Aggression originates in the acute stress response found in all vertebrate creatures.  This instinct is sometimes referred to as the “fight or flight” response, and was first described by Walter Cannon in 1929.  It is perhaps the most basic survival instinct of the animal world.  When an animal perceives a change in its environment, such as the presence of another animal or an abrupt change in light levels, adrenaline and other hormones flood the body, resulting in heightened mental alertness and increased physical ability.  The animal is therefore better prepared to analyze the threat level posed by the stressor and to physically respond if the danger is real.  This heightened mental and physical state is better known as stress.  In both humans and animals, short-term stress is beneficial, as it provides additional resources to the mind and body when most needed for survival.

          While this stress instinct is commonly known by Cannon’s terminology of “fight or flight”, the actual behaviors caused by stress are somewhat more varied.  Many animals, for instance, freeze in place when threatened, essentially “fleeing” into the background environment.  Others might play dead.  An armadillo or a hedgehog might curl into a ball, or a skunk might spray its musk.  These are all variations of fighting or fleeing, as they either confront a danger or seek to avoid it.  An entirely third option is to befriend the stressor, perhaps through a non-threatening posture or a relaxed facial expression, in order to share resources and to establish that neither animal currently threatens the other. 

          Human psychology and intelligence vastly increases the range of behaviors caused by this stress, but they are all rooted in the same three choices of fighting, fleeing or befriending.  Fighting, for instance, may take the form of violence, passive aggression, social betrayal or verbal assault.  These are all ways of confronting a stressor.  Fleeing includes the whole range of psychological avoidance behaviors, including denial, conformity, and escapism.  Befriending includes the many ways in which we interact with the people and forces that threaten us in order to resolve those threats peacefully.  Despite our human sophistication, we ultimately have the same choices available to deal with our stressors as animals do with theirs; we may fight, we may flee, or we may seek to befriend.

          The next question, then, is why one person chooses to fight in a given situation while others choose to flee or befriend.  This is where the mathematical discipline of game theory comes into play.  Game theory examines behavior in strategic situations in which an individual’s success depends upon the choices made by another.  The results of various choices are often presented as a grid.  Here is a very simplistic one that models the possible stress responses: 

          As the grid shows, choosing the path of aggression maximizes the chance of winning – choosing to fight wins any time the opponent flees or befriends, and has a 50% chance of either winning or losing if the opponent is also aggressive.  Fleeing, on the other hand, minimizes the chance of losing.  A fleeing tactic will never win, but it will not lose, either.  Befriending is a bit more complex; it will lose against an aggressive opponent and win against a fleeing one, but the big payoff comes when both sides choose to befriend.  When the two sides manage to work together, everyone benefits.

          This simplistic model demonstrates the reasons different people choose different ways to respond to stress.  Those who choose the path of aggression do so because they want to win, and they fear that their opponent will also be aggressive.  Those who choose to flee are more concerned with avoiding a loss than in securing a gain, so they select a tactic that is least likely to result in a loss.  Those who choose to befriend are looking beyond the immediate conflict to try to establish a positive winning cycle that will benefit both players.  Of the three choices, befriending is the only one not rooted in fear, and it is the only one that creates a long-term solution to the conflict.  It is riskier than either fighting or fleeing because its results depend heavily upon the opponent’s choice, but once both sides learn the benefits of working together, befriending creates a positive cycle that keeps both sides winning.