Enter the Agricultural Revolution. A variety of innovations, none particularly complex-they included higher-yielding crops, better tools, and a more efficient use of capital-changed farming and, subsequently, the face of the earth. In late eighteenth-century America, "it took 19 out of 20 workers to feed the country's inhabitants and provide a surplus for export," wrote the economist Milton Friedman. Two hundred years later, only 1 of 20 American workers was needed to feed a far larger population while also making the United States "the largest single exporter of food in the world."

The Agricultural Revolution freed up millions of hands that went on to power the Industrial Revolution. By 1850, worldwide population had grown to 1.3 billion; by 1900, 1.7 billion; by 1950, 2.6 billion. And then things really took off. Over the next fifty years, the population more than doubled, reaching well beyond 6 billion. If you had to pick a single silver bullet that allowed this surge, it would be ammonium nitrate, an astonishingly cheap and effective crop fertilizer. It wouldn't be much of an overstatement to say that ammonium nitrate feeds the world. If it disappeared overnight, says the agricultural economist Will Masters, "most people's diets would revert to heaps of cereal grains and root crops, with animal products and fruits only for special occasions and for the rich."

Or consider the whale. Hunted since antiquity, by the nineteenth century it had become an economic engine that helped turn the United States into a powerhouse. Every square inch of it could be turned into something, so the whale afforded one-stop shopping for a fast-growing nation: material for the manufacture of paint and varnish; textiles and leather; candles and soap; clothing and of course food (the tongue was a particular delicacy). The whale was especially beloved by the finer s.e.x, surrendering its body parts for corsets, collars, parasols, perfume, hairbrushes, and red fabric dye. (This last product was derived from, of all things, the whale's excrement.) Most valuable was whale oil, a lubricant for all sorts of machinery but most crucially used for lamp fuel. As the author Eric Jay Dolin declares in Leviathan, "American whale oil lit the world."

Out of a worldwide fleet of 900 whaling ships, 735 of them were American, hunting in all four oceans. Between 1835 and 1872, these ships reaped nearly 300,000 whales, an average of more than 7,700 a year. In a good year, the total take from oil and baleen (the whale's bonelike "teeth") exceeded $10 million, today's equivalent of roughly $200 million. Whaling was dangerous and difficult work, but it was the fifth-largest industry in the United States, employing 70,000 people.

And then what appeared to be an inexhaustible resource was-quite suddenly and, in retrospect, quite obviously-heading toward exhaustion. Too many ships were hunting for too few whales. A ship that once took a year at sea to fill its hold with whale oil now needed four years. Oil prices spiked accordingly, rocking the economy back home. Today, such an industry might be considered "too big to fail," but the whaling industry was failing indeed, with grim repercussions for all America.

That's when a retired railway man named Edwin L. Drake, using a steam engine to power a drill through seventy feet of shale and bedrock, struck oil in t.i.tusville, Pennsylvania. The future bubbled to the surface. Why risk life and limb chasing underwater leviathans around the world, having to catch and carve them up, when so much energy was just waiting, in the nation's bas.e.m.e.nt, to be pumped upstairs?

Oil was not only a cheap and simple fix but, like the whale, extraordinarily versatile. It could be used as lamp oil, a lubricant, and as a fuel for automobiles and home heating; it could be made into plastic and even nylon stockings. The new oil industry also provided lots of jobs for unemployed whalers and, as a bonus, functioned as the original Endangered Species Act, saving the whale from near-certain extinction.

By the early twentieth century, most infectious diseases-smallpox, tuberculosis, diphtheria, and the like-were on their way out. But polio refused to surrender.

It would be hard to invent a more frightening illness. "It was a children's disease; there was no prevention; there was no cure; every child everywhere was at risk," says David M. Oshinsky, author of the Pulitzer Prizewinning Polio: An American Story. "And what this really meant was that parents were absolutely frantic."

Polio was also a great mystery, spiking in summertime, its cause unknown. (In a cla.s.sic case of mistaking correlation and causality, some researchers suspected that ice cream-consumed in far greater quant.i.ties in the summer-caused polio.) It was first thought to target immigrant slum children, especially boys, but it struck girls too, as well as kids in the leafiest suburbs. Even Franklin Delano Roosevelt, who was far removed from immigrant slums and, at thirty-nine, from childhood as well, contracted the disease.

Every outbreak prompted a new round of quarantines and panic. Parents kept their kids away from friends, from pools and parks and libraries. In 1916, the worst polio epidemic to date struck New York City. Out of 8,900 reported cases, 2,400 people died, most of them children under five. The disease roared on. Nineteen fifty-two was the worst year yet, with 57,000 reported cases nationwide, 3,000 of them fatal and 21,000 resulting in permanent paralysis.

Surviving a bad case of polio was only marginally better than dying. Some victims lost the use of their legs and lived in constant pain. Those with respiratory paralysis practically lived inside an "iron lung," a huge tank that did the work of their failed chest muscles. As the population of living polio victims grew, the cost of their medical care was staggering. "At a time when less than ten percent of the nation's families had any form of health insurance," Oshinsky writes, "the expense of boarding a polio patient (about $900 a year) actually exceeded the average annual wage ($875)."

America was by now the most powerful country on earth, the victor in two world wars, possessor of a blindingly bright future. But there was legitimate concern that this single disease would consume such a large share of future health-care dollars that it would cripple the nation.

And then a vaccine was developed-a series of vaccines, really-and polio was effectively stamped out.

To call the vaccine a "simple" fix might seem to discount the tireless efforts of everyone who helped stop polio: the medical researchers (Jonas Salk and Albert Sabin chief among them); the fund-raising volunteers (the March of Dimes, Oshinsky writes, was "the largest charitable army the country had ever known"); even the non-human martyrs (thousands of monkeys were imported to be given experimental vaccines).

On the other hand, there is no simpler medical fix than a vaccine. Consider two of the major ways we try to thwart disease. The first is to invent a procedure or technology that helps fix a problem once it's arisen (open-heart surgery, for instance); these tend to be very costly. The second is to invent a medicine to prevent the problem before it happens; in the long run, these tend to be extraordinarily cheap. Health-care researchers have estimated that if a polio vaccine hadn't been invented, the United States would currently be caring for at least 250,000 long-term patients at an annual cost of at least $30 billion. And that doesn't even include "the intangible costs of suffering and death and of averted fear."

Polio is a stark example, but there are countless cheap and simple medical fixes. New ulcer drugs reduced the rate of surgery by roughly 60 percent; a later round of even cheaper drugs saved ulcer patients some $800 million a year. In the first twenty-five years after lithium was introduced to treat manic depression, it saved nearly $150 billion in hospitalization costs. Even the simple addition of fluoride to water systems has saved about $10 billion per year in dental bills.

As we noted earlier, deaths from heart disease have fallen substantially over the past few decades. Surely this can be attributed to expensive treatments like grafts, angioplasties, and stents, yes?

Actually, no: such procedures are responsible for a remarkably small share of the improvement. Roughly half of the decline has come from reductions in risk factors like high cholesterol and high blood pressure, both of which are treated by relatively cheap medicines. And much of the remaining decline is thanks to ridiculously inexpensive treatments like aspirin, heparin, ACE inhibitors, and beta-blockers.

By the early 1950s, automobile travel had become fantastically popular in the United States, with about 40 million cars on the road. But at the thirty-fifth annual convention of the National Automobile Dealers a.s.sociation, held in January 1952, a vice president of the BFGoodrich tire company warned that the smooth ride might be over: "If the death rate continues upward it will seriously hurt the automobile business, as many people will quit driving."

Nearly 40,000 people died in U.S. traffic accidents in 1950. That's roughly the same number of deaths as today, but a straight-up comparison is very misleading, because far fewer miles were driven back then. The rate of death per mile driven was five times higher in 1950 than it is today.

Why so many fatalities then? Suspects were legion-faulty cars, poorly designed roads, careless drivers-but not much was known about the mechanics of car crashes. Nor was the auto industry exactly burning to find out.

Enter Robert Strange McNamara. Today he is best remembered as the much-maligned secretary of defense during the Vietnam War. One reason McNamara was so maligned was that he tended to make decisions based on statistical a.n.a.lysis rather than emotion or political considerations. In other words, he behaved like an economist.

This was no coincidence. He studied economics at Berkeley and went on to the Harvard Business School, where he stayed on as a young professor of accounting. McNamara volunteered when World War II broke out, and his a.n.a.lytical skills landed him in the Statistical Control Office of the Army Air Forces.

His team used data as a weapon to fight the war. For example, the abort rate among American bombers leaving England for daytime sorties over Germany was found to be unnaturally high, about 20 percent. The pilots gave a variety of explanations for failing to reach the target: a malfunctioning electrical system, a spotty radio, or illness. But a closer a.n.a.lysis of the data led McNamara to conclude that these reasons were "baloney." The real explanation, he said, was fear. "A h.e.l.luva lot of them were going to be killed, they knew that, and they found reasons to not go over the target."

McNamara reported this to the commanding officer, the notoriously headstrong Curtis LeMay, who responded by flying the lead plane on bombing missions and vowing to court-martial any pilot who turned back. The abort rate, McNamara says, "dropped overnight."

After the war, the Ford Motor Company asked McNamara and others from his unit to bring their statistical wizardry to the auto industry. McNamara wanted to return to Harvard, but he and his wife had both racked up huge medical bills-from polio, of all things. So he took the job at Ford. He quickly rose through the ranks even though he wasn't a "car guy" in any traditional sense. "Instead," as one historian later wrote, "he was absorbed by such novel concepts as safety, fuel economy, and basic utility."

McNamara was particularly concerned with the deaths and injuries from automobile accidents. He asked the car guys what caused the problem. There were few statistics available, he was told.

Some aeronautical researchers at Cornell were trying to prevent airplane deaths, so McNamara commissioned them to look into auto crashes. They experimented by wrapping human skulls in different materials and dropping them down the stairwells in Cornell's dormitories. It turned out that human beings were no match for the hard materials used in car interiors. "In a crash, the driver was often impaled on the steering wheel," McNamara says. "The pa.s.senger was often injured because he'd hit the windshield or the header bar or the instrument panel." McNamara ordered new Ford models to have a safer steering wheel and a padded instrument panel.

But the best fix, he realized, was also the simplest one. Rather than worrying about what a pa.s.senger's head would hit when he was flung about during an accident, wouldn't it be better to keep him from being flung at all? McNamara knew that airplanes had seat belts; why not cars?

"I calculated the number of deaths we'd prevent each year, which was very high," he says. "And this came at essentially no cost, with no great penalty for wearing them."

McNamara had all of Ford's company cars outfitted with seat belts. "I flew down to visit an a.s.sembly plant in Texas," he recalls. "The manager met me at the plane. I buckled my seat belt, and he said, 'What's the matter, you afraid of my driving?'"

That manager, it turned out, reflected a widespread sentiment about seat belts. McNamara's bosses saw them as "inconvenient, costly, and just a bunch of d.a.m.n nonsense," he says. Even so, they followed his lead and put seat belts in the new Ford models.

McNamara was of course right: the seat belt would eventually save many lives. But the key word here is "eventually."

The brilliant rationalist had encountered a central, frustrating tenet of human nature: behavior change is hard. The cleverest engineer or economist or politician or parent may come up with a cheap, simple solution to a problem, but if it requires people to change their behavior, it may not work. Every day, billions of people around the world engage in behaviors they know are bad for them-smoking cigarettes, gambling excessively, riding a motorcycle without a helmet.

Why? Because they want to! They derive pleasure from it, or a thrill, or just a break from the daily humdrum. And getting them to change their behavior, even with a fiercely rational argument, isn't easy.

And so it was with the seat belt. Congress began setting federal safety standards in the mid-1960s, but even fifteen years later, seat belt use was laughably low: just 11 percent. Over time, the numbers crept upward, thanks to a variety of nudges: the threat of a traffic ticket; expansive public-awareness campaigns; annoying beeps and flashing dashboard lights if the belt wasn't buckled; and, eventually, a societal acceptance that wearing a seat belt wasn't an insult to anyone's driving ability. Seat-belt use rose to 21 percent by the mid-1980s, 49 percent by 1990, 61 percent by the mid-1990s, and today it is over 80 percent.

That's a big reason the per-mile auto fatality rate has fallen so much in the United States. Seat belts reduce the risk of death by as much as 70 percent; since 1975 they have saved roughly 250,000 lives. Traffic fatalities still claim more than 40,000 lives a year, but relatively speaking, driving isn't all that dangerous anymore. What makes the death toll so high is that so many Americans spend an enormous amount of time in their cars, racking up some 3 trillion miles per year. That translates into one death for every 75 million miles driven-or, put another way, if you drove 24 hours a day at 30 miles per hour, you could expect to die in a car accident only after driving for 285 straight years. Compared with the death rates in many countries in Africa, Asia, and the Middle East, where seat-belt use is far less prevalent, driving in the United States isn't much more dangerous than sitting on your couch.

And seat belts, at about $25 a pop, are one of the most cost-effective lifesaving devices ever invented. In a given year, it costs roughly $500 million to put them in every U.S. vehicle, which yields a rough estimate of $30,000 for every life saved. How does this compare with a far more complex safety feature like air bags? At an annual U.S. price of more than $4 billion, air bags cost about $1.8 million per life saved.

Robert McNamara, who recently pa.s.sed away at the age of ninety-three, told us shortly before his death that he still wanted to get to 100 percent compliance on seat belts. "A lot of women often don't use the shoulder belt because they're uncomfortable, they're not designed to take account of the b.r.e.a.s.t.s," he said. "I think with very little thought, belts could be designed that are more comfortable and therefore increase the percentage of use."

He may or may not be right about women and seat belts. But without doubt there is one group of people for whom seat belts are poorly designed: children.

Sometimes it pays to be low status. When a family of four goes for a drive, the kids usually get shunted to the backseat while the mom or dad rides shotgun. The kids are luckier than they know: in the event of a crash, the backseat is far safer than the front. This is even truer for adults, who are larger and therefore more likely to smack into something hard when they sit up front. Unfortunately, while it's okay to consign the low-status kids to the rear seat, if the parents go out for a drive alone, it's a bit awkward for one of them to ride in the back while leaving the other up front in the martyr's seat.

Seat belts are now standard issue in the rear seat of all cars. But they were designed to fit grown-ups, not kids. If you try to strap in your three-year-old darling, the lap belt will be too loose and the shoulder belt will come across his neck or nose or eyebrows instead of his shoulder.

Fortunately, we live in a world that cherishes and protects children, and a solution was found: the child safety seat, commonly known as a car seat. Introduced in the 1960s, it was first embraced by only the most vigilant parents. Thanks to the advocacy of doctors, traffic-safety experts, and-surprise!-car-seat manufacturers, it came into wider use, and the government eventually joined the party. Between 1978 and 1985, every state in the United States made it illegal for children to ride in a car unless they were buckled into a safety seat that met federal crash-test standards.

Motor-vehicle accidents were the leading cause of death for U.S. children back then, and they still are today, but the rate of death has been falling dramatically. Most of the credit has gone to the car seat.

Safety isn't free, of course. Americans spend more than $300 million a year buying 4 million car seats. A single kid will typically inhabit three different seats over time: a rear-facing seat for infants; a larger, front-facing seat for toddlers; and a booster seat for older children. Moreover, if that kid has a sibling or two, his parents may have to buy an SUV or minivan to accommodate the width of the car seats.