[madcaptenor]

This is a technical definition in terms of "elasticity of demand": see https://en.wikipedia.org/wiki/Income_elasticity_of_demand . Unfortunately it doesn't agree with what normal people hear when they hear the word "luxury".

[sharemywin]

so the article basically restated the definition of luxury. The rich spend more than the poor on luxury items.

[cbanek]

Well the interesting part is the ratios they give, not the definition of luxury.

It also assumes people are rational consumers, which obviously isn't true. Why would people go into (credit card) debt for something they don't need (luxury)?

Impulse control? Bad financial education? People declare bankruptcy anyway?

[sharemywin]

if your life sucks anyway why would you care.