Feedback loops: what they are and why they matter

If you’ve ever been on a swing, or triggered a deafening squeal from a speaker by putting it too close to the microphone, then you’ve witnessed a feedback loop in action.  You can find feedback loops almost everywhere, from nuclear power stations to viral videos, and they are particularly relevant to sustainability, the economy, and society.  Here’s what feedback loops are and why they are vital for understanding today’s world and designing the future.

 

What are feedback loops?

A feedback loop is a mathematical way of describing how something behaves over time.  Feedback loops can be either positive and negative.

A positive feedback loop means that a change in a system reinforces itself.  For example, think of a landslide: one rock falling can knock two others out of place, which in turn dislodge many other rocks until the whole hillside is on the move.  Similarly, because bacteria can reproduce by splitting in two, the total number of bacteria increases only slowly when there are few bacteria, and faster when there are more bacteria.

Positive feedback loops tend to change a system away from its original state – first slowly, then rapidly.  If you made a graph of how something in a positive feedback loop behaves over time, it might look like this:

Positive feedback loop, increasing over timeOr this:

Positive feedback loop, decreasing over time

A negative feedback loop, in contrast, means that a change in a system weakens itself – therefore, negative feedback loops tend to return a system to its original state.  For example, when you sit on a swing and kick yourself away from the centre, gravity brings you back towards the centre.  If you stretch a spring, then it becomes more tense – when you let go, that tension shortens the spring again.

Something in a negative feedback loop might behave like this over time:

Negative feedback loop, oscillating

Or like this:

Negative feedback loop, damped

Why do feedback loops matter?

Here are just a few examples of important feedback loops.

 

Runaway climate change

Image: US Geological Survey, Collapsed permafrost block of coastal tundra on Alaska’s Arctic Coast, Wikimedia Commons, Public Domain
Image: US Geological Survey, Collapsed permafrost block of coastal tundra on Alaska’s Arctic Coast, Wikimedia Commons, Public Domain

To an extent, negative feedback loops let the Earth’s climate recover from small changes.  Slightly higher levels of carbon dioxide, for example, can trigger plants to grow more and remove some of that carbon dioxide from the atmosphere again1.

However, there are also positive feedback loops that could amplify warming trends.  For example, sea ice normally reflects 50% to 70% of the sunlight that hits it, keeping the ice cool.  Climate change might melt some of the ice, exposing the ocean surface.  The dark ocean reflects only 6% of the sunlight that hits it, absorbing the other 94% as heat, which causes further warming and further melting of the ice2.

A paper published last month warns that, like a line of falling dominoes, a cascade of positive feedback loops could lead the climate into a “Hothouse Earth” state for thousands of years3.  Ice sheets melting at 1°C to 3°C of warming could cause more warming, tipping ocean currents and forests into causing more warming at 3°C to 5°C, which in turn could trigger the melting of permafrost (which could release large amounts of greenhouse gases) at over 5°C of warming.  5°C of climate warming, by the way, can be translated as “really bad news”.

 

Stable market equilibrium, or bursting bubbles?

Image: Rafael Matsunaga, That was supposed to be going up, wasn't it? São Paulo Stock Exchange (Bovespa), Flickr, Creative Commons Attribution 2.0 Generic
Image: Rafael Matsunaga, That was supposed to be going up, wasn’t it? São Paulo Stock Exchange (Bovespa), Flickr, Creative Commons Attribution 2.0 Generic

Markets are governed by both negative and positive feedback loops, which can affect whether prices are stable or unstable.

Normally, the price of items can’t move very far from an equilibrium point at which supply and demand are equal4.  High prices mean that fewer people will buy, while low prices mean that sellers have less incentive to sell5.  Both cases push prices back towards the equilibrium point.  These negative feedback loops should, in theory, keep prices fairly stable over time.  Read more in my post What’s the profit motive and how is it supposed to work?

But prices aren’t always stable in the real world.  Think of the price of the cryptocurrency Bitcoin, which saw a sharp rise followed by a sharp crash earlier this year, swinging wildly along the way6.  Why?

Speculators hope to make money by buying something (such as company shares) cheaply and then selling it at a higher price.  They’ll therefore buy things that are rising in price, hoping that the price will rise even further.  That act of buying increases demand and pushes the price up – which encourages other speculators to join in, creating a positive feedback loop.  The direction of the positive feedback loop can switch rapidly: any hint of prices dropping will scare the speculators into selling, leading to prices crashing even further.  Speculative bubbles and their inevitable bursting have occurred many times throughout history, from tulip bulbs in 17th century Holland to the dot-com bubble of the 1990s7.

The Economist explains how speculative activity can affect the housing market8.  Buy-to-let “investors”, compared to people who will live in the house they buy, are more likely to buy when prices are rising and sell when prices are falling, because higher rents mean more profit.  Buy-to-let activity can therefore contribute to unstable house prices.

 

Filter bubbles and echo chambers

Image: Colin, A backlit laptop computer keyboard, Wikimedia Commons, Creative Commons Attribution-ShareAlike 4.0 International
Image: Colin, A backlit laptop computer keyboard, Wikimedia Commons, Creative Commons Attribution-ShareAlike 4.0 International

YouTube, Pinterest, Twitter and other websites will often suggest new things for you to watch, read or follow depending on what you have looked at recently.  Watch a cat video, for example, and YouTube’s recommendation engine might automatically play another cat video for you.  That’s helpful – or is it?

Because these algorithms want you to click on as much as possible, they also take into account what other users have watched after viewing one particular video.  Watch one angry political video, and the algorithm might show you a series of ever more extreme or hate-filled videos, because it thinks you will like them.  You could end up in a “filter bubble” where you only see one side of a debate.

Similarly, if you mostly follow people on Twitter who have similar viewpoints and interests – I’ve often done this myself – then you might find yourself in an “echo chamber” where most people appear to agree with the viewpoints you already hold.

These are positive feedback loops with some very negative consequences: filter bubbles and echo chambers have been linked to political polarisation, radicalisation, and violent extremism9.

Happily, however, some recent research suggests that fewer people than previously thought are influenced by online echo chambers10.  I recommend reading Tim Harford’s thoughtful piece How to burst your political filter bubble.

 

How can we harness feedback loops for good?

Changing social norms

Image: Ludovic Bertron, Rainbow flag and blue skies, Wikimedia Commons, Creative Commons Attribution 2.0 Generic
Image: Ludovic Bertron, Rainbow flag and blue skies, Wikimedia Commons, Creative Commons Attribution 2.0 Generic

Many environmental problems would be easier to solve if people behaved differently – for example, eating significantly less red meat (particularly in richer countries, where meat consumption is higher than in poorer countries), taking fewer flights, or cycling or taking public transport instead of driving.  But it’s more effort to do things that are not currently mainstream: restaurants tend to serve meat and dairy, long-distance trains and ferries are expensive, and cities are often dangerous to cycle in because of the heavy traffic.

The good news is that, as a previously unpopular lifestyle choice or viewpoint grows in popularity, there is less stigma, businesses start to provide options for the growing market, and there is more information available to guide people.  All of this makes it easier, more appealing, and more socially acceptable for other people to join in and contribute to the growing trend.

This self-reinforcing growth of a social norm might have a “tipping point” – a recent study found that once 25% of people in a group adopt a new norm, the remaining 75% also tend to flip and the new norm becomes the mainstream11.

While that study was based on experiments, rapid social changes can also happen in the real world.  Before 1967, you could go to jail in the UK for having a gay relationship, but today, LGBT rights are protected by law and same-sex marriage is legal in England, Wales and Scotland12.  In 1987, only 11% of the British public thought same-sex relationships were “not wrong at all”, but by 2016, that figure had risen to 64%13.

Gay rights campaigner Peter Tatchell has said14, “Compared to two decades ago, Britain is almost a different country. All the main anti-gay laws have been abolished. We are now one of the best countries in the world for gay equality.”

Just think what the power of social change could achieve.  How much sooner might practical alternatives to carbon-intensive flights become available if flying were socially unacceptable?  The process already appears to be underway with veganism and flexitarianism, as more and more people are cutting down on eating meat and shops and restaurants begin to offer more options15.

 

Tiered pricing

Image: Dave Pickersgill, Fountains in Sheffield, Geograph, Creative Commons Attribution-ShareAlike 2.0 Generic
Image: Dave Pickersgill, Fountains in Sheffield, Geograph, Creative Commons Attribution-ShareAlike 2.0 Generic

Have you ever walked out of a supermarket with three packets of biscuits when you only wanted one – just because that multibuy offer was too good to resist?  Discounts for buying in bulk are great news for retailers, but they can encourage excessive consumption.  Scotland has even banned multibuy discounts on alcohol16.

What if we flipped that pricing structure on its head?  The city of Santa Fe in California did just that with its tiered pricing model for water.  Customers could buy a certain amount of water for a relatively low price, but any additional units were charged at a higher price17.  This negative feedback loop was designed to discourage customers from using too much water, without either making it too expensive for poorer people to access water or imposing water rationing.

It worked: daily water use per person in Santa Fe fell from 140 gallons in 2001 to about 100 gallons in 201518.

Tiered pricing is not perfect: it has been challenged in the Californian courts19, and poorer customers might be disadvantaged if several households share the same metered water source20.  Nevertheless, tiered pricing has successfully reduced water consumption in several places21 and the principles could be extended to other things such as electricity or even flights22.

 

Innovation and investment

Image: National Cancer Institute, Scientist Looking Through Microscope, Wikimedia Commons, Public domain
Image: National Cancer Institute, Scientist Looking Through Microscope, Wikimedia Commons, Public domain

Technology isn’t the answer to everything and usually has unintended consequences, but it almost certainly has a role to play in solving many of the world’s current problems – think of renewable energy, atmospheric carbon dioxide removal or a floating system to clean plastic from the ocean.

But there’s a problem: helpful technologies can be expensive, particularly in the early stages of development.  However, as a new technology gets cheaper, more businesses and people will want to use it, which will lead to further investment, innovation and cost savings from economies of scale.

The result?  New technologies that might appear impractically expensive at first could become viable options over time, transforming lifestyles and our environmental impacts.

Take cellular agriculture: the world’s first lab-grown hamburger cost hundreds of thousands of pounds to create in 201323.  The process was very inefficient, using equipment that was only designed for tiny quantities of cells.  Nevertheless, the prototype showed that growing meat in the lab was a real possibility and attracted a lot of media attention.

Several years later, the cellular agriculture industry is growing, as many startups have received millions of pounds in funding.  Investment, new ideas, media attention and public awareness can all strengthen each other – and as producers scale up their facilities, the lab-grown meat could become more efficient, cheaper and more environmentally friendly.  Personally, I hope this set of positive feedback loops will soon make animal farming obsolete and reduce the food system’s burden on the planet.

 

Time for some feedback!

I’d love to hear your comments.  Which feedback loops do you observe in your daily life or on the news? Are they harmful or helpful?

 

 

  1. Boston University (2016), CO2 fertilization greening the Earth
  2. The technical term for how much light a surface reflects is albedo. An albedo of 1 means that all of the light is reflected (a perfectly white surface) while an albedo of 0 means that none of the light is reflected (a perfectly black surface). National Snow and Ice Data Center, Thermodynamics: Albedo
  3. Read my summary of the paper here: Food Climate Research Network (2018), Domino effect could cause “Hothouse Earth”
  4. Economics Online, Market equilibrium
  5. The technical terms for these interactions of price, demand and supply are, respectively, price elasticity of demand and price elasticity of supply. See Economics Help, Price Elasticity of Supply and Price Elasticity of Demand
  6. CoinDesk, Bitcoin (USD) Price. Select “all” to see price history from 2010.
  7. Elvis Picardo, Investopedia, Five Of The Largest Asset Bubbles In History
  8. The Economist (2017), Britain’s buy-to-let boom is coming to an end, “Property investors buy when prices are rising but sell when they are falling, making house prices more volatile.”
  9. Ryan Broderick (2016), This Is How Facebook Is Radicalizing You; James E. Hawdon (2017), Over the years, Americans have become increasingly exposed to extremism; Kelly and François (2018), This is what filter bubbles actually look like
  10. Dubois and Blank (2018), The myth of the echo chamber
  11. Science Daily (2018), Tipping point for large-scale social change
  12. Not, however, in Northern Ireland. BBC Newsround (2017), Decriminalisation of homosexuality: History of gay rights in the UK.
  13. Schraer and D’Urso (2017), Gay rights 50 years on: 10 ways in which the UK has changed
  14. BBC Newsround (2017), Decriminalisation of homosexuality: History of gay rights in the UK
  15. Dan Hancox, The Guardian (2018), The unstoppable rise of veganism: how a fringe movement went mainstream; Mintel (2017), Brits carve their meat intake: 28% of Brits have cut back their meat consumption over the last six months
  16. BBC News (2011), Scots ban on supermarket alcohol deals comes into force
  17. This form of pricing is also known as an increasing block tariff.
  18. Nelson D. Schwartz, New York Times (2015), Water Pricing in Two Thirsty Cities: In One, Guzzlers Pay More, and Use Less
  19. Janny Choy, Stanford Woods Institute for the Environment (2015), Stanford professor developing water usage model that could help California meet conservation goals
  20. Water & Sanitation for the Urban Poor (2016), Pro-poor tariff systems: what if ‘fair’ pricing isn’t fair enough?
  21. Examples of tiered pricing successfully reducing water consumption include: farms in Broadview, California (Michael Hanemann, 2006, Tiered pricing of water); residential water use in the Eastern Municipal Water District of Southern California (Baerenklau et al., 2013, Do Increasing Block Rate Water Budgets Reduce Residential Water Demand? A Case Study in Southern California); and the city of Zaragoza in Spain (Kayaga and Smout, 2014, Tariff structures and incentives for water demand management).
  22. See the campaign group a free ride, which promotes taxing frequent flyers at a higher rate than people who take only (say) one flight a year.
  23. Pallab Ghosh, BBC News (2013), World’s first lab-grown burger to be cooked and eaten

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