Rational Animations made this video collaborating with 80,000 Hours. The script has been written by Benjamin Hilton as an adaptation of part of the 80,000 Hours career guide, by Benjamin Todd. I have included the full script below.

It’s easy to feel like one person can’t make a difference.

The problems the world faces seem so vast, and we seem so small. Sometimes even the best of intentions aren't enough to make the world budge.

Now, it’s true that many common ways people try to do good have less impact than you might think at first.

But some ways of doing good have allowed certain people to achieve an extraordinary impact. How is this possible? And what does it mean for how you can make a difference?

We’ll start by looking at doctors — and end up at nuclear war.

Many people train to become doctors because they want to save lives! And, of course, their work is very important. But how many lives does a doctor really save over the course of their career? You might assume it's in the hundreds or thousands. But the surprising truth is that, according to an analysis carried by Dr Greg Lewis — a former medical doctor —  the number is far lower than you'd expect.

Since the 19th century, life expectancy has skyrocketed. But that’s not just because of medicine. There are loads of contributing factors, like nutrition, improved sanitation, and increased wealth. Estimating how many years of life medicine alone saves is really difficult. 

Despite this difficulty, one attempt — from researchers at Harvard and King’s College London — found that medical care in developed countries increases the life expectancy of each person in these countries by around 5 years.

Most developed countries have around 3 doctors per 1,000 people. So, if this estimate is right, each doctor saves around 1,666 years of life, over the course of their career. Using the World Bank’s standard conversion rate of 30 extra years of healthy life to one “life saved,” that’s around about 50 lives saved per doctor!

But that’s actually a substantial overestimate.

Doctors are only one part of the medical system, which also relies on nurses and hospital staff, as well as overhead and equipment. And more importantly, there are already a lot of doctors in the developed world. So if you don’t become a doctor, someone else will be available to perform the most critical procedures. Additional doctors only allow society to carry out additional, usually less significant, procedures.

Look at this graph, from the analysis by Dr. Greg Lewis we quoted earlier. Each point is a country — The vertical axis shows disability-adjusted-life-years per 100-thousand people. You can think of that figure as roughly how many years of disability a group of 100-thousand people has to endure on average. Therefore, the fewer, the better. Each country has a different figure. The horizontal axis shows the number of doctors per 100-thousand people in each country. As you can see, countries with more doctors suffer lower disability.

But notice how the curve goes nearly flat once you have more than 150 doctors per 100-thousand people. After this point (which almost all developed countries meet), additional doctors only achieve a small impact on average. In fact, at 300 doctors per 100-thousand people, an additional doctor saves only around 200 years of life throughout their career.

So, when you take all this into account, including some accounting for the impact of nurses and other parts of the medical system, it looks more like each doctor saves only around 3 lives through the course of their career. Still an admirable achievement, but perhaps less than you may imagine.

But that’s an ordinary doctor. Some doctors have had much more impact than this.

By 1968, we knew how to prevent deaths from cholera. The main cause of death was dehydration due to bad diarrhoea. The treatment for this was relatively cheap: just salt and glucose, dissolved into water. But millions of people were still dying every year.

The problem was that this salt and glucose solution needed to be administered by a feeding tube, or an intravenous drip — straight into the bloodstream. Delivering this kind of treatment is far from simple, which meant that many cholera victims died from extreme dehydration before they could even reach medical care.

Enter Dr. David Nalin. He was an American doctor sent to work at a refugee camp, just on the border of Bangladesh and Myanmar.

In a groundbreaking study, Dr. Nalin showed if everything was dissolved at the right concentration, and consumed at the right rate, just drinking a solution of salt and glucose could rehydrate cholera patients pretty much as effectively as IV fluids. This meant the treatment could be used anywhere, even remote villages, and using only extremely cheap and widely available ingredients.

Since then, this astonishingly simple treatment has been used all over the world, and the annual rate of child deaths from diarrhoea has plummeted from around 5 million to 1.5 million.

If Dr Nalin had not been around, someone else would probably have discovered this treatment eventually. However, even if we imagine that he sped up the roll-out by only five months, his work alone would have saved about 500,000 lives. This is a very approximate estimate, but it makes his impact more than 100,000 times greater than that of an ordinary doctor:

And even just within medical research, Dr Nalin is far from the most extreme example of a high-impact career.

Take blood transfusions. Today, they’re a routine, lifesaving procedure. But over a century ago, transfusions were risky endeavours that often ended in death. At the dawn of the 20th century, doctors simply didn't understand why some blood transfusions worked while others were fatal. Lives were being lost and medicine was stumped.

That all changed thanks to the work of Austrian biologist Karl Landsteiner. In 1901, after years of meticulous laboratory research, Landsteiner identified three blood groups: A, B, and O (although, shockingly, he called “O” “C”). This groundbreaking discovery unlocked the mystery behind the deaths caused by early blood transfusions.

Landsteiner found that transfusions with incompatible blood types often led to potentially fatal outcomes. But when blood types matched between donor and patient, transfusions were safe and effective.

With this knowledge, doctors could begin matching donor and patient blood types for the first time. Landsteiner's insights allowed blood transfusions to save lives instead of endangering them. It was a pivotal advancement in medicine.

The value of Landsteiner's research became abundantly clear during the First World War. Setting up blood banks near field hospitals allowed for rapid access to typed blood to treat wounded soldiers. Transfusions went from risky last resorts to standard life-saving procedures over the course of the war.

In the decades since, Landsteiner's system has enabled millions of successful blood transfusions. Patients with blood loss from trauma or surgery, those undergoing cancer treatment, and many more have benefitted. The analysis by Greg Lewis puts Karl Landsteiner’s discovery of blood groups as saving tens of millions of lives.

This is a very uncertain estimate. It could easily be off by a factor of 10 or more, and seems more likely to be too high than too low. It's possible that speeding up the discovery wouldn't have had much impact at all — most of the lives saved were in the modern era, when people were rapidly gaining access to other medical advancements which could save their lives, or prevent the need for transfusions completely. On the other hand, the discovery of blood groups probably made other scientific advances possible, and we're ignoring their impact. Nevertheless, the basic point stands: Landsteiner's impact was likely vastly greater than even David Nalin’s.

There are also many more examples you don’t hear about, like Viktor Zhdanov. In the 20th century, smallpox killed around 400 million people — far more than died in all the century’s wars and political famines. Zhdanov single-handedly lobbied the World Health Organization to start a campaign which eventually led to the complete elimination of smallpox. Without his involvement, smallpox would not have been eliminated until much later, and possibly not at all.

But who saved the most lives in history?

One top candidate is Stanislav Petrov, a little-known Soviet Lieutenant Colonel who may have saved your life.

It was just another late night shift for Stanislav Petrov in the secret Serpukhov-15 bunker outside Moscow. As a duty officer, he settled into the commander's chair to monitor the Soviet Union's early-warning satellites over the United States. Little did he know this routine shift was about to thrust him into one of the most harrowing incidents of the Cold War.

Petrov’s instructions were clear: if the United States launched nuclear missiles, he was to report that immediately to his superiors, who had a policy of striking back, immediately, with their own nuclear weapons.

Shortly after midnight, the screens showed five ICBMs launched from the United States, heading towards the Soviet Union.

For five excruciating minutes, he turned over the evidence again and again in his mind.

Petrov reasoned that, if the US were starting a nuclear war, they’d probably launch hundreds of missiles, not just five. So he reported it as a false alarm. And he was right — no missiles had in fact been launched. The world was safe, at least for now.

If Petrov had triggered a strike, it’s possible that someone else could have called off the launch process. But there’s at least a reasonable chance that the Soviet Union would have retaliated, and hundreds of millions would have died. The two countries may have even ended up engaged in an all-out nuclear war, leading to billions of deaths.

And Petrov's decision may have done much more than "just" save a billion people from nuclear catastrophe. As we saw in our video on longtermism, focusing on potential risks to the long-term future can be even more high-impact than saving lives today. The nuclear war that Petrov prevented could have devastated scientific, artistic, economic, and all other forms of progress, and possibly even lead to the end of civilisation itself. Every human who will live in the future will be alive, in part, because of Petrov.

So we’ve seen people’s careers have had huge positive effects, some vastly more than others.

A large component is luck — Nalin, Landsteiner, and Petrov were in the right place at the right time. You can’t guarantee you’ll make an important medical discovery.

But it wasn’t all luck: Landsteiner and Nalin chose to use their medical knowledge to solve some of the most devastating health problems of their day. And it was somewhat foreseeable that someone high up in the military of a nuclear power could have the opportunity to prevent nuclear conflict.

What does this mean for you?

People often wonder how they can “make a difference,” but if some careers can result in thousands of times more impact than others, this isn’t the right question. Two different career options can both “make a difference,” but one could be dramatically better than the other.

Instead, the key question is: What are some of the best ways to make a difference? In other words, what can you do to give yourself a chance of having one of the highest-impact careers?

This video was adapted from part of the new 80,000 Hours Career Guide — which attempts to answer exactly that question. 

The guide covers everything you need to know about how to find a fulfilling career that does good, from why you shouldn’t just “follow your passion”, to why medicine and charity work aren’t always the best way to help others.

It’s full of practical tips and exercises, and at the end, you’ll have a draft of your new career plan.

Just go to 80000hours.org/rational to find out more.

We all want to make a difference. If you think carefully about how, the best ways to help others could end up doing a lot more good than you’d ever think possible. Identify society's most pressing problems, develop expertise to address them, and who knows? You might, like Petrov, help to literally save the world.