William Petty

The other William you need to know about if you are interested in data. While Petty didn't use charts of maps (the furthest he went were tables) we think he did enough to make our list of data heroes. Born in 1623 he came up with the first national balance sheet and population estimates for the UK in ways no-one before him had thought of, carefully counting the number of chimneys to gauge the number of houses, and sifting through data on grave stones to estimate birth and death rates. Like Playfair, a character that history has somehow missed, it was Petty (not Richard Stone) that invented GDP, and, as Richard Davies argues in the essay below published in The Economist, he is also the founding father of economics. Pretty heroic, we think.

Meet Sir William Petty, the man who invented economics

Born in 1623, the young Petty showed an early interest in clockmaking and joinery but did not enter either trade. His hometown, Romsey, is close to the sea and he left home on a ship, as a cabin boy, aged 14. That brought the first of two mishaps that changed the course of his life. After breaking his leg in a nautical accident he was put ashore for treatment in Normandy. As the injury healed he enrolled in a Jesuit college in Caen and developed an interest in anatomy, which he went on to study in Amsterdam and Paris. In Paris he met and worked for Thomas Hobbes, whose empiricism was a deep influence. Collecting data and making real-world observations became central to his work.

Back in England from 1646, Petty continued his medical studies at Oxford, rising to become professor of anatomy at Brasenose College by the time he was 27. Then came the second upheaval. After witnessing a bungled hanging Petty appeared to bring the criminal, a woman, back to life. She was pardoned, his reputation soared and he received a lucrative post, as physician general, in Ireland. His travels explain why Petty's economics are full of international comparisons: stagnant Ireland, more prosperous England and its great rival at the time, the Netherlands.

Success in Ireland meant Petty was granted an estate. He puzzled over how to value it. He first calculated its benefits: if cattle grazing in a field put on a certain amount of weight each year, the market price of the extra meat was a logical measure. Next he needed to work out how many years' income to tally. Men cared about their children and grandchildren, he reasoned, but concerns for the future were finite. Using data on the extent to which generations overlapped, Petty reckoned 21 years was right. He had jumped from a blank sheet of paper to an embryonic version of the "present value" calculation at the heart of modern finance.

Petty's landholding also provided the spur for his most important invention, GDP. England fought the Dutch three times between 1652 and 1674, and landowners faced high taxes. Petty thought this unjust and, to explain why, he set out the first set of national accounts for England and Wales. First, he asserted that total income must equal total spending. Since 4.5 pence a day was needed for food, housing, clothing and "all other necessities", and the population was 6m, spending was Pounds 40m a year. Next he tallied the income from a long list of assets--land, houses in London, ships--for a sum of Pounds 15m. If Pounds 15m of the Pounds 40m in spending was income from assets, the remaining Pounds 25m must be wages. The tax burden, he argued, should be shifted accordingly.

Other ideas were less self-serving. Out of concern that high interest rates were holding back trade in Ireland, Petty developed a sophisticated monetary theory, according to James Ullmer of Western Carolina University. He calculated the cash needed for all the transactions in Ireland each year, and how quickly it circulated, to derive an estimate of the amount of money needed to keep a lid on interest rates. This "quantity theory of money" is core to monetary economics, and Petty's version of it came a century before the one published by David Hume in 1752 that is usually credited as the first proper treatment.

Petty's monetary theories prompted him to contest criticism of England's fledgling banking system. Take, he argued, the problem of an economy that needs a money supply of Pounds 100,000 but has only Pounds 60,000 in cash. It could keep Pounds 20,000 as currency, with Pounds 40,000 put into banks. Because banks could lend the money out, loans and deposits would run close to Pounds 80,000. Add back the coins, and you have Pounds 100,000. Since such "fractional reserve" banking could help multiply money, Petty was a supporter: banks could help England compete with the Dutch.

Pyramid schemer

Petty also drew conclusions from his national accounts, which pointed to the importance of income from labour. He worried that unemployment would reduce men's facility to work--precisely the job-market scarring or "hysteresis" modern economists fret about. He pre-empted a rhetorical proposal made centuries later by John Maynard Keynes: since pointless work was better than none, the unemployed could be paid to build a pyramid on Salisbury Plains, or transport Stonehenge to London. Petty's real point was that deficient demand was a threat, and that in times of slump public investment could help offset it.

To read the full article, continue to The Economist.