The drift runs one way, which inconsistency alone would not produce

The argument

The strongest argument offered for the vanity-sizing explanation is about the shape of the pattern rather than about anyone's motives. Inconsistency is a scattering process: if brands were simply applying their own standards badly, the body measurements behind any given size number would spread in both directions, with some brands labelling generously and others meanly, and the middle would hold still. Proponents of this explanation argue that what size charts actually show is a trend, in which the measurements attached to a given number have moved steadily in one direction, and a trend is not what independent error produces. From there the argument is that a one-way drift implies a shared incentive. The retailers concerned do not coordinate, but they all face the same customer, and that customer prefers the smaller number. Any brand whose sizes run tighter than its competitors is punished in the fitting room, so each has reason to move the same way and none has reason to move back. On this reading the absence of a standard is the permission and flattery is the motive, and it is the motive that explains the direction. Proponents argue that the distinction matters for the remedy. If the problem were untidiness, publishing a standard would resolve it, since nobody gains from being untidy. If the problem is a shared incentive, a voluntary standard will be adopted in name and undercut in practice, because the first brand to label honestly loses sales to those that do not. What follows, on this account, is that fixing sizing requires labels stating body measurements directly, so that the flattering number has nothing left to hide behind.

Premises

Counter-arguments

Critics answer that a one-way drift has an innocent explanation the argument does not consider: fit models are drawn from the customers a brand serves, and if that population has changed, grading faithfully to it produces exactly the trend described with no intent to flatter. They add that the inference from direction to motive is weak, because any shared external pressure produces a shared direction. They also question the evidence itself, since older size charts were built on different measuring conventions and are not straightforwardly comparable with current ones. On that view the drift is real, its cause remains open, and the remedy of printing measurements is one both explanations endorse.

Rejecting the premises

[Rejecting P1] Independent error scatters only when the influences on it are independent, and brands grading against the same customer population are not independent at all. [Rejecting P2] A drift in the measurements behind a size number is equally consistent with faithful grading against a reference body that has itself changed, so it does not select between the two explanations. [Rejecting P3] Shoppers who find a garment too large return it, which is a counter-pressure on generous labelling that the argument leaves out of account. [Rejecting C] At most it follows that a common pressure acts on sizing, not that the pressure is flattery rather than a moving reference body.