Cyril Northcote Parkinson derived the dictum from his extensive experience in the British Civil Service.
A current form of the law is not the one Parkinson refers to by that name in the article, but a mathematical equation describing the rate at which bureaucracies expand over time. Much of the essay is dedicated to a summary of purportedly scientific observations supporting the law, such as the increase in the number of employees at the Colonial Office while Great Britain's overseas empire declined (he shows that it had its greatest number of staff when it was folded into the Foreign Office because of a lack of colonies to administer). He explains this growth by two forces: (1) "An official wants to multiply subordinates, not rivals" and (2) "Officials make work for each other." He notes that the number employed in a bureaucracy rose by 5–7% per year "irrespective of any variation in the amount of work (if any) to be done".
Putting the 'Merry' in Merry-Go-Round.
London Underground - 1977