Big Think's Top 25 +1 Videos

Mama, Don't Let Your Babies Grow Up To Deny Evolution

If adults want to deny evolution, sure. That’s fine. Whatever. But those adults better not make their kids follow in step because we as society need them to be better. Bill Nye, everyone's favorite Science Guy, explains the importance of promoting evolution education for America's future voters and lawmakers.

My Man, Sir Isaac Newton

Are you at least 26 years-old? If so, you are older than Isaac Newton was when he invented calculus... on a dare! (If you're younger than 26, better hurry up.) Big Think expert and overall cool guy Neil deGrasse Tyson explains why Newton is the greatest physicist who ever and likely will ever live.

Will Mankind Destroy Itself?

Theoretical physicist Michio Kaku sees two major trends today. One eventually leads to a multicultural, scientific, tolerant society that will expand beyond Earth in the name of human progress. The other trend leads to fundamentalism, monoculturalism, and -- eventually -- civilizational ruin. Whichever of these two trends wins out will determine the fate of mankind. No pressure, everyone.

Ricky Gervais on the Principles of Comedy

Comedy isn't just about making people laugh, says actor Ricky Gervais. It's about making people think. And while different forms of comedy require different approaches, the crux of any good performance will always be rhythm.

Reading the Bible (Or the Koran, Or the Torah) Will Make You an Atheist

Author and magician Penn Jillette was asked to leave his Christian youth group by a pastor who told his parents: "He's no longer learning about the Bible from me. He is now converting everyone in the class to atheism." The reason? Jillette did his homework and was turned off by the hostilities of the text. It can be intimidating to come out as an atheist, especially in a religious community. Jillette found that having "out" atheist role models helped him feel unalone.

Henry Rollins: The One Decision that Changed My Life Forever

Punk legend Henry Rollins describes the biggest turning point in his life: the moment he decided to leave his job as manager of a Häagen-Dazs store and eventually become the lead singer of Black Flag. It was the courage to take a risk, plus a whole lot of luck, that got Rollins to where he is today.

5 Programming Languages Everyone Should Know

Java is "heavyweight, verbose, and everyone loves to hate it," but programmer Larry Wall still thinks you should know it. In this video, he offers suggestions for people interested in learning languages, as well as suggestions for those significantly less invested in computer programming.

The Importance of Unbelief

If you assume there’s no afterlife, Stephen Fry says, you’ll likely have a fuller, more interesting "now" life. The actor and comedian details the positive influence philosophers have had on his life, as well as his journey of understanding both what he believes and why he believes it.

Why be happy when you could be interesting?

We don't really want what we think we desire, says philosopher Slavoj Žižek.

James Gleick on the Common Character Traits of Geniuses

This video is part of a series on female genius, in proud collaboration with 92Y's 7 Days of Genius Festival.

The personalities of Isaac Newton and Richard Feynman were, on one level, extremely different. Biographer and former New York Times reporter James Gleick says Newton was argumentative, had few friends, and likely died a virgin. Feynman, on the other hand, loved dancing and going to parties, and had many friends in the scientific community. But in regards to their working habits, both men were solitary and had the ability to concentrate with a sort of intensity that is hard for mortals to grasp. At bottom, Gleick says geniuses tend to have a yearning for solitude which, though fruitful for their professional work, made the task of daily living more burdensome.

The Importance of Doing Useless Things

From poetry and ballet to mathematics and being clever, life is laden with frivolous pursuits that hold no bearing on our ability to survive. Yet, insists Richard Dawkins, if it weren’t for the development of these impractical activities, we wouldn’t be here.

Why monogamy is ridiculous

Dan Savage: the idea that one instance of infidelity should ruin a relationship is a new—and misguided—notion.

Dan Harris: Hack Your Brain's Default Mode With Meditation

Dan Harris explains the neuroscience behind meditation, but reminds us that the ancient practice isn't magic and likely won't send one floating into the cosmic ooze. He predicts that the exercise will soon become regularly scheduled maintenance, as commonplace as brushing your teeth or eating your veggies. Harris, an ABC News correspondent, was turned on to mediation after a live, on-air panic attack. His latest book is 10% Happier: How I Tamed the Voice in My Head, Reduced Stress Without Losing My Edge, and Found Self-Help That Actually Works--A True Story.

How Intellectuals Betrayed the Poor

For 40 years academics were duped into idolizing the idea of unfettered markets, says Cornel West, and now our society is paying a terrible price.

Why Some Races Outperform Others

A psychologist explains the latest research into education disparity.

Why It's So Hard for Scientists to Believe in God

Some scientists see religion as a threat to the scientific method that should be resisted. But faith "is really asking a different set of questions," says Collins.

Why Facebook Isn't Free

Internet pioneer Jaron Lanier argues that free technologies like Facebook come with a hidden and heavy cost – the livelihoods of their consumers.

How to Tell if You’re a Writer

For John Irving, the need for a daily ration of solitude was his strongest "pre-writing" moment as a child.

Your Behavior Creates Your Gender

Nobody is born one gender or the other, says the philosopher. "We act and walk and speak and talk in ways that consolidate an impression of being a man or being a woman."

Are You a Liberal Snob? Take The Quiz

Charles Mrray designed this quiz to have a salutary effect on bringing to people’s attention the degree to which they live in a bubble that seals them off from an awful lot of their fellow American citizens.

Why You Should Watch Filth

John Waters defends the creation and consumption of obscene films, and recommends some of his personal favorites.

What Are You Worth? Getting Past Status Anxiety.

Writer Alain De Botton says that status anxiety is more pernicious and destructive than most of us can imagine, and recommends getting out of the game altogether.

Sheila Heen on the Psychology of Happiness and Feedback

Sheila Heen, a Partner at Triad Consulting Group and a lecturer on Law at Harvard Law School, explains the psychology behind feedback and criticism. Heen is co-author of "Thanks for the Feedback: The Science and Art of Receiving Feedback Well."

Are You a Psychopath? Take the Test.

Psychologist Kevin Dutton presents the classic psychological test known as "the trolley problem" with a variation. Take the test and measure you response on the psychopathic spectrum.

Here's How to Catch a Liar, If You Really Want To

It’s very complex as to whether or not we really want to catch a liar. We think we do. What if we find out that both of our presidential candidates are lying? Then what do we do? I’m not saying they are; I never comment on anyone in office or running for office. Only after they’re out that they’re fair game. . . . Clinton said, "I didn’t have sex with that woman" and then gave her name. "That woman" is putting her at a distance from himself.

Why I Came Out at Age 81

As a teenager in the '40s, James Randi "would have gotten stoned" for being gay. But when he outed himself to the world in 2010, the reaction was "wonderful."

More playlists
  • From politics to every day life, humans have a tendency to form social groups that are defined in part by how they differ from other groups.
  • Neuroendocrinologist Robert Sapolsky, author Dan Shapiro, and others explore the ways that tribalism functions in society, and discuss how—as social creatures—humans have evolved for bias.
  • But bias is not inherently bad. The key to seeing things differently, according to Beau Lotto, is to "embody the fact" that everything is grounded in assumptions, to identify those assumptions, and then to question them.
  • People have been digging up limestone and gypsum from below Paris since Roman times.
  • They left behind a vast network of corridors and galleries, since reused for many purposes — most famously, the Catacombs.
  • Soon, the ancient labyrinth may find a new lease of life, providing a sustainable form of air conditioning.

Ancient mining areas below Paris for limestone (red) and gypsum (green).Credit: Émile Gérards (1859–1920) / Public domain

"If you're brave enough to try, you might be able to catch a train from UnLondon to Parisn't, or No York, or Helsunki, or Lost Angeles, or Sans Francisco, or Hong Gone, or Romeless."

China Miéville's fantasy novel Un Lun Dun is set in an eerie mirror version of London. In it, he hints that other cities have similar doubles. On the list that he offhandedly rattles off, Paris stands out. Because the City of Light really does have a twisted sister. Below Paris Overground is Paris Underground, the City of Darkness.

Most people will have heard of the Catacombs of Paris: subterranean charnel houses for the bones of around six million dead Parisians. They are one of the French capital's most famous tourist attractions – and undoubtedly its grisliest.

But they constitute only a small fragment of what the locals themselves call les carrières de Paris ("the mines of Paris"), a collection of tunnels and galleries up to 300 km (185 miles) long, most of which are off-limits to the public, yet eagerly explored by so-called cataphiles.

The Grand Réseau Sud ("Great Southern Network") takes up around 200 km beneath the 5th, 6th, 14th, and 15th arrondissements (administrative districts), all south of the river Seine. Smaller networks run beneath the 12th, 13th, and 16th arrondissements. How did they get there?

Paris stone and plaster of Paris

It all starts with geology. Sediments left behind by ancient seas created large deposits of limestone in the south of the city, mostly south of the Seine; and gypsum in the north, particularly in the hills of Montmartre and Ménilmontant. Highly sought after as building materials, both have been mined since Roman times.

The limestone is also known as Lutetian limestone (Lutetia is the Latin name for ancient Paris) or simply "Paris stone." It has been used for many famous Paris landmarks, including the Louvre and the grand buildings erected during Georges-Eugène Haussmann's large-scale remodelling of the city in the mid-19th century. The stone's warm, yellowish color provides visual unity and a bright elegance to the city.

The fine-powdered gypsum of northern Paris, used for making quick-setting plaster, was so famed for its quality that "plaster of Paris" is still used as a term of distinction. However, as gypsum is very soluble in water, the underground cavities left by its extraction were extremely vulnerable to collapse.

Like living on top of a rotting tooth: subsidence starts far below the surface, but it can destroy your house.Credit : Delavanne Avocats

In previous centuries, a road would occasionally open up to swallow a chariot, or even a whole house would disappear down a sinkhole. In 1778, a catastrophic subsidence in Ménilmontant killed seven. That's why the Montmartre gypsum quarries were dynamited rather than just left as they were. The remaining gypsum caves were to be filled up with concrete.

The official body governing Paris down below is the Inspection Générale des Carrières (IGC), founded in the late 1770s by King Louis XVI. The IGC was tasked with mapping and, where needed, propping up the current and ancient (and sometimes forgotten) mining corridors and galleries hiding beneath Paris.

A delightful hiding place

Also around that time, the dead of Paris were getting in the way of the living. At the end of the 18th century, their final destination consisted of about 200 small cemeteries, scattered throughout the city — all bursting at the seams, so to speak. There was no room to bury the newly dead, and the previously departed were fouling up both the water and air around their respective churchyards.

Something radical had to happen. And it did. From 1785 until 1814, the smaller cemeteries were emptied of their bones, which were transported with full funerary pomp to their final resting place in the ancient limestone quarries at Tombe-Issoire. Three large and modern cemeteries were opened to receive the remains of subsequent generations of Parisians: Montparnasse, Père-Lachaise, and Passy.

Who says stacking skulls and bones can't be fun?Credit: Rijin via Wikimedia and licensed under CC BY-SA 4.0

The six million dead Parisians in the Catacombs, from all corners of the capital and across many centuries, together form the world's largest necropolis — their now anonymized skulls and bones methodically stacked, occasionally into whimsical patterns. The Catacombs are fashioned into a memorial to the brevity of life. The message above the entrance reads: Arrête! C'est ici l'empire de la Mort. ("Halt! This is the empire of Death.")

That has not stopped the Catacombs, accessible via a side door to a classicist building on the Avenue du Colonel Henri Rol-Tanguy, making just about every Top 20 list of things to see in Paris.

An underground economy

However, while the Catacombs certainly are the most famous part of the centuries-old network beneath Paris, and in non-pandemic times draw thousands of tourists each day, they constitute just 1.7 km (1 mile) of the 300-km (185-mile) tunneling total.

Subterranean Paris wasn't just used for mining and storing dead people. In the 17th century, Carthusian monks converted the ancient quarries under their monastery into distilleries for the green or yellow liqueur that still carries their name, chartreuse.

Because the mines generally keep a constant cool temperature of around 15° C (60° F), they were also ideal for brewing beer, as happened on a large scale from the end of the 17th century until well into the 20th century. Several caves were dug especially for establishing breweries, and not just because of the ambient temperature: going underground allowed brewers to remain close to their customers without having to pay a premium for real estate up top.

Overview of the Paris Catacombs.Credit: Inspection Générale des Carrières, 1857 / Public domain.

At the end of the 19th century, the underground breweries of the 14th arrondissement alone produced more than a million hectoliters (22 million gallons) per year. One of the most famous of Paris' underground breweries, Dumesnil, stayed in operation until the late 1960s.

In that decade, the network of corridors and galleries south of the Seine, long since abandoned by miners, became the unofficial playground for the young people of Paris. They explored the fantastical world beneath their feet, in some cases via entry points located in their very schools. Fascinated, these cataphiles ("catacomb lovers") read up on old books, explored the subterranean labyrinth, and drew up schematics that were passed around among fellow initiates as reverently as treasure maps.

As Robert Macfarlane writes in Underland, Paris-beneath-their-feet became "a place where people might slip into different identities, assume new ways of being and relating, become fluid and wild in ways that are constrained on the surface."

Some larger caves turned into notorious party zones: a 7-meter-tall gallery below the Val-de-Grâce hospital is widely known as "Salle Z." Over the last few decades, various other locations in subterranean Paris have hosted jazz and rock concerts and rave parties — like no other city, Paris really has an "underground music scene."

Hokusai's Great Wave as the backdrop to the "beach" under Paris.Credit: Reddit

Cataphiles vs. cataphobes

With popularity came increased reports of nuisance and crime — the tunnels provided easy access to telephone cables, which were stolen for the resale value of their copper.

The general public's "discovery" of the underground network led the city of Paris to officially interdict all access by non-authorized persons. That decree dates back to 1955, but the "underground police" have an understanding with seasoned cataphiles. Their main targets are so-called tourists, who by their lack of knowledge expose themselves to risk of injuries or worse, and degrade their surroundings, often leaving loads of litter in their wake.

The understanding does not extend to the IGC. Unlike in the 19th century, when weak cavities were shored up by purpose-built pillars, the policy now is to inject concrete to fill up endangered spaces — thus progressively blocking off parts of the network. That procedure has also been used to separate the Catacombs to prevent "infiltration" of the site by cataphiles.

Many subterranean streets have their own names, signs and all. This is the Rue des Bourguignons (Street of the Burgundians) below the Champs des Capucins (Capuchin Field), neither of which exists on the surface.Credit: Jean-François Gornet via Wikimedia and licensed under

The cataphiles, however, are fighting back. In a game of cat and mouse with the authorities, they are reopening blocked passages and creating chatières ("cat flaps") through which they can squeeze into chambers no longer accessible via other underground corridors.

Catacomb climate control

Alone against the unstoppable tide of concrete, the amateurs of Underground Paris would be helpless. But the fight against climate change may turn the subterranean labyrinths from a liability into an asset — and the City of Paris into an ally.

The UN's 2015 Climate Plan — concluded in Paris, by the way — requires the world to reduce greenhouse gas emissions by 75 percent by 2050. And Paris itself wants to be Europe's greenest city by 2030. More sustainable climate control of our living spaces would be a great help toward both targets. A lot of energy is spent heating houses in winter and cooling them in summer.

This is where the constant temperature of the Parisian tunnels comes in. It's not just good for brewing beer; it's a source of geothermal energy, says Fieldwork, an architectural firm based in Paris. It can be used to temper temperatures, helping to cool houses in summer and warming them in winter.

One catch for the cataphiles: it also works when the underground cavities are filled up with concrete. So perhaps one day, Paris Underground, fully filled up with concrete, will completely fall off the map, reducing the city's formerly real doppelgänger into an air conditioning unit.

Cool in summer, warm in winter: Paris Underground could become Paris A/C.Credit: Fieldwork

Strange Maps #1083

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  • A new study finds that the contents of an infants' first stool, known as meconium, can predict if they'll develop allergies with a high degree of accuracy.
  • A metabolically diverse meconium, which indicates the initial food source for the gut microbiota, is associated with fewer allergies.
  • The research hints at possible early interventions to prevent or treat allergies just after birth.

The prevalence of allergies arising in childhood has increased over the last 50 years, with 30 percent of the human population now having some kind of atopic disease such as eczema, food allergies, or asthma. The cause of this increase is still subject to debate, though it has been associated with a number of factors, including changes to the gut microbiomes of infants.

A new study by Canadian researchers published in Cell Reports Medicine may shed further light on how these allergies develop in children by examining the contents of their first diaper.

The things you do for science

The research team examined the first stool of 100 infants from the CHILD Cohort Study. The first stool of an infant is a thick, green, horrid-looking substance called meconium. It consists of various things that the infant ingests during the second half of gestation. Additionally, it provides not only a snapshot of what the infant was exposed to during that time, but it also reveals what the food sources will be for the initial gut bacteria that colonize the baby's digestive tract.

The content of the meconium was examined and found to contain such varied elements as amino acids, lipids, carbohydrates, and myriad other substances.

A graph of the comparative, summed abundance of different elements in a metabolic pathway after scaling to median abundance of each metabolite. The blue figures are those children without atopy, the yellow ones show the data for those with an atopic condition. Petersen et al.

The authors fed this information into an algorithm that used this data, along with the identities of the bacteria present as well as the baby's overall health, to predict which infants would go on to develop allergies within one year. The algorithm got it right 76 percent of the time.

A way to prevent childhood allergies?

Infants whose meconium had a less diverse metabolic niche the initial microbes to settle in the gut were at the highest risk of developing allergies a year later. Many of these elements were associated with the presence or absence of different bacterial groups in the digestive system of the child, which play an increasingly appreciated role in our overall health and development. The findings were summarized by senior co-author Dr. Brett Finlay:

"Our analysis revealed that newborns who developed allergic sensitization by one year of age had significantly less 'rich' meconium at birth, compared to those who didn't develop allergic sensitization."

The findings could be used to help understand how allergies form and even how to prevent them. Co-author Dr. Stuart Turvey commented on this possibility:

"We know that children with allergies are at the highest risk of also developing asthma. Now we have an opportunity to identify at-risk infants who could benefit from early interventions before they even begin to show signs and symptoms of allergies or asthma later in life."

A model for early childhood allergies

Petersen et al.

As shown above, the authors constructed a model of how they believe metabolites and bacterial diversity help prevent allergies. Increased diversity of metabolic products in the meconium encourage the development of "healthy" families of bacteria, like Peptostreptococcaceae, which in turn promote the development of a healthy and diverse gut microbiome. Ultimately, such diversity decreases the likelihood that a child will develop allergies.

  • The Chinese Room thought experiment is designed to show how understanding something cannot be reduced to an "input-process-output" model.
  • Artificial intelligence today is becoming increasingly sophisticated thanks to learning algorithms but still fails to demonstrate true understanding.
  • All humans demonstrate computational habits when we first learn a new skill, until this somehow becomes understanding.

It's your first day at work, and a new colleague, Kendall, catches you over coffee.

"You watch the game last night?" she says. You're desperate to make friends, but you hate football.

"Sure, I can't believe that result," you say, vaguely, and it works. She nods happily and talks at you for a while. Every day after that, you live a lie. You listen to a football podcast on the weekend and then regurgitate whatever it is you hear. You have no idea what you're saying, but it seems to impress Kendall. You somehow manage to come across as an expert, and soon she won't stop talking football with you.

The question is: do you actually know about football, or are you imitating knowledge? And what's the difference? Welcome to philosopher John Searle's "Chinese Room."

The Chinese Room

Credit: via Wikipedia and licensed under CC0 1.0

Searle's argument was designed as a critique of what's called a "functionalist" view of mind. This is the philosophy that argues that our mind can be explained fully by what role it plays, or in other words, what it does or what "function" it has.

One form of functionalism sees the human mind as following an "input-process-output" model. We have the input of our senses, the process of our brains, and a behavioral output. Searle thought this was at best an oversimplification, and his Chinese Room thought experiment goes to show how human minds are not simply biological computers. It goes like this:

Imagine a room, and inside is John, who can't speak a word of Chinese. Outside the room, a Chinese person sends a message into the room in Chinese. Luckily, John has an "if-then" book for Chinese characters. For instance, if he gets <你好吗>, the proper reply is <我还好>. All John has to do is follow his instruction book.

The Chinese speaker outside of the room thinks they're talking to someone inside who knows Chinese. But in reality, it's just John with his fancy book.

What is understanding?

Does John understand Chinese? The Chinese Room is, by all accounts, a computational view of the mind, yet it seems that something is missing. Truly understanding something is not an "if-then" automated response. John is missing that sinking in feeling, the absorption, the bit of understanding that's so hard to express. Understanding a language doesn't work like this. Humans are not Google Translate.

And yet, this is how AIs are programmed. A computer system is programmed to provide a certain output based on a finite list of certain inputs. If I double click the mouse, I open a file. If you type a letter, your monitor displays tiny black squiggles. If we press the right buttons in order, we win at Mario Kart. Input — Process — Output.

Can imitation become so fluid or competent that it is understanding.

But AIs don't know what they're doing, and Google Translate doesn't really understand what it's saying, does it? They're just following a programmer's orders. If I say, "Will it rain tomorrow?" Siri can look up the weather. But if I ask, "Will water fall from the clouds tomorrow?" it'll be stumped. A human would not (although they might look at you oddly).

A fun way to test just how little an AI understands us is to ask your maps app to find "restaurants that aren't McDonald's." Unsurprisingly, you won't get what you want.

The Future of AI

To be fair, the field of artificial intelligence is just getting started. Yes, it's easy right now to trick our voice assistant apps, and search engines can be frustratingly unhelpful at times. But that doesn't mean AI will always be like that. It might be that the problem is only one of complexity and sophistication, rather than anything else. It might be that the "if-then" rule book just needs work. Things like "the McDonald's test" or AI's inability to respond to original questions reveal only a limitation in programming. Given that language and the list of possible questions is finite, it's quite possible that AI will be able to (at the very least) perfectly mimic a human response in the not too distant future.

Credit: Oli Scarff via Getty Images

What's more, AIs today have increasingly advanced learning capabilities. Algorithms are no longer simply input-process-output but rather allow systems to search for information and adapt anew to what they receive.

A notorious example of this occurred when a Microsoft chat bot started spouting bigotry and racism after "learning" from what it read on Twitter. (Although, this might just say more about Twitter than AI.) Or, more sinister perhaps, two Facebook chat bots were shut down after it was discovered that they were not only talking to each other but were doing so in an invented language. Did they understand what they were doing? Who's to say that, with enough learning and enough practice, an AI "Chinese Room" might not reach understanding?

Can imitation become understanding?

We've all been a "Chinese Room" at times — be it talking about sports at work, cramming for an exam, using a word we didn't entirely know the meaning of, or calculating math problems. We can all mimic understanding, but it also begs the question: can imitation become so fluid or competent that it is understanding.

The old adage "fake it, 'till you make it" has been proven true over and over. If you repeat an action enough times, it becomes easy and habitual. For instance, when you practice a language, musical instrument, or a math calculation, then after a while, it becomes second nature. Our brain changes with repetition.

    So, it might just be that we all start off as Chinese Rooms when we learn something new, but this still leaves us with a pertinent question: when, how, and at what point does John actually understand Chinese? More importantly, will Siri or Alexa ever understand you?

    Jonny Thomson teaches philosophy in Oxford. He runs a popular Instagram account called Mini Philosophy (@philosophyminis). His first book is Mini Philosophy: A Small Book of Big Ideas.

    • A treasure trove of mobility data from devices like smartphones has allowed the field of "city science" to blossom.
    • I recently was part of team that compared mobility patterns in Brazilian and American cities.
    • We found that, in many cities, low-income and high-income residents rarely travel to the same geographic locations. Such segregation has major implications for urban design.

    Almost 55 percent of the world's seven billion people live in cities. And unless the COVID-19 pandemic puts a serious — and I do mean serious — dent in long-term trends, the urban fraction will climb almost to 70 percent by midcentury. Given that our project of civilization is staring down a climate crisis, the massive population shift to urban areas is something that could really use some "sciencing."

    Is urbanization going to make things worse? Will it make things better? Will it lead to more human thriving or more grinding poverty and inequality? These questions need answers, and a science of cities, if there was such a thing, could provide answers.

    Good news. There already is one!

    The science of cities

    With the rise of Big Data (for better or worse), scientists from a range of disciplines are getting an unprecedented view into the beating heart of cities and their dynamics. Of course, really smart people have been studying cities scientifically for a long time. But Big Data methods have accelerated what's possible to warp speed. As "exhibit A" for the rise of a new era of city science, let me introduce you to the field of "human mobility" and a new study just published by a team I was on.

    Credit: nonnie192 / 405009778 via Adobe Stock

    Human mobility is a field that's been amped up by all those location-enabled devices we carry around and the large-scale datasets of our activities, such as credit card purchases, taxi rides, and mobile phone usage. These days, all of us are leaving digital breadcrumbs of our everyday activities, particularly our movements around towns and cities. Using anonymized versions of these datasets (no names please), scientists can look for patterns in how large collections of people engage in daily travel and how these movements correlate with key social factors like income, health, and education.

    There have been many studies like this in the recent past. For example, researchers looking at mobility patterns in Louisville, Kentucky found that low-income residents tended to travel further on average than affluent ones. Another study found that mobility patterns across different socioeconomic classes exhibit very similar characteristics in Boston and Singapore. And an analysis of mobility in Bogota, Colombia found that the most mobile population was neither the poorest nor the wealthiest citizens but the upper-middle class.

    These were all excellent studies, but it was hard to make general conclusions from them. They seemed to point in different directions. The team I was part of wanted to get a broader, comparative view of human mobility and income. Through a partnership with Google, we were able to compare data from two countries — Brazil and the United States — of relatively equal populations but at different points on the "development spectrum." By comparing mobility patterns both within and between the two countries, we hoped to gain a better understanding of how people at different income levels moved around each day.

      Mobility in Brazil vs. United States

      socioeconomic mobility map US Brazil cities Socioeconomic mobility "heatmaps" for selected cities in the U.S. and Brazil. The colors represent destination based on income level. Red depicts destinations traveled by low-income residents, while blue depicts destinations traveled by high-income residents. Overlapping areas are colored purple.Credit: Hugo Barbosa et al., Scientific Reports, 2021.

      The results were remarkable. In a figure from our paper (shown above), it's clear that we found two distinct kinds of relationship between income and mobility in cities.

      The first was a relatively sharp distinction between where people in lower and higher income brackets traveled each day. For example, in my hometown of Rochester, New York or Detroit, the places visited by the two income groups (e.g., job sites, shopping centers, doctors' offices) were relatively partitioned. In other words, people from low-income and high-income neighborhoods were not mixing very much, meaning they weren't spending time in the same geographical locations. In addition, lower income groups traveled to the city center more often, while upper income groups traveled around the outer suburbs.

      The second kind of relationship was exemplified by cities like Boston and Atlanta, which didn't show this kind of partitioning. There was a much higher degree of mixing in terms of travel each day, indicating that income was less of a factor for determining where people lived or traveled.

      In Brazil, however, all the cities showed the kind of income-based segregation seen in U.S. cities like Rochester and Detroit. There was a clear separation of regions visited with practically no overlap. And unlike the U.S., visits by the wealthy were strongly concentrated in the city centers, while the poor largely traversed the periphery.

      Data-driven urban design

      Our results have straightforward implications for city design. As we wrote in the paper, "To the extent that it is undesirable to have cities with residents whose ability to navigate and access resources is dependent on their socioeconomic status, public policy measures to mitigate this phenomenon are the need of the hour." That means we need better housing and public transportation policies.

      But while our study shows there are clear links between income disparity and mobility patterns, it also shows something else important. As an astrophysicist who spent decades applying quantitative methods to stars and planets, I am amazed at how deep we can now dive into understanding cities using similar methods. We have truly entered a new era in the study of cities and all human systems. Hopefully, we'll use this new power for good.