[highfreq]

While I think the articles most point is reasonable. The choice of stocks as an example is a poorly one. Because prices are reacting in large part to news. To the degree that the news presents something material and unsuspected it influenced peoples trading. Of course good news doesn't always lead a stock to increase, because sometimes most investors are expecting better news. Certainly there is a lot of randomness, but for heavily traded stocks the impact of news is real and substantial. Unexpected good news most often leads to an increase in stock price, and unexpected bad new most often leads to a decrease.

[SixSigma]

Here's an example: http://m.bbc.co.uk/news/business-27058146

Google's share price changes the same day as an announcement. The BBC attributes the change to the news as though they are monotonic. But who is to say that the news didn't stop the share price falling twice was far?

Also, every trade requires two parties, so one side thought they were getting a good deal.

I'm also amused that the share traders are called "investors" rather than "gamblers".