Consider a straightforward experiment I conducted last year: Over two weeks, I sent pairs of Latino men in their 20s to ride commuter trains in the greater Boston area, often cited as one of the nation’s most liberal regions.

These people were not asked to do anything out of the ordinary, just to wait for the train and ride it. The pairs I sent were native Spanish speakers, so when they spoke to each other, it was probably in Spanish. To gauge other riders’ attitudes about Latinos, I surveyed them before the experiment and two weeks into the tests. In each case, the trains and times were randomly selected and were later compared with a control group of riders on different trains. These trains originated in communities with very few Latino residents, and the men I sent to ride the trains were often the only Latinos at those stations on a day-to-day basis. In this sense, the experiment was testing how people react when a very small group of Latinos moves to a new community.

The results were clear. After coming into contact, for just minutes each day, with two more Latinos than they would otherwise see or interact with, the riders, who were mostly white and liberal, were sharply more opposed to allowing more immigrants into the country and favored returning the children of illegal immigrants to their parents’ home country. It was a stark shift from their pre-experiment interviews, during which they expressed more neutral attitudes.

Political scientists, economists, sociologists and psychologists have long noted that, under most circumstances, when people from different ethnic, racial and religious groups come into new contact, conflict ensues. Just look at the battles over busing students from different neighborhoods into public schools in the 1960s and ’70s.

And those conflicts often change the way people vote.