For mobile foragers, sharing is insurance. Hunting especially is very chancy, requiring both luck and skill, so it’s adaptive to share if you succeed today, on condition that others share with you when you fail. Targeting plants and small animals is more of a sure bet, though in some forager communities even these are shared, as the social rewards of generosity are important, and the social costs of refusing are high since the intimacy of forager camps makes success hard to conceal.
Storage, however, tends to erode sharing. Storing, like sharing, is a way of managing risk, and farmers are more likely to store than to share. Variation in supply within the community is likely to flow from variation in commitment and effort, not differences in luck. Local bad luck – unfavourable weather, a plague of pests – will probably affect everyone in a community, which makes sharing a poor form of insurance. It’s to my advantage to share with you, if my good years are your bad ones, and vice versa (so long, of course, as you return the favour). Not so if we’re both having it tough at the same time, as we have no surplus to share; and not so if we both have good years together, as then we don’t need one another (emphasis added).