The last several days in the US market has been a bit of a shock to lots of investors. History has had many surprise events such as the unexpected 1987 super US Market crash. Anyone connected to the markets in some way back then can remember it as easily as those who remember the assassination of President Kennedy. Not too many years ago, we had the "Flash Crash" that saw a 1,000 point swing in the Dow Jones. The main difference I see in markets from years ago and the markets today is the movement of markets around the world all trending in the same direction.
I've talked about this for several years now but I thought a graph below of today's market returns would do a better job of describing what happens when one country's market goes down. Many many years ago this was not the general case. When one country's market went down, other countries markets might be down or up.
The simple reason for all markets trending the same has to do with the fact that companies are now doing business in other countries. When business slows down in a country they are doing business then this affects their bottom line and investors care about the bottom line. So when expectations of a company's revenue are perceived to go down then investors in the parent country trade the stock with those expectations. Essentially now everybody is in everybody's business around the world. The world economy is now acting like what one country's economy used to do.
Trade according to the big movements and investing will be less of a surprise.