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Hmm ... has the average american woman grew new limbs, extra appendixes, wings, gills or something like that?

No, but she might be older or younger, live in a different part of the country, be more likely single or married, have different ancestry, have a different career, live in a household with a different fractional number of children, ...

Any of these things could have an impact at least as great as the raw passage of time. IMHO, much more interesting to look at comparable individuals. Parents and their children and grandchildren at the same ages, for example.

A woman is still a woman. Neither of the factors you mentioned causes weight gain. Let alone 20%

Yep, age will. If the average age of an American woman has changed from, say, 30 years to 50 years (aging population, changing demographics etc.) then this could contribute to weight differences because people tend to become more overweight as they age and get less active.

14.2 extra gills, actually.

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