Zerosum
From Academic Kids

Zerosum describes a situation in which a participant's gain (or loss) is exactly balanced by the losses (or gains) of the other participant(s). It is so named because when you add up the total gains of the participants and subtract the total losses then they will sum to zero. Cutting a cake is zero or constantsum because taking a larger piece for yourself reduces the amount of cake available for others. Situations where participants can all gain or suffer together, such as a country with an excess of bananas trading with an other country for their excess of apples where both benefit from the transaction, are referred to as nonzerosum.
The concept was first developed in game theory and consequently zerosum situations are often called zerosum games though this does not imply that the concept, or game theory itself, applies only to what are commonly referred to as games. Optimal strategies for twoplayer zerosum games can often be found using minimax strategies.
In 1944 John von Neumann and Oskar Morgenstern proved that any zerosum game involving n players is in fact a generalised form of a zerosum game for two persons; and that any nonzerosum game for n players can be reduced to a zerosum game for n + 1 players, the (n + 1) th player representing the global profit or loss.
This means that the zerosum game for two players forms the essential core of mathametical game theory.
(The two paragraphs above are translated from the French article on zerosum games)
To treat a nonzerosum situation as a zerosum situation, or to believe that all situations are zerosum situations, is called the zerosum fallacy.
Economics and nonzerosum
Nonzerosum situations are an important part of economic activity due to production, marginal utility and valuesubjectivity. Most economic situations are nonzerosum, since valuable goods and services can be created, destroyed, or badly allocated, and any of these will create a net gain or loss.
If a farmer succeeds in raising a bumper crop, he will benefit by being able to sell more food and make more money. The consumers he serves benefit as well, because there is more food to go around, so the price per unit of food will be lower. Other farmers who have not had such a good crop might suffer somewhat due to these lower prices, but this cost to other farmers may very well be less than the benefits enjoyed by everyone else, such that overall the bumper crop has created a net benefit. The same argument applies to other types of productive activity.
Trade is a nonzerosum activity because all parties to a voluntary transaction believe that they will be better off after the trade than before, otherwise they would not participate. It is possible that they are mistaken in this belief, but experience suggests that people are more often than not able to judge correctly when a transaction would leave them better off, and thus persist in trading throughout their lives. It is not always the case that every participant will benefit equally. However, a trade is still a nonzerosum situation whenever the result is a net gain, regardless of how evenly or unevenly that gain is distributed.
Complexity and nonzerosum
It has been theorized by Robert Wright, and among others, that society becomes increasingly nonzerosum as it becomes more complex, specialized, and interdependent. As one supporter of this view states:
 The more complex societies get and the more complex the networks of interdependence within and beyond community and national borders get, the more people are forced in their own interests to find nonzerosum solutions. That is, winwin solutions instead of winlose solutions.... Because we find as our interdependence increases that, on the whole, we do better when other people do better as well  so we have to find ways that we can all win, we have to accommodate each other  Bill Clinton, Wired interview, December 2000.[1] (http://www.wired.com/wired/archive/8.12/clinton.html)
See also
Topics in game theory 

Evolutionarily stable strategy  Mechanism design  Nowin  Winner's curse  Zerosum 
Games: Prisoner's dilemma  Chicken  Stag hunt  Ultimatum game  Matching pennies ... 
Related topics: Mathematics  Economics  Behavioral economics  Evolutionary biology  Evolutionary game theory  Population genetics  Behavioral ecology 
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