A zero-sum game is one in which all participants are competing at the expense of other participants. If you are in a zero-sum game with one other person, then should you be "+1", your opponent must be "-1".

All closed systems are zero-sum: nothing can be added to the system, and thermodynamics states that energy (and therefore mass, and therefore pretty much anything) cannot be created, so in order for any one party to gain, a loss has to be measured somewhere else.

Most humans observe most conflicts as zero-sum games; this is most likely related to the dualistic view of reality that we have. Everything comes in pairs: "good/bad","right/left","high/low","true/false". Thesis + antithesis = synthesis.

The zero-sum game also occurs in our biology - most stimulus/response systems operate on a zero-sum threshold for discomfort: a being is in stasis; said being experiences negative stimulation, such as prodding a amoeba with a stylus; the being adjusts to remove the negative stimulation, such as the amoeba moving away from the stylus. Simple stimulus/response discomfort avoidance.

As a result, the default assumption for most people is that every situation is zero-sum: for someone to win, someone has to lose. This helps to shape our attitudes and policies on a large range of things, subtly influencing our decision-making by influencing the domain of the problem.

An example of this effect: most people imagine the happiest state they can be in as the goal. The problem is, if they achieve this, they then start thinking of this state as the zero point. If you are extremely happy around your current significant other, you can start to think of this extreme happiness as "zero". When you s.o. is no longer around, you then become depressed - you're at a negative state - because being at zero and losing something means going negative. It's likely you won't be happy again until s/he comes back.

But, this process is flawed: "zero" isn't when you're at your happiest, it's when you have no real stimulation - up or down - at all. Being around a positive then lifts you positive; when that is lost, you're just returned to zero, neutral.

The psychological implications between the two understandings is significant. Differences between these two determine whether or not someone can be content by his- or herself. It also has ramifications in general perception: if you "shift your zero", the contents of a room can become background noise, and only those things actively seeking your attention are visible (versus any contents at all being +'s on top of the neutral of an empty room - or even the room being a + on top of a neutral void).

All in all, most people psychologically handicap themselves by shifting their zero and, thus, losing out on a lot.