There's a new work-in-progress on my website. It's with Ro'i Zultan and is called "Brothers in Arms: Cooperation in Defence". A poster is here.
We started this paper to try and resolve a puzzle in social science and biology. Often, unrelated individuals help each other when they face an external attacker. For example, an estimated 200,000 people were active members of the French Resistance in World War II. In the laboratory, we can observe a similar phenomenon under controlled conditions: cooperation increases in the presence of an external threat. Non-humans do it too. For instance, musk oxen band together to drive off wolves.
This is a puzzle because, economically, helping others seems to have costs but no benefits. Or, biologically speaking, this behaviour should decrease the fitness of the animals who display it.
We explain this cooperation by invoking signalling theory. Some groups are prepared to cooperate, for example because they are kin, or because they are in long-term cooperative relationships. Other groups consist of self-interested individuals. Attackers are strategic: they prefer to attack groups who will not help each other. (Think of the bandits in the Magnificent Seven.) If they start to think they are attacking a very cooperative group, they will look for another target. Then, even self-interested individuals have an incentive to help each other during an attack, as this will deter further attacks.
So far you might get with common sense. However, economists and others may be asking "surely the group's reputation for cooperativeness is a public good, just like defence itself, and will be underprovided". Here, our theoretical model shows one possible answer: the series of repeated attacks gives this public good a "weakest-link technology". That is, if you are the first person not to help during an attack, then the attacker immediately recognises that previous helping was fraudulent and can no longer be deterred. This in turn gives a strong incentive to help when it's your turn.
The paper is here.