I got the data straight out of the BLS Consumer Expenditure Survey . Specifically the numbers below are from the tables for Size of consumer unit.
First family size and spending on monthly basis :
Pretty obviously, bigger families spend more.
But what about the amount spent per person?
There the trend reverses and for larger families they spend less per person. I assume this is due to a couple factors. First I'm assuming that the larger the family the more smaller children and I'd assume smaller children consume less food (usually). So a typical family of 4 with 2 little kids isn't going to eat as much per person as 2 adults would. Second I'd also guess that larger families are better at economizing on their food spending due to necessity. Its one thing to splurge on the steak if you're a single person but buying 5 steaks is going to hit your pocket book a lot harder.
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