Since this thread is getting boring and repetative, I figured I would introduce a new concept, the negative sum game, when most (not all, but the majority) of the participants lose. (Playing poker in a casino is a negative sum game, because of the rake, which is the house's comission.)
Example:
I am a doctor in cancer research. I come up with a new way that MIGHT treat cancer. I found a company based on my promising research and we go public. At the IPO we take in 100 million from the original investors. The stock start to trade, we start the research, use the money to hire doctors, laboratories, etc.
After 2 years, we realize that we used up pretty much all of the money and my original idea wouldn't work for one reason or other. I hold a pressconference announcing the news. Stock start to fall, in the next few weeks we burn through all the 100 mill and eventually go bankrupt. The company gets unlisted, stock trades at 2 cents a piece, if at all. The company never earned a dime, never paid any dividens.
In this case from the stocktraders' view it was a negative sum game, because basicly 100 million got siphoned out of the investors pockets. Overall it was zero sum, because that money was used up by my company, it went to reserach and salaries, nevertheless stocktraders traded the stock down until it got to zero, except shorters, anybody who bought it after the announcement had to sell it at a lower price. (Just for the argument sake, if the stockprice was less than $5 originally, you couldn't even short it.)
This just shows, that :
1. The stockmarket itself is not a closed system, because beside buyers and sellers there is a 3rd entity, called the company. That fact greatly influences the whole game set up.
2. It is possible that there are more money lost than made, thus a stock can be a negative sum game.
Conclusion: (in my opinion)
The stockmarket is not a zero sum game (because it is not a closed system, which is a requrement for zero sum games), but that doesn't mean that it is always a positive sum game. Stocks can be negative sum games too.
On the investopedia website, when they write about this, they say:"... it is not a zero sum game because wealth CAN BE created outside of the stocktraders."
Please note that they use CAN and not IS. Thus it is a conditional, it doesn't mean that wealth is ALWAYS created by the underlying companies, just as my example showed. Or just recall pets.com,webvan.com and other very promising tech start ups from the bubbletimes. They were all negative sum games eventually as far as stocktraders were concerned, although some traders made money by trading them.
Now for extra fun, I mention again, that stocks CAN BE zero sum games. When there is no dividends paid AND the company's value doesn't change in either way (if that actually happens is a different question, let's say my company in its first 2 years in the example ), then it is a zero sum game....