The portable contraption is not much bigger than a toaster oven. Pop in the cheek swabs, and in just 90 minutes it can determine whether two people are related. Originally developed in collaboration with the departments of Justice and Defense, the technology, called Rapid DNA, is already in use in police departments for forensic purposes. This month, the Department of Homeland Security announced the technology’s latest application: a pilot program to test asylum-seeking families at the U.S.-Mexico border.

The verification of kinship is of interest to immigration officials because family plays a central role in immigration policy. Citizens can sponsor close relatives to immigrate, and under certain circumstances refugees have a right to join family members in the United States. The family status of Central American asylum seekers currently arriving at the border matters because families enjoy certain protections that individual adults do not. For example, a legal settlement limits the government’s power to incarcerate migrant children, and by extension their parents.

But proving kinship is not necessarily a straightforward task. The standard methods for doing so — interviewing applicants or soliciting documentary evidence like birth certificates — are time-consuming and imperfect. DNA, advocates contend, provides an objective, expedient and fail-safe method for verifying family claims. So far, however, its use has been limited by the high cost and complex logistics of testing people in far-flung places. Rapid DNA technology promises to change that.

The device itself may be cutting-edge, but testing immigrants to establish family relationships is not new at all. Immigration authorities have been experimenting with genetic testing since long before DNA. The history of their efforts, however, counsels caution. Immigration officials tout the “simplicity and certainty” of DNA, yet no matter how simple or certain, genetic tests are indelibly marked by the political context of their use. Too often, that context is characterized by discriminatory, anti-immigrant agendas.

U.S. immigration authorities first adopted genetic testing on a mass scale during the Cold War. In the early 1950s, civil war and revolution in China spurred many Chinese to emigrate. Tens of thousands of applicants sought entry into the United States claiming to be the children of Chinese American fathers. Consular officials were convinced that many of these applicants were “paper sons,” people falsely claiming kinship with a Chinese-American citizen. But paper immigration was itself a response to racist Chinese exclusion laws first promulgated in the late 19th century. While European immigrants were allowed to enter through Ellis Island in droves, virtually the only way for Chinese to gain entry to the United States was as the children of citizens.

By 1950, an estimated one-quarter of the Chinese population of the United States was made up of paper children, people whose legal status relied on fictitious relations with a citizen parent. And officials began to argue that this paper population had become dangerous. Chinese Communists, they warned, might infiltrate the country by posing as the sons of citizens. Inspired by the growing use of blood-group testing in paternity cases, the consulate and then the Immigration and Naturalization Service began using scientific methods to verify the kinship claims of would-be migrants. Within months, officials declared the Hong Kong blood program a stunning success. Testing had revealed that some 40 percent of applicants were not the children of their citizen sponsors. Given that only about half of noncompatible kin could be revealed with the rudimentary method, the true number of false claims was estimated at around 80 percent. Individuals who tested negative faced criminal charges for “blood fraud.” The program, it seemed, was an unalloyed scientific — and administrative — triumph.

But testing was neither innocuous nor neutral. For one thing, it assumed a definition of kinship at odds with the family practices of those being tested. Among southern Chinese migrants, adoption was common, fueled by high rates of infant mortality and a preference for sons. Remarriage and polygyny, in which a man had multiple female partners, were also practiced. Part of what blood tests revealed, then, was not fake families but non-biological ones. What is more, even as the government insisted on biological kinship for the Chinese, its definition of kinship for other racial-national groups was more flexible. At the very moment officials began using tests to root out Chinese paper sons, Congress passed legislation that created another sort of paper child: the international adoptee. A series of laws in the 1950s facilitated the adoption of foreign children by (mostly white) American families. The government thus applied different definitions of family for different kinds of migrants.

Ultimately, blood testing could not be separated from the broad history of anti-Chinese discrimination. After all, paper immigration had developed in response to more than half a century of Chinese exclusion laws. The government’s perceived need to test the Chinese also arose from that history of exclusion. Scientific methods might be touted as neutral, but their application could not be separated from their political context.

Today DNA has eclipsed blood-group testing, but politics continues to determine its use. In an undated policy memo from around 2009, U.S. Citizenship and Immigration Services officials called for the expansion of genetic testing. They warned that al-Qaeda operatives could slip into the country posing as kin, a fear that echoed with perfect pitch those of their Cold War predecessors. Except now the dreaded foreign infiltrators were terrorists, not Communists.

Until recently, the application of DNA to immigration and refugee proceedings was limited by cost and accessibility. Rapid DNA now appears poised for use in global refugee camps, in local immigration offices and at the nation’s border.

Even some immigrant rights advocates may be inclined to welcome this development. In the wake of last summer’s catastrophic “zero-tolerance” policy, in which the government separated more than 3,000 Central American children from their parents with no plan to reunite them, some lawmakers called for DNA testing to facilitate reunifications. Over the last month, the administration has floated the possibility of resuming family separations. Legal volunteers advising asylum seekers waiting to cross into the United States from Tijuana have begun using Sharpies to write parents’ names and birth dates on their children’s backs in case they are separated.

As a technique of kinship verification, DNA is surely preferable to a Sharpie. Yet immigration advocates should be cautious about its expanding uses among migrant populations, including those from Central America. While most family units would no doubt test positive, a small minority likely would not. Child fosterage and informal adoption among poor and working-class Central American families are common. Amid epic levels of violence, orphaned children may be raised by or flee with someone other than their biological parents.

It is not hard to predict how the administration will spin cases of non-compatibility. Officials have repeatedly and baselessly charged that Central Americans bringing children across the border are “child traffickers,” and negative DNA results would no doubt be trumpeted to reinforce that claim. As in the past, genetic testing will be used to criminalize immigrant families.

Indeed, the racist impetus that framed Cold War blood testing is still with us. Today, some 20 receiving countries use DNA tests in immigration and refugee proceedings. In practice, it is almost always nonwhite migrants from the global south who are subjected to them. The result is a two-tiered definition of family. Customs like adoption that are perfectly legitimate when practiced by citizens are vilified as fraudulent and criminal when practiced by foreigners.

Genetic tests may promise objectivity, but the history of their application in immigration policy reflects less a desire to establish kinship than an impulse to exclude. We should be wary.