Nonlinear stock-flow-feedback systems frequently behave in ways that are counterintuitive or different from that which a decision maker's unaided mental model would suggest -- particularly in response to the dynamics of a policy change. Forrester notes, for example, that both the places where "high leverage points" (i.e., places at which a policy change can permanently alter a system's behavior) are located, and the direction in which a decision maker must "push" a system to change it's behavior, are counterintuitive.
Frequently, the counterintuitive behavior of social systems is due to the existence of negative feedback loops. As an example, consider a small sailboat with a rudder in the stern. If the sailor operating the boat wishes to turn the bow (front) to the starboard (right), he or she must turn the rudder, located in the stern, to port (left). In other words, he or she must intervene with a policy change (turn the rudder) in a place (the stern) different from where the change will manifest itself (the bow) and in a direction (port) opposite from the one desired for the entire system (starboard). A sailboat is a negative feedback system in the sense that there is a desired direction the sailor wishes to point the bow of the boat and an actual direction the bow of the boat is pointing. If a discrepancy develops between the desired direction and the actual direction, corrective action (moving the rudder) is taken.
A second example of a simple negative feedback loop system that can exhibit counterintuitive behavior is a thermostat. Consider the following thought experiment. If a person is given a cigarette lighter (i.e., a small heat source) and an ice cube (i.e., a small source of cold) and asked to apply one to the sensor on the thermostat to make a room warmer, which one should he or she choose? The answer, of course, is the ice cube because applying a cold source to the thermostat's sensor would cause the furnace to turn on and heat the room.