At the Democratic debate in Des Moines, former Maryland Gov. Martin O’Malley shared a statistic that he said you would never hear out of one of the leading Republican presidential candidates.

"Let’s say it in our debate because you’ll never hear this from that immigrant-bashing carnival barker Donald Trump: The truth of the matter is net immigration from Mexico last year was zero. Fact-check me. Go ahead, check it out," O’Malley said.

Challenge accepted.

We actually wrote about this back in June when former President Bill Clinton said something similar. When we did that fact-check, we didn’t have the 2014 data yet. So let’s revisit what we found with the addition of the most recent data, which is the crux of O’Malley’s claim.

One way to measure net migration -- the number of new arrivals versus the number who leave -- is the count of people living in America who were born in Mexico. The Census Bureau tracks this. We used two sets of numbers from the American Community Survey -- an estimate based on three years of surveys and an estimate based on just the year the survey took place. (For the 2014 data, a three-year estimate is not currently available.)

Year Born in Mexico 3-year average estimate (1,000s) 1-year estimate (1,000s) 2014 n/a 11,714 2013 11,604 11,585 2012 11,650 11,563 2011 Featured Fact-check Democrats and Joe Biden “want to abolish the suburbs altogether by ending single-family home zoning.” 11,700 11,673 2010 11,642 11,711

Source: American Community Survey

According to Census Bureau data, the number of U.S. residents born in Mexico dropped each year from 2010-13. It bounced back up in 2014, but it remains lower than the peak in 2007, said Alex Nowrasteh, an immigration policy analyst at the libertarian Cato Institute.

"That is not a small statistical difference, but it's small compared to the days of rapid Mexican immigration from 1994 to 2007," Nowrasteh said. "O'Malley was mostly correct. The population increased but it was small. Mexican immigration certainly isn't surging anymore."

The margin of error is large enough that the actual number of Mexican-born people living in America in 2014 could be lower than it was in 2013.

Steven Camarota, director of research at the Center for Immigration Studies, a group that favors reduced immigration, told us back when we looked into Clinton’s claim that the Mexican-born figures are a valid indicator of the overall trend.

While there’s a little variation, both estimates show fewer people from 2010 to 2013.

"This strongly implies zero net (migration)," Camarota said.

Four reasons for the decline

Why is migration from Mexico to the United States slowing down?

Demographics is possibly the biggest driver. The birth rate in Mexico has fallen steadily since at least 1990 and now stands at 2.2 children per woman. Marc Rosenblum at the Migration Policy Institute said that translates into fewer people coming into the workforce.

Rosenblum told us that apprehensions at the border strongly suggest that fewer people are trying to sneak into the country. Data from the U.S. Border Patrol show a drop of more than 50 percent since 2009. It used to be border agents stopped over half a million people coming in from Mexico. Last year, it was about 230,000.

More surveillance, stiffer penalties and more barriers at the border have had the intended effect, he said.

Other drivers include the hit on the construction industry during the recession. That was a sector that drew many immigrants, and while it has recovered a bit, it is not nearly as robust as it once was.

Plus, according to one survey, Mexicans who return to their native country generally find that they make more money than when they left.

Our ruling

O’Malley said, "Net immigration from Mexico last year was zero. Fact-check me."

Census data from 2010 up to 2013 support O’Malley’s point, as the population of people living in America but born in Mexico fell by about 40,000 since 2010. Experts said this is a strong indication of a prevailing trend. That figure actually rose slightly in 2014 -- though remaining significantly lower than the peak number of Mexican-born Americans in 2007. And the 2014 number had a large enough margin of error that it’s possible there was an actual drop from 2013.

Other data, including a rapid decline in apprehensions at the border and changing demographics in Mexico add weight to the conclusion that O’Malley’s point is correct.

With a caveat that the measurements aren’t perfect, we rate this statement Mostly True.