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One thing I notice about the debates on both tax policy and income inequality is that a lot of people seem to have relatively hazy ideas about how income is earned in America, and how much of each kind of income there is. For example, if someone is earning $300,000 a year, where is their income likely to be coming from? How much revenue is drawn from capital gains taxes? And how is income distributed between corporate shareholders and workers? People don't tend to have very strong priors about the answers to these questions, because they simply haven't yet learned what the relative sizes of different sources of income actually are.
For a person working for a wage, their source of income is their job, or their labor (aka their time that they sell to someone). But you can own a house, and rent it out, and when your renters pay you renter every month, the source of that income check is your rental property, or your house. It would just be called rental income. If you have money in the bank in an account that pays interest, those interest payments are another source of income. We can say the source is interest payments, or we can identify the asset that is generating rent, and say your money in the bank that is generating “rent” (aka interest) is your source of income.