FAA Mulls Adding 10 Pounds To Estimated Weight of Fliers discusses the changing estimate of passenger weight:
A draft proposal circulating within the FAA suggests the government’s assumptions of how much the average flier weighs — 180 pounds in summer and 185 pounds in winter, including clothes and carry-on items — errs on the light side. The document, prepared by the FAA’s Flight Standards Office, suggests adding at least 10 pounds to the estimated passenger weight, which airlines add up to determine whether a plane is too heavy to fly.
That average is, presumably, of all passengers, male and female (and children?), including shoes, clothes, and one “personal item” (e.g. a purse, but not a full-size carry-on). That doesn’t seem like a particularly light estimate. According to JAMA, the average American man weighs 187 pounds, and the average American woman weighs 151 pounds — without heavy clothes and a bag.
If weight is such an issue, would it be that hard to weigh passengers and their carry-ons while they’re checking in and going through all that security? Well, probably it would be an issue — even with all that junk added in, people don’t like to get weighed, and they certainly don’t want to get charged a premium for weighing too much.