The Art of Thinking Clearly: The Secrets of Perfect Decision-Making

by Rolf Dobelli

‘Each year, the number of traffic accidents rises by 7%,’ warns a politician. Let’s be honest: we don’t intuitively understand what this means. So, let’s use a trick and calculate the ‘doubling time’. Start with the magic number of 70 and divide it by the growth rate in per cent. In this instance: 70 divided by 7 = 10 years. So what the politician is saying is: ‘The number of traffic accidents doubles every 10 years.’ Pretty alarming. (You may ask: ‘Why the number 70?’ This has to do with a mathematical concept called logarithm. You can look it up in the notes section.) Another example: ‘Inflation is at 5%.’ Whoever hears this thinks: ‘That’s not so bad, what’s 5% anyway?’ Let’s quickly calculate the doubling time: 70 divided by 5 = 14 years. In 14 years, a dollar will be worth only half what it is today – a catastrophe for anyone who has a savings account. Suppose you are a journalist and learn that the number of registered dogs in your city is rising by 10% a year. Which headline do you put on your article? Certainly not: ‘Dog registrations increasing by 10%.’ No one will care. Instead, announce: ‘Deluge of dogs: twice as many mutts in 7 years’ time!’ Nothing that grows exponentially grows for ever. Most politicians, economists and journalists forget that. Such growth will eventually reach a limit. Guaranteed. For example, the intestinal bacterium, Escherichia coli, divides every twenty minutes. In just a few days it could cover the whole planet, but since it consumes mo...