What would happen if all borders opened?

The economic literature on open borders is unusual in the size of the estimated effect: several published models put the gain in global output at anywhere from 50 to over 100 per cent, on the grounds that the same worker produces far more in a high-productivity economy and that current restrictions are the largest single distortion in the world economy. Those models generally assume institutions stay intact under very large population movement, which is precisely the assumption critics regard as doing all the work. Observed migration behaviour also suggests movement would be far smaller than the theoretical maximum, since most people do not migrate even when legally free to, as within the European Union. Housing, public services and wage effects on the lowest-paid domestic workers are the contested empirical questions, and the evidence is mixed rather than absent. The honest classification is speculative, because the physical resources exist and the political and institutional response is unmodelled.

How confident this is

This is speculation. The scenario is far enough outside anything observed that the answer depends heavily on which assumptions you start from — treat it as a thought experiment, not a prediction.

Society

• What would happen if everyone stopped having children? Schools would empty within a decade, the workforce would begin shrinking in about twenty years, and the last human would die in roughly a century.
• What would happen if everyone stopped eating meat? A large drop in agricultural emissions and land use, offset by disruption to a billion livelihoods and some regions with no alternative.
• What would happen if antibiotics stopped working? Routine surgery, chemotherapy and childbirth would all become far more dangerous — this is already happening slowly.
• What would happen if all jobs were automated? The production problem would be solved and the distribution problem would not — nobody has demonstrated the second half.
• What would happen if half the world's population disappeared? Not abundance — the collapse of specialised supply chains, because modern systems need specific people rather than a certain number of them.
• What would happen if the world's population doubled overnight? Food is the binding constraint — global production feeds around 8 billion, and doubling demand cannot be met from existing stocks.
• What would happen if everyone lived to 150? It depends entirely on whether those are healthy years — extended lifespan without extended healthspan makes almost everything worse.
• What would happen if everyone worked a four-day week? Trials generally show output holding steady with better wellbeing — but the results come from office work, not from hospitals or factories.
• What would happen if everyone drove an electric car? Urban air quality would improve immediately; the grid would need roughly a quarter more electricity, and mining demand would rise sharply.
• What would happen if everyone spoke the same language? Trade and science would get modestly easier, and roughly seven thousand languages worth of knowledge would be lost.
• What would happen if nobody paid taxes? Everything funded collectively stops — roads, courts, defence, schools, health systems and pensions — within weeks to months.