We all deal with systems—computers, cars, or communities, for example—and a few concepts may help see things a little more clearly.
This is an excerpt from another book of mine, Future Hype: The Myths of Technology Change.
 

Perfection means not perfect actions in a perfect world,
but appropriate actions in an imperfect one.
— R. H. Blyth

Systems are difficult to work with, and seeing things for what they are is an essential first step.  Horst Rittel in the late 1960s distinguished between “tame” and “wicked” problems.  This is not the distinction between easy and hard problems—many tame problems are very hard.  But wicked problems, while not evil, are tricky and malicious in ways that tame problems are not.  The unexpected consequences we’ve seen have been because systems problems are wicked.  We will understand systems better—and why they spawn unexpected consequences—if we understand a little more of the properties of wicked problems and approach them with appropriate respect.
Tame problems can be clearly stated, have a well-defined goal, and stay solved.  They work in a Newtonian, clockwork way.  The games of chess and go are tame.  Wicked problems have complex cause-and-effect relationships, human interaction, and inherently incomplete information.  They require compromises.
For example, mass transit is a wicked problem.  Everyone likes mass transit—unless it comes through their neighborhood, it consumes road lanes, or they have to pay for it.  The difference between something that works in the lab, on paper, or in one’s head versus something that works in the real world and is practical to real people is a characteristic only of wicked problems.
Tame and wicked problems differ in many ways.*  See if the traits of wicked problems as described below sound familiar, either with the examples mentioned here or with situations you have experienced yourself.

• Problem Definition.  A tame problem can be clearly, unambiguously, and completely stated.  Math problems are tame.  By contrast, there is no absolute statement of a wicked problem.  To state a wicked problem means to also state its solution.  That is, the problem can’t be stated without a proposed solution in mind, and coming up with a new solution means seeing the problem in a new way.  Avoid locking in a problem definition too soon.
• Goal.  A tame problem has a well-defined goal, such as the QED in a proof or the checkmate in chess.  With a wicked problem, you could keep iterating and refining your solution forever—or go back and consider other solutions.  After all, if a wicked problem is something you can’t define, how can you tell when it’s resolved?  You don’t stop because you’re done (you’ve reached the goal) but rather because of external constraints (you’ve run out of money, time, or patience, for example).  You must strive for an adequate solution, not a perfect one.
• Solutions.  Solutions are unambiguously correct or incorrect with tame problems.  The solution to a wicked problem is not judged as correct or incorrect but somewhere in the range between good and bad.
• Time.  The solution to a tame problem can be judged immediately (that is, there is no maturation time), and the problem stays solved.  Euclid’s geometry proofs are still valid today.  Evaluating the solution to a wicked problem takes time (because the results of implementing the solution take time to be appreciated) and is subjective.  Is that a good design?  Maybe, but maybe not.  Like the response to art, different people will have different answers, and the solution causes many side effects (unintended consequences), like medicine in the body.  Additionally, a “solved” wicked problem may not stay solved—wicked problems aren’t solved but are only addressed; they’re treated, not cured.  Your perception of how good the solution is may change over time.
• Consequences.  Trial and error may be an inefficient approach with a tame problem, but it won’t cause any damage.  Implementing or publicizing a proposed solution doesn’t change the problem.  With a wicked problem, however, every implementation changes reality—it’s no longer the same problem after an attempted solution.  After a failed attempt, the solution you realize you should have tried may now not work.
• Reapplying Past Solutions.  A class of tame problems can be solved with a single principle.  A general rule for finding a square root or applying the quadratic formula will work in all applicable cases.  By contrast, the solution to a wicked problem is unique.  We can learn from past successes, but an old solution applied unchanged to a new problem won’t produce the old result.  Many unexpected consequences arise when we rush to reapply (without customization) a particular solution we’ve seen before—there will likely be unseen differences between the old and new problems.
• Problem Hierarchy.  A tame problem stands alone.  It is never a symptom of a larger problem, but a wicked problem always is.  For example, if the cost of something is too high, this can be a symptom of the higher-level problem that the company doesn’t have enough money.  Often, we can’t see the higher-level problem (“This new software is terrific!  I can’t imagine what could be better.”).

Systems are large, complex, and real-world, and they are the domain in which technology is applied.  Industry’s dreams and expectations for its new high-tech products are formed in the lab, but it is in the system of society that they’re put to use.  This brief summary of wicked problems as well as these cautionary examples give some insight into the inherent difficulty of meddling with systems.  This is not to say that we can’t address systems problems but that they should be approached with caution and respect.
Let’s end this chapter with a final example of unexpected consequences due to technology.  In the 1954 short story “Answer,” Fredric Brown envisions many great scientists working for years to build a giant computer network, connecting the computing power of billions of planets.  As the inaugural question for this technological marvel, the gathered dignitaries ask, “Is there a God?”
The computer doesn’t hesitate before answering, “There is now!” 

Everything has both intended and unintended consequences. 
The intended consequences may or may not happen;
the unintended consequences always do.
— Dee Hock, president of VISA

* Rittel and Webber, “Dilemmas in a General Theory of Planning,” Policy Sciences, 4:155–169, 1973.

Photo credit: Wikimedia
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• See all the definitions in the Cross Examined Glossary.