How does the creation of the "four-hundred list" contribute to the idea of "keeping up with the joneses? "

The "four-hundred list" is a list of 400 important people that were the elite in New York during the Gilded Age.

"keeping up with the joneses"  is an idiom referring to the comparison to one's neighbor as a benchmark for social class or the accumulation of material goods.

The idea of not being in that list is what could have contributed to create the "keeping up with the Joneses" idiom. You had to have prestige and material things to be in the "four-hundred list" and that meant having more than others (the "Joneses"). The "keeping up" would mean to tolerate others having less than you do. For example: a neighbour.

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