The most common theory: A sizable percentage of poll respondents, though willing to punch a phone key to say they support Trump, are still too embarrassed to actually tell another human being.

Last week, research firm Morning Consult put this hypothesis to the test. Recruiting 2,397 registered Republicans and Republican-leaning voters online, the company split the sample into thirds—sending one group to answer election questions on a web site, another to an automated interactive voice response phone line, and the rest to a call center staffed by live interviewers.

Critically, this survey drew its respondents from the same general pool, which had all answered demographic questions beforehand. The only variation was the mode of interview.

The findings, released today: The Trump mode effect is definitely real. Just over 38 percent of people who answered via a web form said they supported Trump, compared to 32 percent of their peers who spoke to a call-center employee, a 6 percentage point gap. But that gap, among college-educated respondents, widened to 9 percentage points.