With two weeks left in 2015, the S&P 500 Index is flirting with its first down year since 2008. Much of the blame for its inability to push higher has been placed on the strength in the U.S. Dollar, which has appreciated against nearly every other global currency.

The argument goes follows: a strong Dollar has been a headwind for U.S. exports, the economy, earnings, and revenues. In a world of slowing growth, currency debasement has increasingly become the economic weapon of choice. The “winners” of this currency war are said to be those countries who can debase their currency at the fastest pace.

I reject the notion that you can debase your way to prosperity, but this seems to be accepted as an absolute truth these days, especially within the ivory central bank towers.

Regardless of whether you believe that debasement is good or bad for long-term economic growth, is this something investors should be focusing on? Stated differently – should U.S. equity investors really be hoping for a weaker Dollar in the months and years to come?

Based on the historical evidence, the answer is far from clear.

We have full-year data on the Dollar Index (Bloomberg Ticker: DXY Index) going back to 1968. On a calendar year basis, the correlation between the return of the Dollar Index and the return of the S&P 500 has been essentially zero (.04).

The S&P 500 has experienced its highest return years with both a weaker Dollar (1985, 1995) and stronger Dollar (1975, 1980) with no predictive pattern. Similarly, it has experienced its worst down years with both a weaker Dollar (1974, 2002) and stronger Dollar (2001, 2008).

The chart below displays the rolling 1-year correlation between the Dollar Index and the S&P 500 using monthly returns. As you can see, there is little in the way of consistency over time.

In the current economic expansion (June 2009 – Today), however, the correlation has been more persistently negative than in prior cycles. Perhaps this can be attributed to the heightened focus on easy money central bank policies and debasement (along with weaker economic data) being viewed as a “positive” for markets. Or perhaps it is spurious, we'll never know.