President Obama's decision to defer the deportation of more than 4 million illegal immigrants will have a significant impact on one nationality in particular: Mexicans, who make up the vast majority of undocumented workers in the United States.

However, while immigration between the two neighbors is often portrayed as a one-way street, in reality the relationship is more nuanced. Not only does the US economy rely on many undocumented workers, Mexico has had its own problems with illegal immigration. Sometimes, they are US citizens.

It's estimated that 1 million or so American citizens live in Mexico, many of them retirees who head south of the border to enjoy the warm weather, great food, and lower cost of living. These retirees are generally welcomed (they bring with them economic benefits,) and Mexico has a visa for such people — the "Residente Temporal" which targets people who do not work in Mexico and are economically self-sufficient.

Whether out of ignorance or a desire to get around the sometimes complicated nature of Mexican immigration law, however, some US citizens in Mexico do not get documentation or end up overstaying their legally allowed period. It's unclear how many illegal immigrants from the United States reside in Mexico, though it is thought to be significant.

"No one really knows how many of them there are in Mexico," Monica Mora, an expert on American migration in Mexico, said in 2012. "They are usually people who live for a while in Mexico and then return home. They do not stay indefinitely."

Mexico has occasionally gone out of its way to politely, yet firmly, remind US citizens heading to major retirement areas - such as Lake Chapala - to get their visas. Following new legislation in 2008 that did away with a maximum 10-year prison sentence for those who could not produce proper documents, the punishments for illegal immigration can be lax. As long as they have not committed a crime, these people are usually not deported, but are required to pay a fine and get legal documentation to stay in Mexico as soon as possible. It is, in effect, an amnesty.

While figures are hard to come by, some argue that US citizens may make up the vast majority of illegal immigrants in Mexico. A large number of illegal immigrants can also come from Central America. These people are often on their way to the United States, though there are also significant numbers of Guatemalans who travel to work illegally in Mexico.

In the past, many have felt that authorities focused unfairly on Central Americans and ignored illegal US migrants.

"The biggest population of foreigners here is Americans, and an important percentage of them live here without documents," Patricia de los Rios, director of the migrant-affairs program at Iberoamerican University in Mexico City, said in 2010. "There, we see racism in reverse, because if you have an American accent and are light-skinned the police are not going to treat you the same as if you look Honduran."

Data from the Mexican Secretariat of the Interior’s Migration Policy Unit seem to support that. In 2013, Mexico deported 80,079 people, with the vast majority coming from Honduras (32,800,) Guatemala (30,005,) and El Salvador (14,427.) In contrast, just 690 Americans were deported last year. Those numbers may well be more askew in 2014.

And following a highly publicized and US-prompted crackdown on illegal immigrants from Central America that saw tens of thousands of people deported this summer, those figures are likely to be even more skewed in 2014.