We knew that.  Advertisers who peddle the notion that only the under-25s have any right to make the beast with two backs should also think again. Or simply think.

There are one or two flies in the lubricant, though. Men, for instance, tend to be rather more satisfied than their (female) partners. This would appear to be an exclusively heterosexual study, but anomalies in the data suggest that something else may be going on. How to account for the difference between those Japanese who say they are happy in their relationships (15 per cent) and the Japanese who say they are happy with their sex lives (50 per cent)? I may not be able to do the maths, but something fishy's going on there.

One can have some fun with the national characteristics: "Austrians aged 40-80 claim to have the highest satisfaction with both their relationships and sex lives, followed by Canadians and Swedes," the report concludes. So it would appear that even the condition of being an 80-year-old Austrian is no bar to having fantastic sex, which may be alarming for the rest of us, but bully for them - even if it is not something we may care to dwell on. And how come the Swedes only come third? (Then again, I know quite a few Canadians, for some reason, and one or two of them are so randy they make me feel like Philip Larkin, so I can't claim surprise there.)

The over-40s in the affluent west have been aware of the improved quality of their sex lives for some time now. For a start, they are often rather grateful to be having sex at all. The young, for whom sex can be an unthinking duty, an obligation foisted upon them by the predominant culture, probably find it as exciting as a flat alcopop. Not that I spent my youth having what I considered a satisfactory amount of nookie; there was a bit of an alarming drought until I left university. The old adage "Who do I have to screw to get a drink round here?" was, for all practical purposes, reversed to "Who do I have to buy a drink for to get a screw round here?" The answer, then, was, "an inordinate number of people".

I asked a few friends in their 40s how their sex lives were shaping up. The results of my survey: 40 per cent of respondents said, "Get lost" (not their exact words), 30 per cent said, "Mind your own business", 20 per cent said, waggishly, "It all depends with whom", and the rest said, "What sex life?" My sample was hardly statistically significant, I suppose, but it did make me wonder what the pollsters from the Global Study of Sexual Attitudes and Behaviours were doing to get so many ready answers to their questions. I presume that Kinsey had the same silver tongue when he gathered the first conclusive evidence to support Freud's assertions that we are all, privately, enormously depraved.

One friend was more usefully forthcoming. "When you're young," she said, "you simply haven't a clue, and that goes for both men and women. Once you've got a bit of experience under your belt you start getting confident and considerate." I could concur with this. Only the fact that I was fizzing with testosterone could explain how I persisted in the search for physical ecstasy in the face of my miserably incompetent attempts to Do It Right. One recalls one's ignorance, not to mention sheer bad manners, and blushes in shame. With the years comes an appreciation of basic sexual etiquette, which for the man means - how shall I put this? - the sexual equivalent of holding the door open for the woman (oh, all right then, letting her come first); and, for the woman, not kicking up a fuss if he suggests you keep your stockings on.

On warning my wife that I was writing about this, she not only told me to be discreet, but said: "I suppose you're going to say, 'It all depends with whom.'"

For while it is true that sexual ennui for married couples may set in after the first seven weeks - I mean years - a certain mutual effort can rekindle the spark. But the Global Survey suggests that a certain amount of over-40 sexual happiness comes as a result of ditching one partner at around that age and finding another one. Which is generally easier for men, hence the gender imbalance.

I leave you with the words of one of my more forthcoming respondents: "I have three small children running round the place, and no locks on the doors. If I want to have sex with my wife we either have to leave the country or hire a babysitter. And not in the way you're thinking.

"And don't forget we're not getting any younger. Thank God we don't feel like doing it every day. We have sex about as often as we have lobster for dinner. And you know what? They're both great."

Comments on this column can be sent to relations@nationgroup.com.