12/07/2014 09:19 pm ET Updated Feb 06, 2015

Buy You a Beer?

Although I am generally cynical about human nature, I am always gratified by displays of generosity. Lately, I have been impressed by the importance of reciprocity. My intermittent hope for our species is thereby renewed.

What I have been doing is this. I fly often. I have started to buy drinks or a snack for the people seated in my aisle. Domestic airlines nowadays charge for everything, and the seating is so cramped that we become better acquainted with our fellow travelers than perhaps we would prefer.

The first time I picked up the tab for a stranger was after he switched rows so I could be seated with my wife. While I felt I should make the gesture, I was happy to do so.

The next few iterations, however, I realized I had created what social scientists refer to as a natural experiment. I had no ulterior motive, though I could not help noticing how the scenario played out. Without any specific expectations, I nonetheless was surprised.

Some people turn down the offer. I take no offense. A woman especially might suppose the fellow next to her was some creep trying to hit on her, and she'd be trapped for hours.

On just about every other occasion, when I succeed in being generous, the recipient of the kindness has returned the favor. This has happened spontaneously.

During a recent flight, for example, two business consultants across the aisle covered a round each. We all enjoyed the trip on the puddle-jumper more than we would have otherwise.

More than once, the flight attendant has taken notice. Instead of my treating someone else, I have ended up being comped the beer or glass of wine.

These random encounters over the course of a three-hour plane ride reveal our unscripted selves. They are as significant as friendships in describing our personalities.

Evolutionary psychologists ponder altruism. They wonder why anybody behaves in a manner that appears less than self-interested. They speculate that charity is a charade; it is not benevolent but serves another purpose. We are willing to sacrifice ourselves for our cousins, if we save a sizable number of them, because our genes may be within certain probabilities propagated by our relatives.

Many of the hypotheses turn on versions of the classic Prisoner's Dilemma. Two persons allegedly participating in the same criminal conspiracy have been captured and are being held in separate cells with modest evidence against them: each is offered the opportunity to inform on the other. If one does but the other doesn't, the former will be set free, the latter will serve a longer sentence; if both do, they each will serve an intermediate sentence; if neither does, they each will serve shorter sentences. In game theory modeling about this hypothetical, the best strategy is called "tit for tat:" an individual remains loyal until betrayed.

The mutuality of obligations is not only rational but also just. We are nice because the social contract is always tacit if not innate. The genuinely solicitous would regard any quid pro quo as obvious but not to be pointed out.

I do right by you. Even if neither of us has said anything about it, you will return the favor sooner or later. There is an informal ledger we maintain on each other, discreet so as to preserve our sense of free will as well as goodwill.

Aware of these dynamics, the more cautious among us avoid overcommitting themselves by happenstance. As a corollary, they recognize the ratcheting aspect of our relationships: abuse returns itself.

Yet the world is as it should be. Cooperation requires the slight risk of initiating a cycle, and then the effort to sustain it should come more readily. The urges, positive and negative, are powerful.