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games' to test whether reciprocity can lead to cooperation between humans. In such
experiments, individuals play games such as the prisoner's dilemma, and the more
points they gain the greater cash reward they are given at the end of the game. It has
been shown numerous times in such games that repeated interactions, with the
potential for reciprocity lead to higher levels of cooperation (Carmerer, 2003).
If reciprocity is important in humans, then we would predict that individuals should
be more likely to cooperate if they are being watched, because this would increase the
chance of reciprocal helping. Evidence for this comes from an elegant field study by
Melissa Bateson and colleagues (2006) in a university coffee room, where there was an
honesty box to pay for tea and coffee. Above the honesty box, there was a notice that
explained the system of payment for drinks. In alternate weeks, Bateson added a small
(150
Humans
cooperate at
higher levels in
repeated
interactions …
35 mm) image of either a pair of eyes or flowers. They found that people paid
approximately three times as much for their drinks when eyes were displayed, rather
than flowers, suggesting an automatic and unconscious tendency to cooperate in
response to cues of being observed (Fig. 12.6). However, caution should be taken when
interpreting this study as evidence for reciprocity, as the results could be explained
equally by a number of other enforcement mechanisms, such as cooperating to avoid
punishment. For example, economic games have shown that if humans are given the
option of punishing free riders, that they do so, and that this leads to higher levels of
cooperation (Fehr & Gachter, 2002). More generally, Trivers (1971) has suggested that
it is selection for enforcement mechanisms such as reciprocity which provide an
evolutionary basis for the human 'sense of fairness'.
×
… and in response
to cues of being
watched
Non-humans
Reciprocity has been suggested to be important in numerous cases, from cooperation
between cleaner fish and their clients, the warning cries given by many birds in response
to predators, the sharing of blood meals in vampire bats, fish inspecting predators and
food sharing in chimpanzees. However, in all or at least most examples given, cooperation
can usually be explained by a more simple mechanism, such as by-product benefit
(Hammerstein, 2003; Clutton-Brock, 2009c). Consequently, whilst it used to be
assumed that reciprocity was of widespread importance, it is now thought to be rare or
even absent in animals. The point here isn't that reciprocity is not possible, just that
despite lots of empirical attention there is a lack of conclusive examples, and so even in
the best case, it is rare. This is illustrated by discussing two specific cases.
Vampire bats
Wilkinson (1984) studied a population of individually marked vampire bats, Desmodus
rotundus , in Costa Rica. They roost during the day in stable groups of 8-12 that include
mothers, their young, relatives and some non-relatives. Adults forage at night for
animal blood but foraging is risky, and around a quarter of all bats return to the roost
without having fed. These unfed individuals beg for food from those in their group
that  have obtained blood, and commonly receive some. Wilkinson discovered that
regurgitation occurred only between close relatives or between unrelated individuals
who were frequent roost-mates, and suggested that these latter cases involved reciprocity.
However, in order to demonstrate that this really represent reciprocity, the following
conditions are needed.
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