How – and why – we chose the best 21 ideas of the 21st century

What separates a good idea from a bad one? It isn’t always easy to tell. Take the invention of vaccination, for example. Drawing pus from a woman infected with cowpox and injecting it into an 8-year-old boy seems utterly reckless, but in doing so, 18th-century physician Edward Jenner found a way to fight the deadly scourge of smallpox.

It is only with hindsight that we can see Jenner was on to something: a principle that has now saved millions of lives. That is why, a quarter of the way into this century, we have decided to look back and celebrate the ideas that have really mattered in the past 25 years – the ones that are already transforming the way we behave, think or understand what’s around us.

In coming up with our list of the 21 best ideas of the 21st century, there was plenty of heated debate within the editorial team. Our first hurdle was the unexpectedly puzzling question of whether the first quarter of the 21st century concluded at the start of 2025 or the end of it. To be safe, we chose the end. Then it was onto the ideas themselves and further discussion of what should really count, from whether the microbiome is actually a 21st-century concept (we decided it is,) to whether social media was a good or a bad idea (after some back and forth, we decided on bad. What makes a good or a bad idea is, after all, subjective.

In the end, we devised a rigorous set of criteria. To make the list, a concept must have already had a transformative impact – whether on our understanding of ourselves, our health or the wider universe. It must have an idea at the heart of it, even if it was backed up by scientific discoveries. And finally, it must have happened in the past 25 years.

You might think that last criterion would be easy to administer, but there were lots of suggestions that didn’t quite make it. Gravitational waves were discovered in the 21st century, bringing us a whole new way of viewing the cosmos, but they were first predicted by Albert Einstein 100 years earlier. Other ideas like weight-loss drugs, personalised medicine and mRNA vaccines hold much promise, but haven’t quite had their time to shine yet. Perhaps they’ll make it into our 2050 list.

In coming up with our picks, we couldn’t ignore the many ideas that sounded great at first, but turned into flops. That’s why we have also compiled a list of the five worst ideas of the century so far. Sometimes, the line between best and worst is surprisingly hard to draw, which is why a few of our choices on the best list might seem controversial – like smartphones, for instance, which many people would rather see removed from the planet, but on balance we see as a positive. Or the 1.5°C global warming target, which could be seen as a failure: a new report has found that the three-year average of global temperatures has just passed 1.5°C for the first time. Despite this, we argue that changing the threshold down from 2°C remains one of the century’s best ideas, setting the benchmark for global climate ambitions.

Getting on with actually transitioning away from fossil fuels is certainly a good idea, and one perhaps surprising hero we have recognised in this area is Elon Musk. In 2016, before he began dabbling in social media and politics, Musk’s car company Tesla opened its first “gigafactory” in Nevada, marking a turning point in the energy transition by using economies of scale to electrify our transport and energy systems. Other attempts to battle climate change, like alternative fuels and carbon offsets, have made our naughty list for doing more harm than good.

One thing we learned in putting our selection together is the extent to which ideas come about by chance. For most of us, finding a working plug socket on a long train journey enables little more than a few extra minutes of smartphone scrolling. But for two physicists back in 2005, it changed the world’s entire decarbonisation strategy. Similarly, it was a eureka discovery that revealed the origins of our most complex thought processes. We learned that brain regions don’t work alone, but coordinate amongst themselves, creating a powerful and complex web. Since then, these neural networks have transformed our understanding of the brain, as well as how we diagnose and treat its problems.

Looking back a quarter of a century, we find that the world was a very different place. We had avoided the millennium bug, the first draft of the human genome had just been completed and the first crew had arrived at the International Space Station. We didn’t know what a Denisovan was and the word “microbiome” wasn’t in our vocabulary. In the pages of New Scientist, we were celebrating new technologies like wireless communication, marvelling at the computer chip no bigger than an aspirin tablet that would let it happen. “Its heart is a device called a Bluetooth chip,” we wrote, “and pundits are tipping it as The Next Big Thing”. A reasonable guess, but headphones that you don’t have to plug in are more of a nice-to-have than a world-changer, so we called that one wrong.

This reflects the fact that while predictions can be appealing, they are all too often wrong and left forgotten in our hurry to move on to the next shiny thing. Rather than trying to forecast the future, this exercise has taught us that it is worth taking the time to reflect on the past. Advances in health, technology and environmentalism have undoubtedly made the world a better place this century, and let us hope – if not predict – that they will continue to do so.