2.5 William Petty: "... if the present trend continues, then..."

At the same time as John Graunt was examining bills of mortality, his colleagues became interested in the very same material. One of them was William Petty (1623-1687), who was John Graunt's adviser in medical matters. It has been claimed that part of Graunt's studies was written by Petty.

William Petty studied the number of population in different cities - such as London, Paris and Rome - and realised that the future population development could be predicted on the basis of past development, that is, "... if the present trend continues, then..." From those days onwards, a number of imaginative means have been used to improve the reliability of population projections - without any success.