The millennial generation in the U.S. has been labeled with all kinds of superlatives: most educated, most racially diverse, most social media-savvy, most liberal.

But one category where American millennials are underperforming compared to other generations is procreation. In 2012, Americans had children at a pace of 948 births per 1,000 women in their 20s, which is “by far the slowest pace of any generation of young women in U.S. history,” according to a new analysis by nonpartisan research group Urban Institute.

The report, released Tuesday (April 28), analyzed data from the Centers for Disease Control & Prevention. Birth rates for American women in their 20s dropped more than 15% between 2007 and 2012.

So what's behind the sluggish numbers? Well, over the past few years, the child free movement has gained steam. It's addressed the social pressure to have kids and introduced the DINK -- Dual Income, No Kids -- lifestyle. Part of the decline can also be traced back to the declining marriage rates among young adults.

But Urban Institute's report suggests the real driving factor is the crappy economy. The drop in birth rates hit just before the recession in 2007 and continued on through 2012, just after the recovery's effects were being felt. The Wall Street Journal reports that six years after the recession ended, fertility numbers have yet to bounce back, perhaps because Americans are still skittish about the economy.

Say what you will about millennials, but it would appear they're especially cautious when it comes to building a family.