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In evolutionary theory individuals are assumed to maximise their Darwinian fitness, which is
defined as an individual’s relative genetic contribution to the gene pool of the next generation.
33, 55, 74, 173 However, while cooperation increases the fitness of the recipient it is costly for
a cooperator in terms of forgoing own reproductive potential128; even though cooperation can
also entail benefits, e.g. if reciprocated by other group members.8, 197, 213 If a non-cooperator has
a higher individual fitness level than a cooperator, although a group of cooperators is better off
than a group of defectors, a social dilemma arises.44, 76 Here, cooperation is undermined by the
conflict between self and group-interest. The goal of this thesis has been to study human cooperation
in such dilemmas. Using the collective risk social dilemma game,122 the studies of this
thesis particularly focus on the influence of risk scenarios and temporal discounting on cooperation.
In the standard variant of the game,122 human subjects can either cooperate at their own
expense in order to avoid simulated ‘dangerous climate change’, or they risk monetary losses
with a predefined probability – hence the collective risk. In terms of evolutionary game theory,
such studies have been designed to allow human subjects to maximise their fitness,127, 192 here
modelled either by monetary payoffs or by indirect benefits through supporting descendants.
A main finding of the thesis is the role of a stepwise risk scenario with a strict threshold as
a mechanism for the evolution of cooperation. In a computerised behavioural experiment with
such a scenario, participants coordinated their group contributions around the social optimum,
where group members achieve maximum joint outcomes; even though in the experiment the
participants have not been restricted exactly to an equal share principle. On the other hand,
risk and investment were balanced out more generously if the risk decays stepwise linearly or
piecewise linearly. In particular the ratio between the first derivative of a risk curve and the
number of players determines the level of contributions, as further demonstrated by a theoretical
model. While cooperation usually decreases with the number of group members, only a
stepwise risk scenario turned out to stabilise cooperation (a) for a large number of players and
(b) independent of individual risk preferences.
However, cooperation can even fail under a stepwise risk scenario when benefits are deferred
to future generations, as it is likely in the prevention of real climate change.161 This was
shown in an inter- and intergenerational study about discounting, where student participants
reduced climate investments when their own payoffs were delayed in time and widely stopped
contributing when they had no prospects of higher expected returns in the future. A different
picture arises in a follow up experiment with elderly subjects where the question was investigated
whether people are more willing to invest in climate change mitigation when they have
descendants. As a result, descendants did not play a decisive role in participants’ climate contributions.
However, participants donated altruistically for a climate change mitigation measure;
especially when they were wealthy in their real life. This demonstrates broader motives than
pure payoff maximisation among the participants. The study further implies an effect of age on
cooperation due to an overall high contribution level among the elderly compared to the former
student experiment.
