to work." On January 21, 2010, President Barack Obama endorsed the Volcker Rule which deals with proprietary trading of investment banks and restricts banks from making certain speculative kinds of investments if they are not on behalf of their customers. Former U.S. Federal Reserve Chairman Paul Volcker, President Obama's advisor, has argued that such speculative activity played a key role in the financial crisis of 2007–2010. Volcker endorsed only the UK's tax on bank bonuses, calling it "interesting", but was wary about imposing levies on financial market transactions, because he is "instinctively opposed" to any tax on financial transactions. Questions of volatility In February 2010, Tim Harford, writing in the Undercover Economist column of the Financial Times, commented directly on the claims of Keynes and Tobin that 'taxes on financial transactions would reduce financial volatility'. Comparing Currency Transaction Taxes (CTT) and Financial Transaction Taxes (FTT) Research evidence In 2003, researchers like Aliber et al. proposed that empirical evidence on the observed effects of the already introduced and abolished stock transaction taxes and a hypothetical CTT (Tobin) can probably be treated interchangeably. They did not find any evidence on the differential effects of introducing or removing, stock transactions taxes or a hypothetical currency (Tobin) tax on any subset of markets or all markets. Researchers have used models belonging to the GARCH family to describe both the volatility behavior of stock market returns and the volatility behavior of foreign exchange rates. This is used as evidence that the similarity between currencies and stocks in the context of a tax designed to curb volatility such as a CTT (or FTT in general) can be inferred from the almost identical (statistically indistinguishable) behavior of the volatilities of equity and exchange rate returns. Practical considerations Hanke et al. state, "The economic consequences of introducing a [currency-only] Tobin Tax are [...] completely unknown, as such a tax has not been introduced on any real foreign exchange market so far". At the same time, even in the case of stock transaction taxes, where some empirical evidence is available, researchers warn that "it is hazardous to generalize limited evidence when debating important policy issues such as the transaction taxes". According to Stephan Schulmeister, Margit Schratzenstaller, and Oliver Picek (2008), from the practical viewpoint it is no longer possible to introduce a non-currency transactions tax (even if foreign exchange transactions were formally exempt) since the advent of currency derivatives