The "30% of your salary" rule for housing stems from U.S. federal housing policy. It was first introduced in 1969 under the Housing and Urban Development Act as part of the government's effort to define affordable housing.
25%-30% was a benchmark used in housing assistance programs to determine eligibility for federal aid. If households spent more than 30% of their income on rent, they were considered "rent burdened" and might qualify for subsidies.
https://www.hud.gov/about/hud_history https://en.wikipedia.org/wiki/Housing_and_Urban_Development_...
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> person who came up with the “3 months salary for a ring” rule
That was good old De Beers. In the 1980s, they promoted the idea that spending two months' salary on a ring was appropriate, which later increased to three months in some markets.
Example of their marketing campaigns: https://i.etsystatic.com/17868656/r/il/2bc5ab/1933571085/il_...