You'll never look at, or reach into, an airline seat-back pocket the same after reading this.

Besides being a repository for magazines, newspapers, books, iPods and air-sickness bags, seatback pockets get stuffed with all kinds of disgusting trash, from toenail clippings to mushy meals.

People do things on airplanes that they would never do in other public settings. They pluck eyebrows, polish nails and pick noses. They stick chewed gum in places only other passengers will discover. They blow noses into blankets that get folded up for the next weary traveler. They prop bare feet up on bulkheads and seats. Sometimes they even engage in sex acts.

One reason frequent fliers and flight attendants perceive an increase in offensive behavior may be the decline in air service — customers seek retaliation for late flights, snippy workers, lost baggage and unavailable upgrades.

"Increasingly, passengers are certain that the airlines are not on their side and actually don't care anything about them," said Irwin Sarason, a University of Washington psychologist in Seattle who has studied passenger behavior. "In that kind of environment, it isn't too surprising that people will not exercise the restraints they normally would."

Though crammed together elbow-to-elbow in more-public conditions than you'd find at a shopping mall, restaurant, church or office, airline passengers sometimes behave as though the cabin were their own small nesting place — and one where they never have to worry about cleanliness, either.

Steve Cuzzone, finance director for a Birmingham, Ala., manufacturer, has found old french fries, a festering baby diaper, half a hamburger, used Kleenex and wet napkins in seat-back pockets. He put a book in once and pulled it out to find the bottom covered in a melted candy bar.

"If you sit in a middle seat, never look in — those are the riskiest ones," he said, noting that children often sit between parents and that passengers will dispose of their grossest things in an unoccupied middle seat.

Patrick Kerr, who like Cuzzone participated in a FlyerTalk.com online discussion among frequent travelers of disgusting things people do aboard airplanes, was flying from Reno, Nev., to Dallas when a nearby passenger put a wad of chewing tobacco in his mouth and spit juice into a plastic cup throughout the flight.

As passengers left the plane, Kerr, a technology manager in St. Louis, said, the man made one last deposit then tucked the cup deep into the seat pocket.

"I said, 'Hey, you left that.' And he said, 'It's OK. They'll get it,"' said Kerr, who then alerted a flight attendant.