In the 1950s, a study by Harvard University of seating on passenger trains found that 18 inches of width was the minimum needed to comfortably accommodate passengers. But narrower 17" seats to add less weight, which makes planes cheaper to fly. Airlines have also found that by spacing seats closer together, they can cram in additional rows.
How do they get away with it? Legacy airlines' best customers are often given complimentary domestic upgrades based on their elite status. First Class seats remain plenty comfortable. If you're an occasional flyer, though, you're going to be stuck in the "cattle car."
Prefer comfort? Low-cost carriers offer the most spacious Coach seats and the greatest reclining seat pitch. JetBlue's seats, 18" wide with at least 34" of seat pitch, stand out at the top of the list.