diagnoses, treatments, and information. In contrast, the health care system in a community is a macrosystem. It consists of numerous microsystems (doctor’s offices, hospitals, long-term care facilities, pharmacies, Internet websites, and so on) that are linked to provide continuity and comprehensiveness of care. Similarly, a thermostat and fan comprise a relatively simple microsystem. Combine many of these, along with various boiler, refrigerant, and computer-control microsystems, and one has a macrosystem that can maintain an office building environment.

A distinction can also be made between systems that are largely mechanical in nature and those that are naturally adaptive (see Table B-1). The distinctions between mechanical and naturally adaptive systems are fundamental and key to the task of system design. In mechanical systems, we can know and predict in great detail what each of the parts will do in response to a given stimulus. Thus, it is possible to study and predict in great detail what the system will do in a variety of circumstances. Complex mechanical systems rarely exhibit surprising, emergent behavior. When they do—for example, an airplane explosion or computer network crash—experts study the phenomenon in detail to design surprise out of future systems.

In complex adaptive systems, on the other hand, the “parts” (in the case of the U.S. health care system, this includes human beings) have the freedom and ability to respond to stimuli in many different and fundamentally unpredictable ways. For this reason, emergent, surprising, creative behavior is a real possibility. Such behavior can be for better or for worse; that is, it can manifest itself as either innovation or error. Further, such emergent behavior can occur at both the microsystem and macrosystem levels. The evolving relationship of trust between a patient and clinician is an example of emergence at the microsystem level. The AIDS epidemic is an example of emergence that affects the macrosystem of care.

TABLE B-1 Mechanical Versus Naturally Adaptive Systems

Type of System


Naturally Adaptable


Thermostat and fan

Patient giving history information to a physician


Office building heating, ventilation and air conditioning

U.S. health care

The distinction between mechanical and naturally adaptive systems is obvious when given some thought. However, many system designers do not seem to take this distinction into account. Rather, they design complex human systems as if the parts and interconnections were predictable in their behavior, although

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement