The cities that scraped the bottom of the cleanliness category may show signs of grit, grime, or muck, but they all have some basic features in common: they’re big, and most have a bustling nightlife. Partiers tend to be on the younger side—and that may add to the disarray. “Studies have consistently found that youths and young adults are the most prone, or willing, to litter,” says Steve Spacek, author of the “American State Litter Scorecard,” which has highlighted the less-than-pristine conditions in such top 20 cities as New Orleans, Chicago, and Las Vegas.

To be fair, though, plenty of “dirty” cities are working hard to mitigate the squalor. Boston recently installed 400 solar-powered trash compactors on city sidewalks to keep cans from overflowing. In Las Vegas, a new city ordinance decrees that those guys handing out adult “advertisements” on the Strip sidewalks now have to pick up dropped flyers every 15 minutes.

Plenty of people, meanwhile, are willing to accept a little dirt in exchange for a great city. “New Orleans is a living, breathing antique,” says Cindy Denney, a talent manager in the Crescent City, which went from No. 1 to No. 2 this year. “What makes it great is also what makes it funky—old wood, mismatched bricks, and peeling paint, from daily heat and moisture. We pay more attention to cuisine, music, and football than we do dirt.”