The following are excerpts from “A Leader’s Framework for Decision Making” published in Harvard Business Review by David Snowden and Mary Boone.

A complex system has the following characteristics:

• It involves large numbers of interacting elements.
• The interactions are nonlinear, and minor changes can produce disproportionately major consequences.
• The system is dynamic, the whole is greater than the sum of its parts, and solutions can’t be imposed; rather, they arise from the circumstances. This is frequently referred to as emergence.
• The system has a history, and the past is integrated with the present; the elements evolve with one another and with the environment; and evolution is irreversible.
• Though a complex system may, in retrospect, appear to be ordered and predictable, hindsight does not lead to foresight because the external conditions and system constantly change.
• Unlike in ordered systems (where the system constrains the agents), or chaotic systems (where there are not constraints), in a complex system the agents and the system constrain one another, especially over time. This means that we cannot forecast or predict what will happen.

Some thinkers and practitioners have started to argue that human complex systems are very different from those in nature and cannot be modeled in the same ways because of human unpredictability and intellect. Consider the following ways in which humans are distinct from other animals:

• They have multiple identities and can fluidly switch between them without conscious thought.
• They make decisions based on past patterns of success and failure, rather than on logical, definable rules.
• They can, in certain circumstances, purposefully change the systems in which they operate to equilibrium states in order to create predictable outcomes.