In the hours after the US President is elected, equity investors need to brace for volatility. What they shouldn't do is panic.

That's because regardless of how prices react on Wednesday, next-day moves in the S&P 500 Index are useless in telling what comes after. While the index swings an average 1.5 percent the day after the vote, gains or losses over the first 24 hours predict the market's direction 12 months later less than half the time.

This matters because the compulsion to act in the vote's aftermath is often very strong－stocks swing twice as violently as normal those days, data compiled by Bloomberg show. They plummeted 5 percent just after Barack Obama beat John McCain in 2008. But while nothing says Wednesday's reaction won't be a harbinger for the year, nothing says it will, either, and investors should think before doing anything rash.

"Trying to trade that is very difficult," said Thomas Melcher, the Philadelphia-based chief investment officer at PNC Asset Management Group. "Even if the market sells off, if you have any reasonable time horizon, that should be a buying opportunity. The dust will settle and people will conclude the economy is OK."

In the 22 elections going back to 1928, the S&P 500 has fallen 15 times the day after polls close, for an average loss of 1.8 percent. Stocks reversed course and moved higher over the next 12 months in nine of those instances, according to data compiled by Bloomberg.

Nothing shows the unreliability of first-day signals more than the routs that accompanied victories by Obama, whose election in the midst of the 2008 financial crisis preceded a two-day plunge in which more than $2 trillion of global share value was erased. It wasn't much better in 2012, when Election Day was followed by a two-day drop that swelled to 3.6 percent in the S&P 500, at the time the worst drop in a year.

Of course, Obama has been anything but bad for equities－or at least, he hasn't gotten in their way. The S&P 500 has posted an average annual gain of 13.3 percent since Nov 4, 2008, better than nine of the previous 12 administrations. Data like that implies investors struggle to process the meaning of a new president just after Election Day, or infuse the winner with greater influence than they have.

Bloomberg