One of the biggest mysteries of women clothing is how a woman weighing roughly the same as she did 20 or more years ago wears smaller-sized clothing than she used to. The explanation is “size inflation” AKA vanity sizing. This phenomenon is described as: 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 pants is more than four inches bigger at the waist today than they were in the 1970s, 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.

So, why do clothing brands do this? It makes shopping for clothes more difficult when manufacturers don’t use the same standards for labeling, and no doubt increases return rates when products don’t fit as expected. The simple answer is that the downsized labels make customers feel good.

A study in 2013, published in the Journal of Consumer Psychology found that smaller sizes boosted the self esteem of the customers, while larger size labels (for the same actual size clothing) negatively affected the customer’s self esteem which transfers badly on the brand and leads to lower sales.

Though size inflation mainly affects women’s clothing, men are not immune to this. Even though most of mens pants are sized in inches rather than in arbitrary units, studies in America and Britain have shown that some brands of men’s pants labelled “waist 36 inches” are infact up to 5 inches larger.

Imagine this in a real world situation, if a consumer was originally a size 8 but has been gradually adding a few pounds, she may be unaware that in the world of standardized accurate fitting dresses she would now fit better in a size 10 dress. In the store, she tries on a dress from 2 brands, A and B. A keeps their clothing sizes consistent and the measurements haven’t changed in decades. While B, has gradually expanded the measurements for each size to the point that what might have been a size 10 dress years ago is now labeled a size 8.

So when the customer tries on a size 8 dress from A it is uncomfortably tight and will need to go up to a size 10. She finds a size 8 dress from B and it fits just fine.

So if the customer keeps finding that only larger sizes of brand A’s fit her then it is likely that her perception of that brand will decline. Is it any surprise that brands are building a few extra inches into their clothing?

In fact, clothing sizing has increased so much that in 2001, the clothing industry introduced size 0 and in 2011, they had to invent size 00 to ensure slimmer individuals could find clothing that fit.

Despite the desire of consumers for honesty and transparency in the marketing process, it seems that they may be willing for brands to lie a little when it comes to telling them their size. This is an important message for all brands: if you give the customers a product that makes them feel good about themselves, they will like the product more and you will sell more. And if your product makes them feel worse, they might as well spend their money elsewhere.