Earlier this morning we considered the burning question of whether regulation of dental hygienists has contibuted to an increase in income inequality. Before I staked out a position on this I wanted to know if hygienist regulation had increased over time, but sadly, uncredentialed proles like me are denied access to the relevant academic paper unless we fork over $5, thus transferring wealth from me to the economics profession and increasing income inequality along the way. Luckily, reader JR bravely defied the relevant IP laws and sent me a copy. So now I have an answer for you.
The specific question at hand is whether hygienists are increasingly being required to work for dentists, which would decrease their earning power and increase the profits of dental practices owned by wealthy dentists. The answer is no. From the paper:
Until 1988, when Colorado first allowed hygienists to practice without the direct supervision of a dentist, hygienists have been required to work for or be under the direction of a dentist. Since that time, seven states have allowed hygienists to be self-employed without the direct oversight of a dentist.
....In order to show the growth in hygienists’ autonomy over time, in Figure 1 we develop and show a box-and-whisker graphic analysis of state regulation, which gives the mean and spread of the regulation of hygienists over the period 2001–2007. Panel A shows the overall ranking of dental hygienists’ professional practice environment that is allowed by statute or legal rulings.
This is followed by lots of Greek letter math that no sane person would try to understand. However, charts are easy to understand, so I've helpfully reproduced Panel A on the right, adding a bright red arrow showing the increase in hygienist autonomy over the past decade. The basic shape of things is clear: despite pushback from the dental profession, over the past couple of decades hygienists have been allowed to perform more and more tasks and have been unshackled entirely from the dental profession in seven states. This is (probably) a triumph of improved public policy and a counterweight to growing income inequality. So now you know.