Mais poupança não implica mais investimento

O Mainly Macro (que está ali na barra da direita e é leitura diária obrigatória) é um dos melhores blogues que andam por aí acerca de teoria macroeconómica pura. Hoje, dei de caras com um post antigo em que o autor desmonta uma falácia popular em economia: interpretar a identidade da Contabilidade Nacional Poupança = Investimento como uma relação causal. O formato FAQ torna a leitura fácil e rápida.

Inaugurar um blogue com um post alheio é inevitavelmente deprimente; mas, se tem mesmo de ser, pelo menos que seja com alguma coisa escrita pelo Simon Wren-Lewis.

Q: If consumers spend less and save more, does this mean investment must increase?

A: Absolutely not. Someone increasing their saving does not automatically imply that some firm will decide to buy more capital goods.

Q: But surely savings equals investment by identity in the national accounts.

A: Indeed. Total output = total income = total expenditure = Y. In the most simple model of a closed economy without government, income (Y) = consumption (C) + saving (S), but also expenditure (Y) = consumption (C) + investment (I). So S=I by definition. But here investment includes what is called ‘stockbuilding’ or ‘inventory accumulation’, which includes goods that firms wanted to sell but could not. To make this clear, lets split measured investment (I) into these two components: I=DK (buying new capital goods) +DS (stockbuilding). So if people consume less (C falls), but investment in new capital (DK) stays the same, measured investment rises because firms accumulate inventories of the goods that consumers did not buy (DS rises).

Q: But this situation cannot continue, as firms may be losing money.

A: Exactly. They will cut back on their output, incomes will fall, consumption may fall further, and savings will also fall, cutting back on the initial increase that we started with.

Q: When will this process stop?

A: When firms stop accumulating inventories i.e. when DS=0. Then, and only then, will S=DK.

Q: But how can this be? We have assumed that DK stayed the same, and we started with an increase in S?

A: You have not been paying attention. Each time firms reduce their output to match lower demand, incomes and savings fall. Eventually the initial rise in savings is reversed, because overall income has fallen.

Q: Got it. But textbooks make a big thing about aggregate savings equalling investment. If it is just an accounting identity, why is it important?

A: What the textbooks really mean is that we eventually end up with a position in which S=DK. And that is important, for the reasons we have just discussed. It is called the paradox of thrift. A desire by consumers to increase savings ends up just reducing output, and savings do not increase at all. (Of course they are still saving more of their income: S/Y has gone up, but because Y has fallen, not because S has increased.)

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