The study had a complicated design, and I invite readers who are so inclined to ascertain the details for themselves. Here is Enos’s description of his experiment:

Under the assumption that people with similar characteristics tend to ride the train at the same time, I selected pairs [of trains] that were close together in time so that the treatment units [train platforms onto which Spanish-speaking confederates had been inserted] within each station would have similar passengers. Within a matched pair of train times at each station, one was randomly assigned to treatment and one to control, resulting in 18 matched pairs of train times. This design means that we should expect subjects in the treatment and control conditions to be, in expectation, identical.

Enos continues:

Subjects were exposed to the same Spanish-speaking persons in a location near their homes for an extended period, as would be the situation if immigrants had moved into their neighborhood and used the public transportation.

The Spanish-speaking confederates reported to Enos that

persons noticed and displayed some unease with them: for example reporting that “Because we are chatting in Spanish, they look at us. I don’t think it is common to hear people speaking in Spanish on this route.” After the experiment, the confederates reported that other passengers were generally friendly to them but also reported that they felt people noticed them for “not being like them and being Latino.”

Members of the treatment groups and control groups were surveyed before and after the two weeklong experiments in an effort to identify the effect of exposure to Spanish-speaking people. In both surveys, respondents were asked three questions about immigration along with other more general questions:

1. Do you think the number of immigrants from Mexico who are permitted to come to the United States to live should be increased, left the same, or decreased?

2. Would you favor allowing persons that have immigrated to the United States illegally to remain in the country if they are employed and have no criminal history?

3. Some people favor a state law declaring English as the official language. Some other people oppose such a law. Would you favor such a law?

How did the respondents’ answers change?

Treated subjects were far more likely to advocate a reduction in immigration from Mexico and were far less likely to indicate that illegal immigrants should be allowed to remain in this country.

The experiment, Enos wrote,

demonstrated that exclusionary attitudes can be stimulated by even very minor, noninvasive demographic change: in this case, the introduction of only two persons.

In his 2017 book, reflecting on the results of his experiment, Enos is more direct:

The good liberal people catching trains in the Boston suburbs became exclusionary.

Exposure to

two young Spanish speakers for just a few minutes, or less, for just three days had driven them toward anti-immigration policies associated with their political opponents.

Enos examined national precinct and county-level voting results in recent elections to see what effect a black president, Barack Obama, had on whites living in segregated areas as opposed to those living in unsegregated areas.

In the 2008 election, Enos found that with a black Democratic nominee,

white voters in the most-segregated counties were between five and six percentage points less likely to vote for Obama than white voters in the least-segregated counties.

That pattern had not emerged in the previous four presidential elections when the Democratic nominee was white.

“Every time a white Democrat had run going back to 1992 — segregation had had no such effect on the vote,” Enos wrote:

In 2008, this was a massive effect of segregation: the gap between the most and least-segregated counties was almost equivalent to the gap between men and women.

Enos then looked at results from 124,034 precincts, almost every precinct in the United States. Again:

A white voter in the least-segregated metropolitan area was 10 percentage points more likely to vote for Obama than a white voter in the most-segregated area.

These voting patterns, according to Enos, reflect what might be called a self-reinforcing cycle of prejudice.