All cooperative relationships between two individuals—whether business partnerships, romantic relationships, or political alliances—share the same commitment problem: one business partner may cooperate one moment and cheat in another; one romantic partner may promise eternal commitment one day and end the relationship the next; and one Republican Congressman may support Mitt Romney's Presidential campaign for months and then switch allegiance to Newt Gingrich the day after Gingrich wins the South Carolina Primary. The commitment problem occurs because the costs and benefits of the relationship may change over time. When the relationship begins, its benefits are presumably higher than its costs to both partners (assuming that the relationship is consensual and not coercive). However, at some point later on, the circumstances might change for one partner such that the costs of the relationship become greater than its benefits. When this happens, the individual that goes in the red wants to call it quits or starts cheating, while the other partner may still benefit from the partnership and wants to keep going (in the rare event that a relationship becomes disadvantageous to both partners at the same time, then a quick consensual separation is obviously the thing to do).

More often than not, the cost/benefit ratios of a relationship are not symmetrical for both partners, so people have come up with creative ways to make a partner stick to a commitment to cooperate even when it would be more advantageous to defect. First, there are reputation effects: when other people find out (for example, through gossip) who is faithful and who isn't, who cooperates and who cheats, there is a boost in reputation that comes from cooperating, and a cost that comes from cheating (just look at what happens to male politicians who cheated on their wives, when they run for President; or ask why some women pay exorbitant sums of money to put their cheating husband's name or face on huge billboards to give him a bad reputation and make sure no other women will marry or even date him in the future). Second, there can be sanctions on cheating that make it unappealing regardless of the rise of favorable circumstances. In some countries, adultery is considered a criminal offense punishable with jail time, or even corporal punishment, or death. And pretty much everywhere in the world there are hefty sanctions for anyone who breaks a cooperative agreement, whether it's a business or a marriage contract. Finally, there are internal mechanisms of control involving morality and feelings. People are more likely to stick to a commitment, even when it becomes disadvantageous to do so, if they think it's morally wrong to cheat or quit. Romantic feelings for the partner or simply empathy for his or her pain may also act as a deterrent against breaking a commitment.

The truth, however, is that none of these mechanisms work perfectly, and there is always a chance that a long-term cooperative relationship between two individuals will end, or at least take a bad turn for one of the individuals involved. The question then becomes one of trying to predict when this might happen. A long-term cooperative relationship is an investment and the golden rule of financial investment is to cut one's losses quickly when the investment is no longer profitable. Since the commitment problem always lurks beneath the surface of any relationship, two partners must frequently assess the strength of each other's commitment, so that one can bail out of the relationship quickly and cut his or her losses when the other starts cheating or gives unequivocal signs that a break-up is impending. In other words, each partner must assess on a regular basis how much the other values their relationship so as to be able to track any changes in costs and benefits that may affect how much the partner is willing to continue to invest.

The Handicap Principle, a theory that blends evolutionary biology and economics proposed by Israeli zoologist Amotz Zahavi, suggests that the most reliable way to assess how much a relationship is worth is to assess its market value, that is, how much someone is willing to pay for it. Your boss at work can tell you that you are a valuable employee and praise your work constantly, but the best indicator of how valuable you are to your boss is the salary he or she is willing to pay you. Words are cheap, but money isn't. Thus, testing the strength of the bond between you and your partner may require behaving in ways that are costly, risky, or stressful to your partner and see whether he or she is willing to pay the price or to put up with it.

In his book The Handicap Principle: A Missing Piece of Darwin's Puzzle, Zahavi has argued that many expressions of love and affection contain stress-producing elements because the recipient's acceptance and tolerance of them provides reliable evidence of his or her current willingness to continue to invest in the relationship. In this view, many of the affectionate behaviors shown by children toward their parents, such as jumping on their laps or on their backs, derive their communicative value from being inherently stress-producing. A devoted father will reassure his child of his love and willingness to continue to invest in parenting by tolerating the child's intrusive or annoying expressions of affection. Clearly, if a child cheerfully jumped on the back of a strange man standing on a bus, the man would probably react with annoyance to this expression of affection.

Zahavi has suggested that all of our love signals are impositions of one sort or another: kisses, hugs, and petting intrude on personal space and impair freedom of movement. "Lovers who hold each other's hand for hours at a time are each giving up the use of a hand for that time, a pretty heavy imposition," Zahavi says. Lovers who exchange long and passionate kisses stick their tongues in each other's mouth; that's quite intrusive and even carries the risk of transmitting disease. Only people who are highly committed to a romantic relationship accept this type of imposition from their partner. You will not be surprised to hear, at this point, that Zahavi thinks that sex is the ultimate mechanism for bond-testing. The intrusiveness of many forms of sexual behavior, according to him, makes sex an ideal handicap signal for conveying and receiving detailed information about each lover's commitment to the relationship. Here I respectfully disagree. Although it is certainly true (if unfortunate) that some people find the intimacy of sexual acts a little uncomfortable, the vast majority of people find the intrusiveness of sex quite pleasant and rewarding. And I doubt that many people would regard holding hands as a heavy imposition.

The Handicap Principle is a theory that explains under what circumstances communication between individuals - people or animals - can be expected to be honest or deceitful. The idea is that signals that are costly to produce are more likely to be honest than signals that are cheap. The cost represents the "handicap".  The bond-testing hypothesis discussed here is a relatively minor extension of the Handicap Principle. Essentially, while the Handicap Principle maintains that individuals have to take on handicaps to prove to others that their signals are honest, the bond-testing hypothesis suggests that individuals must impose handicaps on others in order to extract reliable information on their attitudes toward themselves. Undeniably, it's an interesting idea. Zahavi's contention that love expressions, including sex, are wasteful, risky, stressful, or even painful impositions - all the properties of handicaps - seems a little pessimistic, but to wonder why romantic partners express their love by sticking their tongues in each other's mouths instead of, say, playing chess together, is to ask a legitimate question that, if nothing else, provides some food for thought. 

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