The Progressive-Democratic Party Presidential candidates (with, for now, the lonely exception of Joe Biden) all want one.  Fred Hatfield, once a (Democrat) commissioner on the Commodity Futures Trading Commission, correctly identified one downside of such a thing.

The tax would be bad for farmers, whose support is critical in the Feb 3 Iowa caucuses.  Farmers manage risk by entering into futures contracts, a type of derivative. Under Mr [Progressive-Democratic Party Presidential candidate and Senator Bernie (I, VT)] Sanders’s proposal, trades of corn and soybeans futures would be taxed at a rate of 0.5 basis point [0.5%].

Even if farmers could somehow be exempted from a financial transaction tax, their cost of hedging would rise because the general cost of trading—of any sort—in commodities would rise, both from the tax and from the reduced liquidity of the derivatives as other traders eschew those markets. Such a tax could only negatively distort the market—as any tax in any market will do.

Moreover, those distortions would extend to other markets: grocery prices would rise from the increased prices faced by farmers and passed on to farm product buyers, beef and chicken prices would rise from the increased cost of feed, alternative farm produce would rise as other crops would substitute in (with their own increased demand-driven prices), and on and on.

And that’s just in agriculture.  The same cascading consequences would occur in all equity and debt markets, in all other commodity futures and forward markets—like metals: iron, aluminum, copper, in addition to “currency” metals—and on and on.

Investment in general, including for plant improvement and innovation, would be depressed by such a tax.

Sanders’, et al., financial transaction tax would have all the broadly negative impact on our economy as does any other social engineering-motivated tax.