Tomorrow, as you cast a vote, you might also gird yourself for rocky markets ahead, especially during the first nine months of 2014.

How so? The second year of a presidential term is traditionally a period of subpar stock performance.

The "presidential stock market cycle" says that stocks perform better or worse depending on the year of the president's term. The second year is the worst, and the third is the best, on average.

Specifically, since 1945, the second year of a president's term saw the S&P 500 gain 5.3% in price on average, versus 16.1% in the third, according to an analysis by S&P Capital IQ. No distinction is made between a president's first or second term. The clock simply starts over.

Of course, the figures are averages, so not all years follow the cycle in lock step. Still, the third year sees the index gain 88% of the time; the second year, only 59%.