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Thanks to him, the plague could finally be halted. He ordered all doctors and students to disinfect their hands in a chlorinated wash after performing autopsies. The death rate in the doctors’ maternity ward fell to barely 1 percent. Over the next twelve months, Semmelweis’s intervention saved the lives of 300 mothers and 250 babies—and that was just in a single maternity ward in a single hospital.

As we wrote earlier, the law of unintended consequences is among the most potent laws in existence. Governments, for instance, often enact legislation meant to protect their most vulnerable charges but that instead ends up hurting them.

Consider the Americans with Disabilities Act (ADA), which was intended to safeguard disabled workers from discrimination. A noble intention, yes? Absolutely—but the data convincingly show that the net result was fewer jobs for Americans with disabilities. Why? After the ADA became law, employers were so worried they wouldn’t be able to discipline or fire bad workers who had a disability that they avoided hiring such workers in the first place.

The Endangered Species Act created a similarly perverse incentive. When landowners fear their property is an attractive habitat for an endangered animal, or even an animal that is being considered for such status, they rush to cut down trees to make it less attractive. Among the recent victims of such shenanigans are the cactus ferruginous pygmy owl and the red-cockaded woodpecker. Some environmental economists have argued that “the Endangered Species Act is actually endangering, rather than protecting, species.”

Politicians sometimes try to think like economists and use price to encourage good behavior. In recent years, many governments have started to base their trash-pickup fees on volume. If people have to pay for each extra bag of garbage, the thinking goes, they’ll have a strong incentive to produce less of it.

But this new way of pricing also gives people an incentive to stuff their bags ever fuller (a tactic now known by trash officers the world around as the “Seattle Stomp”) or just dump their trash in the woods (which is what happened in Charlottesville, Virginia). In Germany, trash-tax avoiders flushed so much uneaten food down the toilet that the sewers became infested with rats. A new garbage tax in Ireland generated a spike in backyard trash burning—which was bad not only for the environment but for public health too: St. James’s Hospital in Dublin recorded a near tripling of patients who’d set themselves on fire while burning trash.

Well-intentioned laws have been backfiring for millennia. A Jewish statute recorded in the Bible required creditors to forgive all debts every sabbatical, or seventh year. For borrowers, the appeal of unilateral debt relief cannot be overstated, as the penalties for defaulting on a loan were severe: a creditor could even take a debtor’s children into bondage.

If you were a creditor, however, you saw this debt-forgiveness program differently. Why loan money to some sandal maker if he could just tear up the note in Year Seven?

So creditors gamed the system by making loans in the years right after a sabbatical and pulling tight the purse strings in Years Five and Six. The result was a cyclical credit crunch that punished the very people the law was intended to help.

But in the history of unintended consequences, few match the one uncovered by Ignatz Semmelweis: medical doctors, while in pursuit of lifesaving knowledge, conducted thousands upon thousands of autopsies, which, in turn, led to the loss of thousands upon thousands of lives.

It is heartening, of course, that Semmelweis’s brilliant data deduction showed how to end this scourge. But our larger point, and the point of this chapter, is that Semmelweis’s solution—sprinkling a bit of chloride of lime in the doctors’ hand-wash—was remarkably simple and remarkably cheap. In a prosperous world, simple and cheap fixes sometimes get a bad rap; we are here to defend them.

There is another powerful, if bittersweet, example from the realm