A Brief History of the Private Investigator
In 1833 a French soldier called Eugène François Vidocq, a French soldier, criminal and privateer, founded the first known private detective agency, "Le Bureau des Renseignements Universels pour le commerce et l'Industrie" (Office of Intelligence) and hired ex-convicts.
In the U.S., the Pinkerton National Detective Agency was a private detective agency established in 1850 by Allan Pinkerton. Pinkerton had become famous when he foiled a plot to assassinate then President-Elect Abraham Lincoln. Pinkerton's agents performed services which ranged from undercover investigations and detection of crimes to plant protection and armed security. It is sometimes claimed, probably with exaggeration, that at the height of its existence the Pinkerton National Detective Agency employed more agents than the United States Army.
Pinkerton agents were also hired to track western outlaws Jesse James, the Reno brothers, and the Wild Bunch, including Butch Cassidy and the Sundance Kid. The Pinkerton agency's logo, an eye embellished with the words "We Never Sleep," inspired the term "private eye."
It was not until the prosperity of the 1920s that the private investigator became a person accessible to the average American. With the wealth of the 1920s and the expanding of the middle class came the need of middle America for private investigators.
Since then the private detective industry has grown with the changing needs of the public. Social issues like infidelity and unionization have impacted the industry and created new types of work, as has the need for insurance and, with it, insurance fraud, criminal defense investigations and the invention of low-cost listening devices. In a number of countries, a licensing process has been introduced that has put criteria in place that investigators have to meet: in most cases, a clean criminal record. This has combined with modern business practices that have ensured that most investigators are now professional in outlook, rather than seeing the PI world as a second career opportunity for retired policemen.
If anyone reading this has any more information on the history of the profession then I would be greatful for your input.