Imagine for a moment that a single company ran Washington Dulles, Chicago O'Hare and New York Kennedy airports. Out of the blue, a Mexican company puts in a takeover bid. It is a reputable company and the bid is for a lot of money - so how does the news go down in the land of the free?

On the face of it, the deal should go through with the minimum of fuss. Globalisation means the days of economic nationalism are dead. It doesn't matter who runs companies, provided they keep the customers happy, the prices low and the profits high. Takeovers, whether hostile or friendly, help keep management on their toes.

This, pretty much, is the argument in favour of the Spanish company Ferrovial in its bid for BAA - the company that runs Britain's biggest airports including Heathrow and Gatwick. The Blair government wears its free-market colours with pride.

Interestingly, though, it was a different story when a company in Dubai wanted to take over the major ports in the US. All of a sudden, the American line on protecting strategic industries looked more similar to that adopted by Jacques Chirac in France than that favoured by Tony Blair in the UK. And a good thing, too.

Here are four reasons why refusing to put key sectors of the economy "in play" might be a good idea. The first is self evident: there are bits of an economy that are important for reasons of national security. If the Americans don't want their ports to fall into foreign hands - even the foreign hands of a friendly state such as Dubai - that is their democratic right.

Secondly, there is not much evidence to back up the claim that takeovers lead to greater efficiency and hence higher growth. A recent comprehensive study from economists at Manchester University showed that companies tended to do no better under new ownership. It was, however, a different story when it came to executive remuneration packages, however.

Thirdly, it is a heck of a lot easier to shut down plants and axe jobs in a satellite plant overseas than it is to take the same sort of tough action at home.

Finally, there is the question of reciprocity. Would Spain allow a British company to run the airports in Barcelona, Madrid and Valencia? No, I don't think so either.