Licensure

Medical licensing only became important 100 years ago with the publishing of the Flexner Report.  Other occupations have been catching up since then.  Due to licensing requirements in Illinois, it took longer to "become a master plumber than for a newly graduated physician to become a Fellow of the American College of Surgeons." Alan Kreuger says licensing is growing in many sectors of the economy:

Morris M. Kleiner, ...questions whether occupational licensing has gone too far. He provides much evidence that the balance of occupational licensing has shifted away from protecting consumers and toward limiting the supply of workers in various professions. A result is that services provided by licensed workers are more expensive than necessary and that quality is not noticeably affected.... Professor Kleiner conservatively estimates that 20 percent of workers in 2000 were in an occupation that was covered by a state licensing requirement, up from 5 percent in the 1950's.
He said that licensing has mushroomed because the service sector, where licensing is more prevalent, has grown rapidly and because more occupations have started requiring licenses.
...A state-by-state list of the occupations covered by licensing requirements is available from www.acinet.org/acinet/licensedoccupations.... Perhaps as many as 3 of every 10 workers nationwide are required to obtain a license to do their job.... Kleiner and a colleague, Robert T. Kudrle, found that stricter state licensing requirements for dentists did not noticeably affect the dental health of 464 Air Force recruits. Other studies have found at best weak evidence that students in classes taught by licensed teachers performed better than those taught by unlicensed teachers.
Summarizing the literature, Professor Kleiner concludes, "there is little to show that occupational regulation has a major effect on the quality of service received by consumers."
At the same time, the hurdles imposed by occupational licensing reduce the supply of workers in many regulated professions, which drives up wages in those jobs and the price of services. Dentists, for example, were found to earn and charge 11 percent more in states with the most restrictive licensing requirements. While tough licensing standards may help higher-income consumers avoid low-quality providers, it also appears to prevent lower-income consumers from gaining access to some services.
Professor Kleiner said that a frequent pattern was for workers who have common interests and provide a homogeneous service to form an association. That association then seeks regulation to restrict the number of people who can work in the occupation. The state legislature and governor have little incentive to resist this pressure because the state gains revenue from license fees.
The most notable opposition to licensing comes from large buyers like hospitals, which object to the monopolization of their suppliers. In most cases, however, consumers are diffuse and have little individual incentive to oppose licensing.
Another factor driving the growth of occupational licensing is the decline of labor unions. Apparently, the labor market abhors a vacuum; it needs some institutions and rules to function. Occupational licensing has replaced unions as the main labor market institution. There are now more than twice as many workers covered by occupational licensing provisions as are covered by a labor union contract.
"There is a lot more flexibility with unions than occupational licensing," Professor Kleiner said. Unions, he pointed out, negotiate at the company level and can be decertified, whereas occupational license requirements are typically statewide and rarely repealed.