While people might like positive feedback from a friend or a job review, it isn't necessarily a good thing.
You'll know what this means if you've heard the screech of a PA system that is turned up too loud and the sound leaks back into the microphone.
This type of feedback can wreck both the PA and your hearing. Something has to give because the amplification obviously can't continue forever.
Modern electronics usually incorporate controls to prevent it happening. Alternatively, it's done manually when someone rushes to turn down the volume.
The general rule is that a system that can't control its positive feedbacks will fail and, in the worst case, physically collapse.
An example of a failing system (though not physically collapsing) is a traffic jam.
As population and traffic grows, disgruntled commuters demand more roads. But then more roads attract more cars, which need more roads. And so on, ad infinitum.
The German mathematician Dietrich Braess recognised the paradox that now bears his name.
A prime example is Boston's "Big Dig" project that ended up costing more than $8 billion, making it the most expensive highway project in US history.
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New bridges and tunnels rerouted the Interstate 93 through the heart of the city.
Did it work? Not according to a 2008 Boston Globe report, which said: "... many motorists ... are spending more time stuck in traffic, not less."
On a global scale we're now seeing positive feedback loops associated with climate tipping points.
Melting icecaps are replaced by dark surface which absorb more heat, which accelerates melting. And so on.
In 1798, Thomas Malthus obliquely recognised another type of positive feedback: population.
It began with a simple observation. If a species reproduces, why do their numbers not increase to infinity?
Parents have offspring, who then have more offspring. With each cycle, the number reproducing increases, which, if left unchecked, grows exponentially.
While the percentage might seem a modest (say) 2 per cent, it's compounded, year on year growth.
Clearly that is impossible. It means the only way to avoid collapse is to stabilise at a number that the natural world can support.
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