The articles guesses as to contributing factors:

> People are exposed to more chemicals that might be weight-gain inducing. Pesticides, flame retardants, and the substances in food packaging might all be altering our hormonal processes and tweaking the way our bodies put on and maintain weight.

> The use of prescription drugs has risen dramatically since the 1970s and ’80s. Prozac, the first blockbuster SSRI, came out in 1988. Antidepressants are now one of the most commonly prescribed drugs in the U.S., and many of them have been linked to weight gain.

> Study authors think that the microbiomes of Americans might have somehow changed between the 1980s and now.