6 ideas
Collier evaluates migration through its distinct effects on the migrants themselves, the societies they leave, and the societies they enter. Good policy must weigh all three rather than a single moralized frame.
The size of an existing diaspora lowers the cost of further migration, so flows tend to accelerate rather than self-limit. Absent policy, migration is a positive-feedback process, not an equilibrium.
Collier argues migration up to a point benefits everyone, but very rapid, large flows can erode the mutual trust that makes host institutions work. The relationship is non-linear, not simply more-is-better or worse.
He centers the argument on social trust and cooperation, the intangible capital that lets prosperous societies function. Migration policy is really about protecting this fragile asset while sharing its benefits.
For the origin society, emigration of the talented can either help via remittances and returns or hurt by draining scarce skills. The effect depends on country size and the rate of departure.
Collier reframes immigration controls as the precondition that keeps migration sustainable and publicly supported. Denying the need for limits, he argues, ultimately undermines the openness liberals want.