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経済学史上最も確立された法則の１つとか言われているんだが

One of the most enduring relationships in economics is that proposed by Ernst Engel in 1857. Even today, it is often referred to as a law, “Engel’s law”.

Engel’s Law Around the World 150 Years Later
By Richard Anker
https://core.ac.uk/display/6307249