Population viscosity

Population viscosity is the tendency for individuals to remain close to where they were born, a pattern that can gather genetic relatives into local neighborhoods. In the book’s account of cooperation, this limited movement creates a route by which a strategy that is rare across a whole population may become common enough locally to prosper. It can therefore help cooperation pass a population-wide threshold that the same strategy might not cross by numbers alone.

Definition and Mechanism

Viscosity does not require individuals to choose relatives as neighbors. It follows from limited dispersal: individuals remain near their birthplace, so brothers, sisters, and cousins are more likely to live nearby than randomly selected members of the population. The book calls this population “viscosity,” treating it as any tendency for people or animals to continue living close to where they were born. For most of human history, and in most parts of the world, individuals seldom strayed more than a few miles from their birthplace. The contrast is with the modern world, where this pattern is less typical.

This spatial clustering matters because genetic relatives tend to resemble one another in more than visible features. They may also share inherited tendencies to adopt a behavioral strategy. Viscosity thus links the geography of a population to its composition: even when a strategy is uncommon in the population overall, it may be comparatively frequent among neighbors who are more closely related.

The Irish Island Example

The argument is illustrated by a visit to a remote island off Ireland’s west coast. Almost everyone there had notably large, handle-shaped ears, a resemblance the author judged unlikely to be an adaptation to the local climate, especially given the strong offshore winds. The proposed explanation was kinship: most island inhabitants were close relatives, so a shared inherited feature was common among them. The island makes the effect of local relatedness visible, though its physical isolation makes it an imperfect model for a broader population where movement is limited but interbreeding continues.

Local Clusters and Cooperation

The main application is to Tit for Tat, a strategy that cooperates first and responds to an opponent’s previous move. If only a small fraction of a population uses it, a population-wide calculation might put it below the critical frequency needed to prosper. But genetic relatives tend to resemble one another in behavioral tendencies as well as appearance. In a local area, Tit for Tat players may therefore meet one another often enough to benefit from mutual cooperation even when the strategy remains rare overall.

Those local successes can build on themselves. Cooperating clusters may grow from small beginnings into larger local clusters, then spread into neighboring areas where Always Defect is numerically dominant. This possibility does not depend on a population being sealed off like the Irish island. It requires only a large population with limited movement, in which individuals tend to resemble immediate neighbors more than distant ones despite continuous interbreeding across the whole area.

The Threshold and Its Asymmetry

In the account’s image, Tit for Tat can cross a “knife-edge”: local clustering supplies a route by which the strategy can gain a foothold on the favorable side of a critical frequency. The route works through the repeated local encounters that allow cooperative individuals to benefit from one another rather than relying on their frequency in the entire population. Population viscosity is therefore not merely a background feature; it changes which strategies are likely to meet, and can alter the prospects suggested by a global calculation.

The effect is asymmetric. Always Defect cannot use clustering in the same way, because local groups of defectors do especially badly in one another’s company. Their proximity does not create a cooperative enclave that can expand at another strategy’s expense. In the book’s comparison, a strategy that resists invasion under ordinary conditions is not necessarily able to exploit local structure to establish itself, while Tit for Tat may gain this additional advantage through kinship and limited dispersal.

Evolutionary Significance

Population viscosity offers a possible explanation for how cooperative strategies might become established without assuming that they are already common throughout a population. It connects relatedness to the local pattern of encounters: limited movement gathers relatives, shared inheritance can make behavioral strategies locally frequent, and repeated cooperation can allow those clusters to expand. The account presents this as a practical way a rare minority of Tit for Tat players could reach critical mass, rather than leaving the outcome to an unexplained appeal to chance.

The result is a distinction between immediate resistance to invasion and longer-term capacity to spread. Always Defect may resist invasion for a long time, but Tit for Tat may eventually assemble enough locally successful clusters to tip the population across the threshold; the reverse transition is not supported by the same clustering advantage. Population viscosity thus supplies a “secret passage” across the knife-edge, but only in the direction favored by local cooperation. It does not guarantee that cooperation will prevail: it describes how neighborhood structure can make a route to that outcome possible.