• A ship scuttled in shallow water becomes an artificial reef
• EU recycling rules produce a mountain of old refrigerators
• The Endangered Species Act leads landowners to "shoot, shovel, and shut up"

All human action is characterized by unintended consequences.  Some are good.  Some are bad.  Some are trivial, some profound.  But none, by definition, are part of the original plan.

To get a sense of how unintended consequences can be positive, consider Adam Smith's "invisible hand," perhaps the most famous metaphor in social science.  Smith maintained that each individual, in seeking his or her own gain, "is led by an invisible hand to promote an end which was no part of his intention."  That unintended end, Smith argued, is the public interest.  "It is not from the benevolence of the butcher, or the baker, that we expect our dinner," Smith wrote, "but from regard to their own self interest."

While Smith's invisible hand illustrates how unintended consequences can be a good thing, it should not be too surprising that our inability to anticipate every effect of our actions also has a down side.

Laws seem especially prone to produce harmful side effects.  Perhaps because they compel or forbid action on a large scale, they often lead to outcomes that were not envisioned by the legislators who crafted them.

The Endangered Species Act of 1973 provides some compelling examples.  As the name implies, the Endangered Species Act was intended to protect animals whose populations are dwindling.  Yet despite this noble goal, the legislation has produced some deleterious effects.  In areas of the country inhabited by endangered species, the Act has created an incentive for landowners to make their property inhospitable to protected animals—in some cases by destroying their habitat.

Consider the case of the red-cockaded woodpecker, which is known to nest in pine trees than can take 80 years or longer to mature.  To avoid running afoul of the Endangered Species Act, landowners in the southeastern United States have been known to cut down the trees to dissuade the rare birds from nesting on their land.

Even worse, landowners sometimes "shoot, shovel, and shut-up" in order to avoid the regulatory pitfalls of playing host to endangered species.  Despite its good intentions, the Endangered Species Act has established a set of incentives that make endangered species a nuisance to landowners.