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.