The airlines have also failed to establish property rights for armrests, but there is a generally understood code for that: The person in the aisle has room to stretch, the person in the window seat has the fuselage to lean against, and the person in the middle has nothing, so he or she gets the armrests.

Most reasonable people agree, although I once had a seat mate on the aisle who insisted the armrest was his. After a little jockeying for ownership, he began pulling hairs from his nose and lining them up on the armrest. “You win,” I said.

But I have a moral code for users of the Knee Defender. If those in front of you never notice or care, then no harm done. When they do notice, what usually happens is that they complain to a flight attendant that the seat is broken. The flight attendant will know immediately what’s up. In that case you remove them at once and, if necessary, apologize. (No major airline condones their use because they can cause a ruckus in the cabin, as happened last Sunday when a pilot had to divert a plane to eject two unruly passengers.)

Mr. Goldman, a former Senate aide who got the idea during a long flight, says people don’t want to use his devices but have no other solution. He offers a card on his website that he suggests you give to the person in front of you. It says, in part:

If you would like to recline your seat at some point during the flight, please let me know and I will try to adjust myself and my Knee Defender so that it can be done safely.

I realize that this may be an inconvenience. If so, I hope you will complain to the airline. Maybe working together we can convince the airlines to provide enough space between rows so that people can recline their seats without banging into other passengers.

Thank you for your understanding.

That card, with its passive-aggressive tone, is just asking for trouble. And it cannot change the reality: The way the system is now designed, the recliner has every incentive to push the button and recline. The Barro pay-me solution gives the recliner even greater incentive to always do so.