In Charlotte, N.C., the lines for the first wave of early balloting last month forced some voters to wait more than two hours. In Las Vegas last weekend, voters were still waiting outside a polling place in a Mexican grocery store two hours after it was set to close. In New York City on Election Day, voters who spilled out of polling sites snaked through schoolyards and around entire city blocks.

There are two ways to interpret these scenes.

“It does give some indication of the health of our democracy that you have all these people who are excited enough to vote that they’ll wait in a long line,” said Stephen Pettigrew, a Ph.D. candidate in Harvard’s department of government who studies polling lines. “But it’s also an indication, at least in some areas, that there is a problem.”

One problem is that some groups are much more likely to face long lines than others. Another, according to Mr. Pettigrew’s research on recent elections, is that the people who do wait are less likely to vote in the future as a result.

Early voters, urban voters and minority voters are all more likely to wait and wait and wait. In predominantly minority communities, the lines are about twice as long as in predominantly white ones, Mr. Pettigrew has found. And minority voters are six times as likely as whites to wait longer than an hour to vote. Those disparities have persisted even within the same town or county, suggesting they don’t reflect simply the greater difficulty of putting on elections in populous cities.