Under the rules of the game, the best collective outcome would be if everyone put in all their credits, and then each player would get back more than they put in. But the best outcome for each individual was to free ride – to keep their original 20 credits, and also get the 40% of what everybody else put in. Of course, if everybody did this then that would be 40% of nothing.
In this scenario what happened looked like a textbook case of the kind of social collapse the free rider problem warns of. On each successive turn of the game, the average amount contributed by players went down and down. Everybody realised that they could get the benefit of the collective pot without the cost of contributing. Even those who started out contributing a large proportion of their credits soon found out that not everybody else was doing the same. And once you see this it's easy to stop chipping in yourself – nobody wants to be the sucker.
Rage against the machine
A simple addition to the rules reversed this collapse of co-operation, and that was the introduction of altruistic punishment. Fehr and Gachter allowed players to fine other players credits, at a cost to themselves. This is true altruistic punishment because the groups change after each round, and the players are anonymous. There may have been no direct benefit to fining other players, but players fined often and they fined hard – and, as you'd expect, they chose to fine other players who hadn't chipped in on that round. The effect on cooperation was electric. With altruistic punishment, the average amount each player contributed rose and rose, instead of declining. The fine system allowed cooperation between groups of strangers who wouldn't meet again, overcoming the challenge of the free rider problem.
How does this relate to why motorists hate cyclists? The key is in a detail from that classic 2002 paper. Did the players in this game sit there calmly calculating the odds, running game theory scenarios in their heads and reasoning about cost/benefit ratios? No, that wasn't the immediate reason people fined players. They dished out fines because they were mad as hell. Fehr and Gachter, like the good behavioural experimenters they are, made sure to measure exactly how mad that was, by asking players to rate their anger on a scale of one to seven in reaction to various scenarios. When players were confronted with a free-rider, almost everyone put themselves at the upper end of the anger scale. Fehr and Gachter describe these emotions as a “proximate mechanism”. This means that evolution has built into the human mind a hatred of free-riders and cheaters, which activates anger when we confront people acting like this – and it is this anger which prompts altruistic punishment. In this way, the emotion is evolution's way of getting us to overcome our short-term self-interest and encourage collective social life.
So now we can see why there is an evolutionary pressure pushing motorists towards hatred of cyclists. Deep within the human psyche, fostered there because it helps us co-ordinate with strangers and so build the global society that is a hallmark of our species, is an anger at people who break the rules, who take the benefits without contributing to the cost. And cyclists trigger this anger when they use the roads but don't follow the same rules as cars.
Now, cyclists reading this might think "but the rules aren't made for us – we're more vulnerable, discriminated against, we shouldn't have to follow the rules." Perhaps true, but irrelevant when other road-users perceive you as breaking rules they have to keep. Maybe the solution is to educate drivers that cyclists are playing an important role in a wider game of reducing traffic and pollution. Or maybe we should just all take it out on a more important class of free-riders, the tax-dodgers.
13/02 UPDATE: We've changed a sentence in the third paragraph that readers said implied all cyclists break rules. This was not the intended implication of the original line, and we thank the readers who pointed this out.