You set out to build social capital in so many other circumstances every day. Every gift you give accumulates it. You bring treats into the office. You may not be thinking this is the reason as you bake the brownies, but should you need to leave early and dump your work on others, they won’t resent you as much because you gave them chocolate goodies. If the guy in the next carrel who occasionally clips his nails also cleans the microwave, it’s harder to get annoyed.

It’s why you shovel the neighbors’ walk after a snowstorm or offer to watch their children. Someday you either need a favor or maybe just some forbearance when you hog three of the parking spots in front of their houses. (You may just be a very kind, altruistic person. If so, good for you. But I have my doubts.)

Favors like these happen a lot in business. Smart contractors help out the neighbors of a home they are reconstructing. It’s a bribe, of sorts, but you may be a bit more willing to put up with the noise and the dust. Clerks at Pret A Manger, the chain of upscale sandwich shops, have been known to give free food to customers. Sure, some people say it is just the clerks flirting, but it’s an accepted policy because the management wants you to come back.

You tip a waiter or waitress because it is social custom in the United States or because we feel guilty that most restaurants don’t pay their employees enough to live on. But if you intend to return and hope to be recognized and treated even better, you might tip more.

It might take time, of course. Time really is not something you have much of on an airplane, even on a long flight. Social capital is most effective when there is a network of connections to people — like an office or a neighborhood. You build social capital with co-workers and neighbors over long periods. You have months or even years.

No time. No network. Just anonymous people jostling. The transience of a plane is also why people tend to be so obnoxious on it. There is no possible penalty. They will never see you again. Nothing incentivizes a person to be nice to you. Economists have noted that social capital is higher among homeowners than among renters. Your neighbor does not go nuts when your dog poops in his yard because he will have to see you nearly every day and can’t risk having bad relations. So you get along.

On a plane, you have no way of making the person in the adjoining seats be nice or even feel guilt or shame for violating commonly held rules. (And repeat after me: The armrests belong to the person squeezed into the middle seat, and your purse or bag belongs under the seat ahead of you, not under your seat.) A bag of treats appears, and suddenly a connection is made. Weak, but it is there. And it is no more judgmental than a wedding gift or a Christmas card.