I mean, it seems pretty obvious, no?

Wealth is concentrated and can skew the averages, but happiness, even if rated on a scale, is not particularly able to skew the number up... so as wealthy americans got spectacularly rich, pulling up the "rich" side, maybe making them equisitely happy... a more widespread shift in sentiments are pulling down the average.

And a lot of this makes sense... Wealth doesn't add much happiness over a certain threshold. A naive happiness maximizing algo would probably do something like cap someone around that number and redistribute the wealth to those below it.

Plus there's the monkey grape/cucumber experiment - https://www.youtube.com/watch?v=-KSryJXDpZo ... Humans are social status sensitive, meaning we're likely to have bad feelings (and make irrational choices) when we feel our place in the herd is falling or below someone else's. Eg: People living near lottery winners are more likely to go bankrupt than similar people who don’t have a winning neighbor, presumed to be a "keeping up with the joneses" kind of issue.