Ireland enters the argument through a brief visit to a remote island off its west coast. The island is not presented as a physically representative model of ordinary populations; instead, it makes visible how limited movement can gather relatives in one place and shape local patterns of resemblance and cooperation.

A local cluster of kin

The island’s inhabitants were strikingly alike in one conspicuous feature: almost everyone had unusually large, jug-handle ears. The account treats this resemblance not as an adaptation to the local climate—the island’s strong offshore winds make that explanation unlikely—but as a consequence of the inhabitants being close kin. Ireland thus supplies a specific, observed illustration of how related people can form local clusters.

Viscosity and cooperation

The island serves as a memorable comparison for population viscosity: a tendency for individuals to remain near where they were born. When movement is limited, genetic relatives are more likely to live near one another than would be expected if individuals were distributed randomly. Their resemblance can extend beyond visible features to inherited tendencies, including a possible tendency to play Tit for Tat. Consequently, a strategy that is rare across the overall population may be common enough within a local cluster for its members to meet and benefit from cooperating with one another.

The limits of the analogy

The island’s physical isolation makes it an imperfect parallel for a large population. The relevant case is not a population cut off from the rest of the world, but one with relatively little movement, where individuals tend to resemble their immediate neighbors more than distant ones despite continued interbreeding across the wider area. Ireland’s role is therefore illustrative rather than evidentiary proof of a general rule: it gives a striking image of local kin clustering, while the argument concerns how such clustering could help cooperative strategies spread through populations.