【正文】 ce of a record, so an AFM can alter the surface it is scanning at the molecular level, in effect writing on that surface. Such writing, if it were fast enough, could be used as a form of lithography for making devices whose ponents had dimensions of nanometres (billionths of a metre). Nanotechnology, as engineering at this scale is known, is all the rage, and nanotech firms could end up using the VideoAFM39。 underlying assumption into question. This study suggests that it may be fruitful to work with the idea that human behaviour, too, can sometimes be governed by evolutionarily stable strategies.Double or quits?Dr Kurzban and Dr Houser were interested in the outes of what are known as publicgoods games. In their particular case they chose a game that involved four people who had never met (and who interacted via a puter) making decisions about their own selfinterest that involved assessing the behaviour of others. Each player was given a number of virtual tokens, redeemable for money at the end of the game. A player could keep some or all of these tokens. Any not kept were put into a pool, to be shared among group members. After the initial contributions had been made, the game continued for a random number of turns, with each player, in turn, being able to add to or subtract from his contribution to the pool. When the game ended, the value of the pool was doubled, and the new, doubled value was divided into four equal parts and given to the players, along with the value of any tokens they had held on to. If everybody trusts each other, therefore, they will all be able to double their money. But a sucker who puts all his money into the pool when no one else has contributed at all will end up with only half what he started with.This is a typical example of the sort of game 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 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。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 l