Lena Edlund, PhD, Associate Professor of Economics at Columbia University, and Evelyn Korn, PhD, Faculty of Economics and Business Administration at University of Marburg in Germany, wrote the Feb. 2002 Journal of Political Economy article "A Theory of Prostitution" that stated:

“Before proceeding, we need to define prostitution. Despite being known as the oldest profession, a workable definition has proven elusive. From a dictionary we learn that prostitution is the ‘act or practice of engaging in sexual intercourse for money’. But a prostitute cannot simply be a woman who sells her body, since ‘that is done every day by women who become wives in order to gain a home and a livelihood’. Promiscuity has been proposed as another candidate. Medieval canon lawyer Johannes Teutonicus suggested that a woman who had sex with more than 23,000 men should be classified as a prostitute, although 40 to 60 would also do. However, promiscuity itself does not turn a woman into a prostitute. Although a vast majority of prostitutes are promiscuous, most people would agree that sleeping around does not amount to prostitution. Moreover, any threshold number of sexual partners, be it 40 or 23,000, fails to identify high end courtesans or call girls as prostitutes, although a reasonable definition would. Instead, we argue that prostitution is the act of rendering, from the client’s point of view, non-reproductive sex against payment.”

Feb. 2002