PSYCHOLOGISTS have known for a long time that economists are wrong. Most economists—at least, those of the classical persuasion—believe that any financial gain, however small, is worth having. But psychologists know this is not true. They know because of the ultimatum game, the outcome of which is often the rejection of free money.

In this game, one player divides a pot of money between himself and another. The other then chooses whether to accept the offer. If he rejects it, neither player benefits. And despite the instincts of classical economics, a stingy offer (one that is less than about a quarter of the total) is, indeed, usually rejected. The question is, why?

One explanation of the rejectionist strategy is that human psychology is adapted for repeated interactions rather than one-off trades. In this case, taking a tough, if self-sacrificial, line at the beginning pays dividends in future rounds of the game. Rejecting a stingy offer in a one-off game is thus just a single move in a larger strategy. And indeed, when one-off ultimatum games are played by trained economists, who know all this, they do tend to accept stingy offers more often than other people would. But even they have their limits. To throw some light on why those limits exist, Terence Burnham of Harvard University recently gathered a group of students of microeconomics and asked them to play the ultimatum game. All of the students he recruited were men.

Dr Burnham's research budget ran to a bunch of $40 games. When there are many rounds in the ultimatum game, players learn to split the money more or less equally. But Dr Burnham was interested in a game of only one round. In this game, which the players knew in advance was final and could thus not affect future outcomes, proposers could choose only between offering the other player $25 (ie, more than half the total) or $5. Responders could accept or reject the offer as usual. Those results recorded, Dr Burnham took saliva samples from all the students and compared the testosterone levels assessed from those samples with decisions made in the one-round game.

As he describes in the Proceedings of the Royal Society, the responders who rejected a low final offer had an average testosterone level more than 50% higher than the average of those who accepted. Five of the seven men with the highest testosterone levels in the study rejected a $5 ultimate offer but only one of the 19 others made the same decision.

What Dr Burnham's result supports is a much deeper rejection of the tenets of classical economics than one based on a slight mis-evolution of negotiating skills. It backs the idea that what people really strive for is relative rather than absolute prosperity. They would rather accept less themselves than see a rival get ahead. That is likely to be particularly true in individuals with high testosterone levels, since that hormone is correlated with social dominance in many species.

Economists often refer to this sort of behaviour as irrational. In fact, it is not. It is simply, as it were, differently rational. The things that money can buy are merely means to an end—social status—that brings desirable reproductive opportunities. If another route brings that status more directly, money is irrelevant.