To find these undocumented people, Passel has to do some detective work. Here's the basic process: For one, we know the number of immigrants the country allows in through the legal process. Also, we know, from the U.S. Bureau of Labor Statistics, how many people in the United States were born in another country. So the formula for finding the number of illegal immigrants in the country becomes, essentially, simple arithmetic.
"It's immigrants in the survey, minus legal immigrants, equals unauthorized immigrants in the survey," Passel says. It's actually a bit more complicated than that, involving a statistical correction for the number of people who are unlikely to make it into the survey.
Under this methodology, we arrive at the 11 million figure. But population size doesn't say much about who these people are. So Passel digs further.
He assigns statistical probabilities to the BLS data set to indicate who among the survey respondents might be illegal (the data does not include personal identifiers). It then becomes a process of logical elimination to find which of the 40 million immigrants in the BLS data are also here illegally.
Passel offers some examples. "We say, this person is a veteran, this person served in the armed forces, so we're going to say they are legal," he says. "This person is a certified public accountant — we're going to say they are legal.... This person is not getting welfare, this person works, this person is a construction worker — so they may be legal or illegal."
Through this roundabout process, we've come to our best guess at the makeup of the undocumented-immigrant population. If a pathway to legalization is affirmed, this picture will only become more clear.
Through his research, here's what we know about the 11 million.
They've come here to settle down.
"The unauthorized immigrant population is, to a degree that I think that is largely unappreciated, a group of families with children," Passel says. Twenty-one percent of nonimmigrant households are couples with children. In other words, nuclear families. In households of illegal immigrants, that number jumps to 47 percent, according to 2008 data.
"More than half of the adult men are in families, are married, or have a partner," he says. "And most of them, 80 percent of that half, have children here. The women represent about 40 percent of the adults, and almost all of them are not here by themselves." Part of this is due to difference of ages in the sample. The undocumented families are younger, and therefore more likely to have children. Nonetheless, the data is indicative of people who have come here to raise a family.
They've been here for a while.
According to 2010 Pew data, 63 percent of unauthorized adults had been in the country 10 years or more.
They're mainly from Mexico.
"Any place that sends us immigrants, we have some unauthorized immigrants from," Passel says. But around 55 to 60 percent are from Mexico, "which translates to over 6 million people. There is no other country that has sent as many as 500,000."