Q: I am 56 and my husband is 58. Over the past two years his sex drive has declined, although we still have enjoyable sex once a week. I'm sure he doesn't have someone else and he has no physical problems - he is very active with lots of interests. But he prefers a good night's sleep most nights, and I don't. Now that I can't get pregnant I would like a more active sex life. Am I being unrealistic?

The Times sex counsellor replies: Possibly - there is a wealth of research into sexual frequency, and pretty much all of it would classify you and your husband in the “normal to damn lucky” category. In 1995 the sociologist Vaughn Call surveyed more than 6,000 married people in the US and found that couples aged between 40 and 50 had sex between four and seven times a month, compared with couples aged between 19 and 29, who had sex about ten to 12 times per month.

In the UK the psychotherapist Brett Kahr has compiled information about sexual frequency that shows that 21per cent of women and 15 per cent of men have no sex at all; 32 per cent of British adults are classified as low sexual frequency practitioners (less than once a month) ; 44 per cent as medium sexual frequency (between once monthly and twice weekly) and only 19 per cent are high sexual frequency (three times a week or more). The percentage of over-50s who had sex more than once daily registered at 0.

Why do older married couples have so little sex? Economics theory explains the phenomenon with the same premise behind “all you can eat” buffet restaurants. “‘The law of diminishing marginal utility” describes how the initial undertaking of an activity (in this case sex) offers us the greatest pleasure and benefit. Because subsequent participation in the same activity cannot compare to the first-time experience, sex (or a third plate of food) loses its utility, and the activity eventually decreases or stops altogether.

When novelty is lost, the costs of sex (physical exertion and emotional investment) can appear to outweigh the benefits (physical pleasure), yet sex is fundamental to our happiness. In 2003 Andrew J. Oswald, who rather sweetly describes himself as a “dull statistical economics professor”, produced a paper entitled Money, Sex and Happiness: An Empirical Study with Professor David G. Blanchflower.

Based on data from 16,000 Americans, the study establishes a close correlation between happiness and sexual activity. Oswald and Blanchflower calculated that the average American over the age of 40 has sexual intercourse two or three times a month, that the “happiness-maximising” number of sexual partners in the previous year is “1”, and that greater income does not buy more sex or more sexual partners.

More recently Daniel Kahneman, the Princeton psychologist and Nobel Laureate, carried out a study involving 909 working women from Texas to try to determine the activity that produced the single largest amount of happiness. In 21st place on the “Happiness in Different Activities Index” was work (6.9 hours a day). Number one? You guessed it. Despite amounting to a paltry 0.2 hours a day, sex came first on the list.

Kahneman's work suggests that when it comes to sex, scarcity does not diminish significance, and as your husband approaches 60, statistics and economics theory would suggest that you are genuinely lucky to be in a monogamous relationship with a man who is healthy and active and has lots of interests.

My advice would be to make the most of what you have, but there would be no harm in trying the following changes. One or all of these is bound to have a positive effect.

1. You (plural) should avoid eating a big meal late at night.

2. Have a couple of drinks to relax, but no more than that.

3. Shower or bath in the evening, rather than in the morning, so that you are clean going to bed.

4. Create the right environment in your bedroom. Eliminate clutter, invest in new linen and get rid of the TV.

5. Go to bed with your husband - not before him and not after him.

6. And ban flannelette nighties and PJs. Naked skin only, please.

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