Tradesports is not zero-sum. And its variation from zero-sum is a good indicator of the rationality of the traders:
Consider an event with two options, A and B. In a rational and huge environment, selling shares of A would have an equivalent effect of buying shares of B. If I believe A is more likely to win than the investing indicates, I will buy shares of A. But in doing so, I drive the price up. The higher I drive the price, the more people SHOULD be willing to compete with the sellers by becoming sellers, and thus, with new sellers coming, the price moderates itself from rising further.
Or I could sell B, or even sell it short. The lower I drive the price, the more people SHOULD be willing to to compete with the buyers by becoming buyers, moderating the price away from the bottom.
That's if people are rational and intelligent.
Suppose everyone who owns A becomes absolutely convinced that A will win. Why sell at 99 if you are sure your stock will become worth 100?
Now let's look at a field like judges. It's nearly impossible to intelligently determine the likelihood of a judge being selected. Several people becoming convinced Karen Williams will be chosen in no ways dissuades people from owning stock in Luttig. Sure enough the value of all nominees does total more than 100.
The only way to make it truly a zero-sum game, given the fact that some people are inevitably going to behave irrationally, is to fix the number of total shares each investor owns at the beginning of the game. That would ensure that each dollar invested in A meant an equal amount of divestiture from B.
Incidentally, if anyone can show me that the buying price of Bush plus the buying price of Kerry is less than 100, I will believe that people were selling candidate A short when they would have made a better investmet by buying candidate B.
Given that a few weeks earlier, the majority of the money on Tradesports predicted a Bush win, and given the "fan" nature of that election, I cannot see how that many people who had invested in Bush could have divested from him... except for the unlikely scenario that Bush supporters were as likely as Kerry supporters to believe silly polls spread by Kerry supporters. If you want to truly invest in a Bush loss, the sensible thing would have been to have bought Kerry enough that Bush+Kerry=100. Any significant variation from that, or a sudden move in that total would seem to signal foul play