Indeed, it is only the potential for people – and capital – to fly to freer countries that prevents governments from being more tyrannical. Governments’ ability to force their citizens to surrender their property is constrained by competition from lower-taxing rivals, while societies that allow successful people to enjoy the rewards of their efforts can attract skilled workers away from less free societies. And this competition also enables societies to learn from one another: the oft-cited examples of superior practice abroad (be it high-welfare Scandinavia, high-growth East Asia or English-speaking North America and Australasia) rely on other jurisdictions trying different things, from which we can learn.
Neither, despite the passionate belief of those behind World Vote Now, does democracy always lead to prosperity. The Social Democratic failure of the mid-20th Century proves that. Bad policies can dominate even in democracies. Indeed, democracies have in-built features that undermine freedom and general welfare. The current economic crisis, caused by broadly similar policies across the developed world, should act as a signal warning that one world government would have the potential to get it wrong on a colossal scale.
It is questionable how democracy could work across 6 billion people, anyway. A legislature the size of the UK House of Commons would be made up of members who each represented 10 million people. I doubt many MPs will hold surgeries or canvass voters across constituencies the size of Beijing. Yet a larger legislature would not function. Consequently, politicians will simply be more remote; more isolated from voters. We should be devolving power, not pushing it ever further away.
Lastly, there is the risk of utter calamity. Recent history has contained far more civil than international wars, and they have proven far more costly in lives. They fall into two categories: wars to control the government; and wars to break away and form a new state. Both have resulted in (sometimes immeasurable) human suffering. Even democracies have not been spared. The potential suffering from a global civil war is too horrifying to contemplate.
There is an alternative. Polities could be small; close to the people: probably the only way that power can be held in check and made to serve, rather than to master, the people. Politicians could be limited to doing only that which individuals cannot achieve either on their own or by cooperating freely with their fellows. Polities could have open borders so that they would benefit from the work and resources of all mankind. They would be free to experiment and so to learn from one another.
Most of all, they would provide havens to which the victims of oppression could escape. Under a world government, minorities, non-conformists and dissidents would have nowhere to hide and nowhere – absolutely nowhere – to run.