A few months ago, the network researchers Sam Arbesman and Nicholas Christakis published a paper looking at how the frequency of good deeds scales with city size.
This was an extension of work by Luis Bettencourt and his colleagues looking at how all kinds of stuff scales with city size. They found that what you might call social productivity — wealth, wages, patents, but also illness and crime — scales superlinearly. That is, big cities have proportionately more of them than small ones. (Bettencourt and Geoffrey West have an article on this in a recent Scientific American.)
Arbesman and Christakis looked at some things that those guys hadn't: the frequency of altruistic acts. They found a more mixed picture.
Some of the things they looked at did scale superlinearly. They include the size and frequency of political donations, and the number of organ donations by deceased people. But some (the number of people returning a census letter) scale sublinearly; you get less of them per person in bigger cities). And others (living organ donation and voting) show a linear pattern. Jonah Lehrer blogged about the work.
Recently, Sam (who I have interviewed and chatted to once or twice, and whose stuff is always worth reading) was kind enough to mail me the paper, and I finally got round to reading it. Having done that, I think that how a particular type of good deed scales with city size might depend on the motivation that underlies the deed.
There are several theoretical explanations for why people act altruistically: they might be helping relatives who share their genes (kin selection), returning a favour (reciprocal altruism), to improve their reputation with onlookers, who will help them when they need it (indirect reciprocity), or to show how great they are as a way of attracting allies, friends and mates (costly signalling, a.k.a competitive altruism).
City size is particularly relevant to the last two of these. In a larger town, you have a bigger potential audience for your good deeds, but it's also easier to remain anonymous, and harder to keep track of other people, and so to know whether you should cooperate with them or not. To seamlessly introduce the theme of my forthcoming book, it's easier to escape your reputation. This also might account for the bystander effect, where people pass by some hideous crime without intervening. The economist/evolutionary biologist Karthik Panchanathan has been doing some experimental work on this.
I'd guess that as our group size increases, it pushes cooperation away from indirect reciprocity — from behaviours that are based on trust, because trust is harder to build and maintain — and towards costly signalling. This doesn't need trust, because altruists are using conspicuous do-gooding as an advertisement of status, like conspicuous consumption. City size, in other words, influences which types of altruism are rewarded, and so influences people's altruism budgets. They stop building a reputation, and start buying one.
This fits with our stereotype of cities as slippery, brash, places full of sharp practice, and of small-town folk as decent, simple and trusting. It also fits, as far as it goes, with the data: Arbesman and Christakis found that a 'doing your bit' form of altruism, returning a census form, became less common in larger cities, whereas a 'showing off' form of altruism, that's dependent on personal wealth - political donation – became more common. The evo psychologist Henry Lyle and his colleagues have already suggested that blood donation is a costly signal, so the same might go for organ donation.
I ran this idea past Sam, and he said that it was possible, but this sort of data is very hard to come by.
There are also some obvious objections. Live organ donation — a pretty damn costly signal of good health, if ever there was — scales linearly. One possibility is that most of these organs go to family members, and so be the result of kin selection, which you wouldn't expect to respond to city size.
Another issue is whether the psychological mechanisms that people use to decide whether to behave altruistically are really that sensitive to groups above a certain size. Can we sense that London is a bigger, more anonymous city than San Francisco, and adjust our behaviour accordingly? I'm not sure — I've lived in London most of my life, and spent a year a long time ago in the Bay Area. I love London, but it doesn't give a monkey's. The Bay Area seemed more like it buoyed people up. I'll stop before speculation becomes anthropomorphism.