ONE of the many mysteries surrounding the female sex is that a woman who weighs roughly the same as she did 20 or more years ago now wears smaller-sized clothes than she used to. The explanation is “size inflation”: clothes with the same size label have become steadily larger over time.

Measurements vary by brand, but research by The Economist finds that the average British size-14 pair of women's trousers is more than four inches bigger at the waist today than they were in the 1970s (see chart), and over three inches wider at the hips. A size 14 today fits like a former size 18, and a size 10 fits like an old size 14. The same “downsizing” has happened in America where, to confuse matters further, a size 10 is equivalent to a British size 12 or 14, depending on the manufacturer.

As the average girth of women has increased, fashion firms have stretched the size of their garments, partly in the belief that women feel better (and so are more likely to buy) if they can squeeze into the same size dress as in their more slender youth. Most women would be shocked to know that they have gone up two whole dress sizes.

Such inflation mainly affects women's clothing, since most men's trousers are sized in inches rather than arbitrary units. But men are not immune. Studies in America and Britain have found that some brands of men's trousers labelled “waist 36 inches”, say, are in fact up to five inches bigger. Size inflation flatters customers, but the danger is that it encourages overweight people to dismiss health risks and reduces the incentive to diet. For the roughly three-fifths of adult Britons who are overweight, size really does matter.