A quirk in the way most states determine food stamp eligibility has resulted in an unusual bias in favor of granting benefits that have been denied to families comprised solely of U.S. citizens to identical families with an illegal immigrant (or other ineligible alien) wage earner.

According to a Center for Immigration Studies report, the semi-complex equation states use to determine food stamp — officially known as the Supplemental Nutrition Assistance Program (SNAP) — benefits is at the heart of this disparate treatment.

“A bias exists against those here legally when calculating eligibility for food stamps,” David North, a CIS fellow and author of the report, said in a statement. “This overt bias, which most legislators are probably not even aware of, translates into an estimated 1.4 billion dollar cost to tax payers.”

So where is the calculation hiccup?

While two families might be identical in size and income, most states will prorate the income of mixed-immigration status families when determining eligibility for SNAP benefits if at least one of the wage earners is an illegal immigrant or ineligible alien. In other words, families that are on the cusp of eligibility based on income are most likely to receive benefits if at least one of the wage earners in the family is in the U.S. illegally or is a legal immigrant who has been in the U.S. on a green card for less than five years.

For example, as North detailed in his report:

Let’s say that the all-citizen family consisted of three people, employed father, stay-at-home mother, and a small child. Dad makes $2,400 a month. The family’s income is too high for food stamps since the maximum monthly income is $2,177 for a family of three. Then next door there is a mixed family, also three people, with the father being the only worker, also earning $2,400 a month. The difference is that the father is an ineligible alien and so, under many states’ regulations, one-third of the family’s income is ignored (prorated is the word in SNAP circles), leaving the family with a nominal income of $1,600 a month that allows the family to get a food stamps allotment, but only for the two citizens, not for all three in the family. There is thus a band of households of three with earnings in the range of $2,177/mo. at the bottom to $2,589/mo. at the top that would be eligible for food stamps but only if the wage earner is a non-eligible alien in the eyes of USDA; all-citizen households in this band would not be eligible for food stamps. (Here’s the math: $2,589 = 150 percent of $1,726, the maximum income allowable for a family of two when there is a 33.3 percent discount on the illegal’s wages).

According to North, the vast majority of states employ this unusual method for determining eligibility. Arizona, Guam, Massachusetts, New Mexico, North Carolina, and Utah are the only states and territories that include ineligible aliens’ income in their eligibility calculations. However, as North detailed in his report, while policy within the five states and Guam use the total income method, even some counties within those states still use the prorated formula.

“In sum, most of America is covered by the proration in food stamps policy,” North wrote in the report.

He estimated that the calculation quirk means that some 460,000 households receiving $1,407,600,000 a year are benefiting from the calculation gimmick.