【文章內容簡介】 e that economists investigating game theory revel in, and both theory and practice suggests that a player can take one of three approaches in such a game: cooperate with his opponents to maximise group benefits (but at the risk of being suckered), freeride (ie, try to sucker cooperators) or reciprocate (ie, cooperate with those who show signs of being cooperative, but not with freeriders). Previous investigations of such strategies, though, have focused mainly on twoplayer games, in which strategy need be developed only in a quite simple context. The situation Dr Kurzban and Dr Houser created was a little more like real life. They wanted to see whether the behavioural types were clearcut in the face of multiple opponents who might be playing different strategies, whether those types were stable, and whether they had the same average payoff.The last point is crucial to the theory of evolutionarily stable strategies. Individual strategies are not expected to be equally represented in a population. Instead, they should appear in proportions that equalise their payoffs to those who play them. A strategy can be advantageous when rare and disadvantageous when mon. The proportions in the population when all strategies are equally advantageous represent the equilibrium.And that was what happened. The researchers were able to divide their subjects very cleanly into cooperators, freeriders and reciprocators, based on how many tokens they contributed to the pool, and how they reacted to the collective contributions of others. Of 84 participants, 81 fell unambiguously into one of the three categories. Having established who was who, they then created “bespoke” games, to test whether people changed strategy. They did not. Dr Kurban and Dr Houser were thus able to predict the outes of these games quite reliably. And the three strategies did, indeed, have the same average payoffs to the individuals who played them—though only 13% were cooperators, 20% freeriders and 63% reciprocators.This is only a preliminary result, but it is intriguing. It suggests that people39。s approaches to cooperation with their fellows are, indeed, evolutionarily stable. Of course, it is a long stretch from showing equal success in a laboratory game to showing it in the mating game that determines evolutionary outes. But it is good to know that in this context at least, nice guys do not e last. They do just as well as the nasty guys and, indeed, as the wary majority.Passage 4Moon river?The latest news from TitanA PICTURE may be worth a thousand words. But when the picture in question is of an alien world, it is difficult to be sure what those thousand words should be. And in the case of the images that have arrived from Titan, Saturn39。s largest moon, that world is very alien indeed.On January 14th Huygens, a space probe built by the European Space Agency (ESA), landed on Titan and began to deliver its precious cargo of data to anxiously waiting scientists. The most striking finding so far is a picture taken as the probe descended. It appears to show pale hills crisscrossed with drainage channels containing dark material, leading to a wide, flat darker region. The landing site itself produced less striking, but still significant images. It is flat, strewn with rounded pebbles andappears to be a dry riverbed.On Earth, or even on Mars, drainage channels and rounded pebbles would be taken as evidence for the erosive effects of liquid water. But at 180176。C, Titan is too cold for water to be liquid. It is, however, not too cold for various hydrocarbons to be so (indeed, the most likely candidates, methane and ethane, are gases at terrestrial temperatures). Many people have suggested that Titan39。s dark regions might be lakes made of such hydrocarbons, or of tar that is posed of hydrocarbons which are too cold to be truly liquid, but have not frozen solid. The presence of hydrocarbons in Titan39。s atmosphere was confirmed on the probe39。s journey through it. Huygens39。s instruments detected both methane and ethane. But the pebbles in the picture probably are made of water—in the form of ice.Because all of the raw images from Huygens were immediately made available to the public via the internet, amateurs have been racing ESA and its American cousin NASA to create processed, posite images. Some scientists say that a glitch led ESA to publish more data than it had originally intended, something that ESA denies. Nevertheless, a few minutes after the Huygens data were published on one website, they were mysteriously yanked off the web again.The availability of the data, though, has led to the publication on the internet of a short movie piled from a series of 80 still images taken of the landing site.