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Mortgage lenders typically use a debt-to-income ratio (i.e. such as 40%) to determine the maximum amount of loan for each borrower. This ratio limit is based on the lender's estimate of "affordability", that is, how much can you afford to spend on housing? Should this ratio be the same for a borrower earning $50,000 per year versus one earning $200,000?
Taxes that impose a greater % of income burden on higher-income individuals is called a "progressive" tax while taxes that impose a greater % of income burden on lower-income individuals is called a "regressive" tax. What types of taxes are regressive? Progressive