Acho que também foi por isso

"Finanças dizem que Moody's ignorou o novo imposto"
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Julgo que o lançamento de mais um imposto numa economia é uma previsão de mais riqueza criada pelo esforço das pessoas e que é roubada pelo Estado.
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Mais riqueza roubada menos consumo, menos crescimento, mais pobreza, menos incentivos para criar riqueza... logo:
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"1) The government's plans to restrain its spending may prove difficult to implement in full in sectors such as healthcare, state-owned enterprises and regional and local governments.

2) The government's plans to improve tax compliance (and, hence, generate the projected additional revenues) within the timeframe of the loan programme and, in combination with the factor above, may hinder the authorities' ability to reduce the budget deficit as targeted.

3) Economic growth may turn out to be weaker than expected, which would compromise the government's deficit reduction targets. Moreover, the anticipated fiscal consolidation and bank deleveraging would further exacerbate this. Consensus growth forecasts for the country have been revised downwards following the EU/IMF loan agreement. Even after these downward revisions, Moody's believes the risks to economic growth remain skewed to the downside.

4) There is a non-negligible possibility that Portugal's banking sector will require support beyond what is currently envisaged in the EU/IMF loan agreement. Any capital infusion into the banking system from the government would add additional debt to its balance sheet.
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A further downgrade could be triggered by a significant slippage in the execution of the government's fiscal consolidation programme, a further downward revision of the country's economic growth prospects or an increased risk that further support requires private sector participation."
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Como é que Tainter explicou o colapso de uma sociedade?
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Trechos retirado daqui.