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It's not just seasonal goodwill: all year round we tend to be generous and trusting, even to strangers. Why?
IF YOU'VE spent hours in the past few weeks battling through the mall, armed only with a roll of wrapping paper and a shopping bag, you are probably wondering why we go to such lengths to buy gifts for each other at this time of year. This-or something like it-is a question that has also intrigued biologists since Darwin's day. Because natural selection favours individuals who do well against their competitors, you would expect all living things to be inherently selfish. But while our seasonal generosity is unique, the animal world is bursting with examples of selfless giving that seem to transcend basic survival instincts. Many animals spend precious time grooming one another. Some will act as lookouts for their group. Others help older adults to raise their young, rather than reproducing themselves. "The most extreme examples of altruism are certainly among the social insects," says Richard Dawkins of Oxford University. "Look at the soldier ants, who dig their jaws into a marauder, and never let go. Or the honeypot ants who fill their bodies with sugar and act as living food larders, to store food for other members of the colony." Over the years biologists have come to realise that things are often not what they seem. Sometimes helping other individuals, or even the group, is the best way to ensure your own wellbeing. Is that why you bought that dishwasher for your wife? Other times, seeming selflessness can be explained as an evolutionary urge to promote the survival of relatives with whom we share some of our selfish genes. This might account for the woolly socks you sent your brother in Alaska. But there's another type of altruism that is a bit more difficult to explain: where the benefits of giving are not obvious and there's no family connection. Why exactly did you give that copy of Cheating Monkeys and Citizen Bees to the workmate with whom everyone knows you don't really see eye-to-eye? Perhaps it's because you expected something in return. Recent studies suggest that not only is reciprocity a source of social cohesion, it also helps to explain the escalating generosity that seems to take hold of us at Christmas. It is three decades since William Hamilton from Oxford University suggested the idea of kin selection and built the foundation for the selfish gene theory. He realised that you could understand why altruism is common within families if you imagined that natural selection is working on an animal's genes, rather than the animal itself. The genes that make an individual likely to promote the survival of its kin will proliferate down the generations because both animals carry copies of the very same genes. But why would someone be nice to a non-relative? Prisoner's Dilemma In 1971, a biologist named Robert Trivers came up with an answer. He suggested that animals might do favours for unrelated individuals because they expect the favour to be returned in the future. He called the concept reciprocal altruism. If reciprocation is to be a convincing explanation for altruism in animals, you need to show that it can benefit individual animals in the long term, even if they run a risk of meeting a cheat who may not return the favour. And biologists have found a clever way to do this, without involving any real animals at all. They use computers to simulate animals that continually play games with one another in which they can choose whether or not to cooperate. The games these cyber-creatures play are based on a concept known as the Prisoner's Dilemma, which was originally developed to model social and military conflicts. The Prisoner's Dilemma encapsulates the risk of reciprocation perfectly, because the reward you get depends on what your partner chooses to do. If both players cooperate, they each get an equal reward. But if the first player cooperates and the second one sells out, the cheat receives the highest possible reward, and the first player is fobbed off with a sucker's payoff, which is the lowest reward available. The temptation to cheat, then, is strong, and an individual who cooperates runs a risk of losing spectacularly. On the other hand, if both players cheat, they each receive a low payoff. The dilemma is that the individual payoff is greatest if one player defects while the other does not, but the average payoff is higher for both players if they cooperate. So what is the best strategy? If we were dealing with humans, we might expect two players, or indeed the entire population of players, to formally agree that everyone cooperates all the time, and that people who cheated would be punished. Which in fact is how civilised society is supposed to work. But can a similar thing occur in the animal world, where signing contracts is not an option? Generation game We can use the Prisoner's Dilemma to model natural selection if the payoff represents fitness-that is an animal's ability to reproduce. In the model, animals follow various strategies such as always cheat or always cooperate. Those that tend to win a high payoff get to reproduce more than those with a less successful strategy, so that the relative frequency of successful strategies increases. If you run the game for a large number of "generations", allowing the cyber animals to play each other over and over again, eventually the population should reach a stable balance where the most successful strategy comes to dominate. And if that dominant strategy turns out to be a cooperative one, then you have shown that cooperation can evolve in the best interests of the individual animal. The first paper showing that altruism could evolve was published in 1981 by Hamilton and Robert Axelrod from the University of Michigan. It was based on a strategy known as tit-for-tat. Individuals adopting this strategy play the tactic their opponent played in the previous round. So assuming all the players start off being generous, a whole population of tit-for-tat players simply cooperate all the time. If a cheat arrives, that player will win the highest prize for cheating against a cooperator once, after which it does consistently badly, because tit-for-tat retaliates. More recently, there have been many models that elaborate on tit-for-tat, making it more forgiving, or incorporating mistakes. "Tit-for-tat explains pretty well what has gone on," says Hamilton. "The latest models which build on it just make the process seem even easier than we thought." The next question is obvious: do animals actually behave in this way? And surprisingly, 17 years on, there is still no evidence that any species is actually caught in a Prisoner's Dilemma in its natural environment, and very little evidence that animals follow a tit-for-tat type strategy. Clever experiments with sticklebacks and guppies show that these fish use tit-for-tat to decide on a partner when they risk their lives to inspect an approaching predator (see "Fish United"). But most cases of reciprocal altruism we see in the wild are more complex. For example, a female vampire bat who has recently eaten will regurgitate a meal to save a nest mate from starvation. She will only do this for another bat with whom she has been a frequent roost mate, and she is more likely to donate blood to a bat that has helped her in the past. But a frequent roost mate could also be a relative, so it is difficult to know whether kin selection or reciprocity explain such a sacrifice. And the more complex the social structure under scrutiny, the more advanced the systems of reciprocity. Primates, for example, barter one type of favour for another of a different kind. "Unrelated vervet monkeys seem to exchange grooming for aid in coalitions," says Lee Dugatkin from the University of Louisville, "while no such exchange exists among related individuals." So it's not really surprising that some researchers doubt whether game theory and tit-for-tat can ever describe nature. "These are fragile explanations because they rely on a number of prerequisites which are not often met in the animal world," says Robert May from Oxford University. "The animals are required to remember specific encounters, individuals and outcomes of each game and they must anticipate future encounters with the same individual." He points out that the theory also gives the same value to games in the future, without taking into account the fact that they may not happen. Theoretical playground Gilbert Roberts from the University of Newcastle also has reservations. "The Prisoner's Dilemma has become a theoretician's playground. It becomes more and more diverged from the behaviours it is trying to explain," he says. But with colleague Thomas Sherrat, he has come up with a finding that he hopes will start to unite the theory with studies in animal behaviour. "I was inspired by the observation that most examples of cooperation are not all-or-nothing behaviours," says Roberts. "Birds that preen one another can vary the amount of time they spend doing so." He wondered how animals decide how much of an investment to make in another individual. Roberts and Sherrat used the Prisoner's Dilemma to see which strategies of variable investment could possibly evolve and lead to stable populations of cooperators. Some players were "short changers", consistently cooperating less than their opponent had in the previous round. Others matched their opponent's investment exactly, or refused to cooperate at all. The strategy that emerged as successful was called "raise-the-stakes", a form of tit-for-tat, where players make a small investment in each other at first, but with each round of the game, become slightly more generous than their opponent was in the previous round. The big advantage of this finding is that it can be tested in nature. "This opens the way to simple experiments to test the theories of how reciprocal altruism might work," says Laurent Keller from the University of Lausanne in Switzerland, "because it is easy to measure the amount of time that animals spend grooming." Christmas card panic Could this also explain why the giving of gifts at Christmas is apt to get out of hand? "Reciprocation is much more important than kin selection in humans," says Hamilton. "As animals, we have devoted a huge amount of resources in the brain to recognising faces. That's why we are such great tit-for-tatters. And gift giving is deeply symbolic in all human cultures. How else do you explain the depth of panic you feel when you receive a Christmas card from someone who you have inadvertently omitted from your list?" There could be more to it than that, however. "Humans will help someone, even though they are extremely unlikely to meet them again," says Martin Nowak from the Institute for Advanced Study, Princeton. Working with colleague Karl Sigmund, Nowak has come up with a model called indirect reciprocity that may explain what looks on the surface like indiscriminate generosity. This time, the Prisoner's Dilemma is played out against a social background. Instead of pairs of players reacting to one another in isolation from the rest of the cyber population, other players witness acts of cooperation and betrayal. Each player is assigned an "image score" which is apparent to all the other players, so an individual can judge whether or not to cooperate with another on the basis of how altruistic they have seen that opponent to be. The beauty of this model is that cooperation evolves even where individuals may never meet each other again. And there is evidence that such a scoring system can occur in the natural world. Arabian babblers, for example, are small birds of unassuming appearance found in the Arabian peninsula. Within their groups, generosity is associated with social status and the highest-ranking babblers are the ones that do the most grooming and guarding. Consequently, the birds fall over one another to do a good turn for a neighbour. Humans may not be quite so generous, but we do have a tendency to be indiscriminately nice and trusting. "We are much nicer to one another than appears to benefit our selfish genes," says Dawkins. So, it's presents for the family to bolster your genetic ties, gifts for friends because you know they will be buying them for you, and something for the office nerd to show everyone that you're a wonderful colleague. And you thought Christmas philanthropy was a selfless pursuit? Bah! Humbug!
Lynn Hunt is a science writer and ecologist at the University of Cambridge |
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From New Scientist magazine, vol 160 issue 2165, 19/12/1998, page 52 |
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