Imagine for a moment that you are Mrs Kylie Kerr of Newton Abbott in the county of Devon. It is a summer's morning, and among your day's chores is that of picking up the laundry from the dry-cleaners. There are, maybe, a blouse or two, possibly a skirt, and a suit belonging to your 41-year-old husband, Don. You hand your little green ticket over the counter, and the shop assistant goes off and rattles hangers somewhere in the back. A minute passes, and she returns with the clothes - and with something else: two gold ear-rings with coloured edges, found (she tells you) in Don's trousers pocket. They are not yours.

Now it's time to change sex and become Don himself. In the warm evening of the same day, you arrive home from the music shop that you run, and immediately sense a certain domestic froideur. The dog is quiet, the cat subdued, the radio is tuned to your least favourite station. All is explained when your wife Kylie produces - from behind her back - a strange pair of ear-rings, and holds them accusingly out to you, in the palm of one hand. These, she informs you, were found in your trousers. And you are lost for words, for you have never, ever seen them before.

Let us step outside Don, and contemplate his plight from the comfort of our own bodies. He is guiltless. It is unlikely that he has had an absent-minded affair with a woman and then pocketed her ear-rings, all without noticing. But his protestations of innocence sound no different to those of the guilty man lying. Can he prove that he hasn't been having it off with a doxy?

You can see the difficulty. There are only two ways that he can empirically convince the world that he is guiltless (yes, the world, for though Kylie accepts his innocence, Newton Abbott is abuzz with the story). The first is to show that it is impossible for him to have committed the crime. The second is for the owner of the ear-rings to come forward and claim them. Both are tall orders. How can you, after all, prove a negative? There is nobody for you to say that you were never with, for nobody exists. And nobody may ever admit that the ear-rings were theirs - or even know that they were lost. It is everybody's nightmare.

In Don's case, the nightmare has been shortlived. The dry-cleaners have admitted that they made an error, and the unfortunate husband has been absolved. But he might just as easily have fallen victim to an obstinate refusal to admit a mistake. And then what would he have done?

Don may be this week's story, but twice in the past month respected periodicals have printed articles suggesting that there are ways - short of discovery en flagrante - to tell if your spouse is betraying the marital bed. Or, indeed, is likely to.

The first, which appeared in a number of august journals - including the Daily Mail - detailed the work of Dr David Buss, an American sociologist who interviewed 107 couples together and separately. He correlated their answers with the incidence of adultery, and concluded that there was a personality profile of the adulterer. Based on this profile (it was claimed), one should be able to discern from other behaviour whether one's partner was likely to stray.

In fact, one might be able to tell simply from going out to dinner. For signs would include arriving late, looking in the hall mirror, interrupting others, going to the loo and leaving the door open, and - most revealing of all - driving past squashed animals and laughing callously.

Clearly, such information is double-edged. Should another man find himself in Don's position, he might be saved from suspicion because he exhibited none of these tendencies. Were he punctual, taciturn, discreet and - when driving - sentimental, he would clearly fail to meet the personality profile.

So far, so good. But suppose that you were entirely innocent of wrongdoing, yet - one dark night - giggled at a joke that you'd heard some hours before, just as you passed a flattened badger? Your wife or husband (an avid Mail reader) slams on the brakes, screeches to a halt, turns to you and snarls, "Who is it? I know you're having an affair!" Very nasty. But not, I'm afraid, as bad as it can get.

This newspaper itself revealed last month that - I quote - "it is a matter of scientific fact that promiscuous men have larger testicles". This is (let me add hastily) a cause, not an effect. Anyway, 80 Manchester University students - my own alma mater - allowed a zoologist, Dr Robin Baker, to measure the volume of their gonads, using a pair of calipers (warm, I hope; in my day student grants were large enough to spare us the necessity of submitting ourselves to such examinations).

Baker's largest gonad was 52 cubic centimetres, the smallest just 8 - and the average 24. Then he set testicle size against sexual behaviour and discovered a clear correlation: the chaps with bigger balls were more likely to use them.

It isn't clear why. I suppose it may be because larger genitals are more obtrusive, and thus assert themselves over the personality of the owner. Or perhaps the the greater amount of sperm-making activity somehow impels the testicle-owner into primeval, instinctual sperm-losing frenzies. Whatever. My point in all this is not to explain the theory, but to share a concern. It is, after all, possible for a man to shun dry-cleaners and thus never find himself having to explain errant ear-rings. He can also avoid mirrors, never interrupt party guests, and force himself to weep over squashed hedgehogs. But what, oh gentle reader, is a chap to do if his testicles are too big?