By Jacob Aron THE greatest shortcoming of the human race is our inability to understand the exponential function.” These are the words of the late Albert Bartlett, a physicist at the University of Colorado, Boulder, whose lectures on the subject became a YouTube hit. Arguably, he’s right. Take saving for retirement. “Start early” is the mantra, but it is easy to overlook just how much difference a few years can make. It all comes down to exponential growth – an often abused term that refers to anything that grows in proportion to its current value. It dictates that a forward-thinking 18-year-old can retire as a millionaire at 65 by investing around £250 a month with an average annual return of 7 per cent. That figure might sound high by today’s standards, but it’s a rough average of the stock market return since 1960. The surprise is that when our saver reaches 55, the savings will amount to a little under £500,000. Thanks to the power of compound interest, however – exponential growth by another name – it will double to £1 million just 10 years later. Wait until you’re 30 to start saving that £250 and you’ll only reach about half as much (see diagram). Starting at 30, you’d actually need to save more than £600 a month to make it to £1 million by 65.FIG-mg30510501.jpg Exponential growth’s stealth factor is nicely illustrated by the story of the man who invented chaturanga, an Indian precursor to chess. He presented his king with a beautifully laid out board divided into 64 squares and when asked to name his reward,