"12 inches of snow. That's about a foot." No, actually that is a foot, you poor, sad imbecile.

Roughly the same proportion could not identify Joseph Stalin, Winston Churchill or Charles de Gaulle. A third thought that Franklin Roosevelt was president during the Vietnam War, and that Columbus sailed to America after 1750. Forty-two per cent - this is my favourite - couldn't name a single country in Asia.

I am always a little dubious about these surveys because I know how easy it would be to catch me out. ("The study found that Bryson couldn't understand simple instructions for assembling a household barbecue and nearly always inadvertently washed both the front and back windscreens when driving round corners.") Still, there is a kind of emptiness of thought at large these days that is hard to overlook. The phenomenon is now widely known as the "Dumbing Down of America".

I first noticed it myself a few months ago when I was watching something called the Weather Channel on TV, and the meteorologist said: "And in Albany today they had 12 inches of snow," then brightly added, "That's about a foot."

No, actually that is a foot, you poor, sad imbecile.

On the same night I was watching a documentary on the Discovery Channel (little realising that I would be able to watch this same documentary on the Discovery Channel up to six times a month for the rest of eternity) when the narrator intoned: "Owing to wind and rain, the Sphinx eroded by three feet in just 300 years," then paused and solemnly added: "That's a rate of one foot a century.

See what I mean? It sometimes feels as if nearly the whole nation has taken Nytol and that the effects haven't quite worn off. This isn't just some curious, occasional aberration. It happens all the time.

Did you say "with their luggage?" Wow!

What the customers wanted, according to the incisive prose of Mr Gordon Bethune, President and CEO, was "a clean, safe and reliable airline that took them where they wanted to go, on time and with their luggage".

Gosh! Let me get a pen and a notebook! Did you say "with their luggage?" Wow!

Now don't get me wrong. I don't for a moment think that Americans are inherently more stupid or brain-dead than anyone else. It's just that they are routinely provided with conditions that spare them the need to think, and so they have got out of the habit.

Partly, you can attribute it to what I call the "London, England" syndrome, after the American newspaper practice of specifying the country as well as the city in datelines. If, say, The New York Times were to report on a British general election, it would dateline the story "London, England," so that no reader anywhere would have to think: "London? Now, let's see, is that in Nebraska?"

American life is full of these little crutches, sometimes to a quite astonishing degree. A few months ago, a columnist in the Boston Globe wrote a piece about unwittingly ridiculous advertisements and announcements - things like a notice in an optometrist's shop saying "Eyes Examined While You Wait" - then carefully explained what was wrong with each one. ("Of course, it would be difficult to have your eyes examined without being there.")

It was excruciating, but hardly unusual. Just a couple of weeks ago a writer in The New York Times magazine did almost precisely the same thing, writing an essay on amusing linguistic misunderstandings and then explaining each in turn.

"It's about the future, because that's where we're going."

The idea is to spare the audience having to think. At all. Ever. I was recently asked by an American publication to remove a particular reference to David Niven, "because he's dead and we don't think he'll be familiar to some of our younger readers". Oh, but of course.

On another occasion, when I made reference to someone in Britain attending a state school, an American researcher said to me: "But I didn't think they had states in Britain."

"I meant state in the rather broader sense of nation state."

"So you mean public schools?"

"Well, no, because public schools in Britain are private schools."

Long pause. "You're kidding."

"It's a well-known fact."

"So let me get this straight. They call private schools public schools in Britain?"

"Correct."

"Then what do they call public schools?"

"State schools."

Another long pause. "But I didn't think they had states in Britain."

But let us finish with my favourite inanity of the moment. It is the reply given by Bob Dole when he was asked to define the essence of his campaign.

"It's about the future," he replied gravely, "because that's where we're going."

The scary thing is, he's right.