According to Engel's law, first formulated by the 19th-century statistician Ernst Engel, the proportion spent on necessities falls as wealth increases, and the share devoted to luxuries rises.

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As the wealth of the population increases, so does its consumption of food and meat.

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As a private good, human capital increases personal income, household wealth, family stability, and healthiness.

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For Rio in general, Prof Neri says, the programme is of great benefit because it increases the wealth of the wider city, especially as a reduction in crime makes tourists less wary of visiting.

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