law.

It was late, and we did not finish the discussion. But it turns out the U.S. faces a birth crisis, just not the one vexing my friends.

American birth rates are plummeting. The Pew Research Center late last year released data showing that births in 2011 were at the lowest level recorded, dropping 8 percent overall from 2007 to '10. Births to immigrant women dropped 14 percent during the same time period and 23 percent for Mexican immigrant women, meaning the people most responsible for increasing U.S. population are no longer making up for American-born women's declining fertility.

Long story short, U.S. women are not having enough children to replace the population and the country is on the path to shrinking in generations to come.

This is going on around the globe, even in places like Brazil that used to be a baby factory. And world population is slated to start declining in the next 50 years.

That's good news to those who see each new child as a competitor for scarce resources.

But as Jonathan Last argues in his new book, What to Expect When No One's Expecting, a falling population could spell economic disaster and make the world a more violent place.

Fewer births mean fewer workers to take care of America's exploding elderly population, for example. Last points out that the number of U.S. citizens over 65 will rise 72 percent between 2005 and '25. This almost certainly means government will spend less money on core services, more on entitlements and hike taxes - things already happening.

Last also notes research showing slower human progress since 1972 - right after fertility in the West started to implode.

On the flip side, countries with lower birth rates tend to be more peaceful. (However, they may also be more likely not to fight for any reason, because losing a child could mean wiping out the family. That could give higher reproducing, more volatile countries the upper hand in conflict.) And it's undeniable that fewer children make life easier and less expensive.

But ominously, he writes about how China's one child policy has led to a huge sex imbalance, with 123 boys born for every 100 girls.

"The inevitable result of this is a large cohort of men who - as a matter of mathematics - cannot marry." He adds, "a skewed sex ratio has often preceded intense violence and instability. So in addition to everything else, the Chinese will have a large cohort of military-aged, unmarried men - tens of millions of them - floating around at precisely the moment when the country is facing the burden of its uncared for elderly."

One of the questions that we don't yet know the answer to is how a society of people with no brothers, sisters, aunts, and uncles will behave. Will they support the elderly? Will they attend religious services or serve in the military like Americans today? Or will they consider self-fulfillment their highest goal?

On the face of it, family life will be diminished.

And if Last is right, the darker consequences will be the destruction of wealth, stability, and ultimately, happiness.

So for the sake of America, let's have more children.

- Marta H. Mossburg is an independent columnist. Contact her at marta@martamossburg.com.