To appear in Heams et al. (eds.), Handbook of Evolutionary Biology (provis. Title), Springer.

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propose are self-evident, or simply follow from common sense. But is this not a simple recourse to the facts? Furthermore, is it really the case that this is so? There are serious reasons to doubt it, given that philosophers do not agree among themselves on the content of what is judicious for our common sense to dictate. In addition, an increasing amount of empirical evidence on human moral psychology seems to go against this faith in the existence of communally shared moral intuitions. It is true that people’s moral reactions follow some rules, but it is quite disconcerting to note that many of these rules do not seem to be anchored in morality. To illustrate this point, I will finish this article by mentioning some results drawn from a series of thought experiments that have recently led to a great deal of ink being spilt them31. over The first is the trolley case. This is a thought experiment where people are asked to decide which of two alternative actions is better. Here is the situation described. You are witness to a grave occurrence: a trolley with broken brakes is speeding along and is about to run over five hikers who are crossing the track further down. By extraordinary chance, you find yourself next to a signal box and, by pushing a lever, you can change the trajectory of the trolley. If you do this, you will save the hikers. But if you opt for this solution the trolley will go onto a track where a railway worker is carrying out some repairs. The trolley will definitely run him over. Do you choose to pull the lever or not? The second thought experiment is the footbridge case. The same trolley is advancing in the direction of the five hikers, but this time, you are next to an obese man on a footbridge that overhangs the track. You know that if you push this man, his weight will be enough to stop the trolley and bring it to a halt, thus saving the five hikers. The obese man will most certainly die though. Do you choose to push the man or not (given that no alternative action is possible)? These thought experiments have been tested many times and the results indicate clearly that in the trolley case (with the signal box), a large majority of the subjects questioned choose to sacrifice the railway worker (thus saving the five hikers), while in the footbridge case, the majority refuses to push the obese man onto the track to stop the trolley. Common sense thus seems to dictate contradictory judgements! To save the coherence of the subjects’ choices, one can resort to the hypothesis of double effect. According to this hypothesis, if the death of a person is an unforeseen side effect of a good action (which is what happens in the trolley case), the subject considers the death to be permissible. In contrast, it is inadmissible to want to kill someone with the aim of saving other people; in the footbridge case, the death of the obese man is a means and not a side effect. This interpretation is interesting from the point of view of a moral approach that rests on the intuitions of common sense because it allows the rationalisation of the subjects’ apparently contradictory choices. The problem is that this interpretation is relativised by a third thought experiment: the loop case. In the third scenario, the subject is able to modify the trajectory of the trolley so that it takes a temporary loop track that returns to the original track. On the loop track, there is an obese man whose weight and mass would stop the trolley. Even in this experiment, it has been found that the majority of subjects choose to pull the lever so that they sacrifice the man and save the five others. The hypothesis of double effect might account for part of the variation observed in people’s moral choices but it is clearly insufficient as an explanation (Cushman et al. 2010; Greene et al. 2009). In fact, it seems that the simple occurrence (or not) of physical contact with the individual to be sacrificed (and not an intuition or a moral kind of reasoning) explains most of the variation observed in subjects’ choices. This idea is confirmed by an experiment carried out by Greene and colleagues (2001) on the trolley and footbridge problems. During this experiment, the

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