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Economists believe that a financial transaction tax would primarily result in reducing low-income employment opportunities and ultimately be paid for by low-income consumers: http://en.wikipedia.org/wiki/Robin_Hood_tax#Evaluation_and_r...

The TL;DR is that a transaction tax would primarily serve to increase bid-offer spread, which slows down the markets and results in every industry selling slightly less at slightly higher prices - which naturally means slightly shifting to favour those with more purchasing power.