Mark Bils, Bariş Kaymak, and Kai-Jie Wu in this NBER paper:
Research on labor markets is increasingly focused on the role of monopsony power, examining its impact on income inequality and labor’s allocation, as well as its implications for policy interventions, specifically minimum legal wages. While traditional approaches associate monopsony power with firm characteristics—especially a firm’s employment share—this paper proposes a novel perspective by attributing monopsony power to comparative advantage at the level of a worker-firm match.
Our study integrates Robinson’s (1933) model of non-competitive firm wage setting with Roy’s (1951) model of worker comparative advantage. The reasoning is as follows. Monopsony power emerges when a firm faces an imperfectly elastic labor supply, implying the firm has workers it could employ at lower wages. The differential between wage received and wage required represents a rent for those inframarginal workers. The Roy model associates rents with a worker’s comparative advantage in their job, i.e., their productivity at their chosen job relative to alternatives. As a result, we can associate monopsony power in wage setting with workers who have a comparative advantage at their firm.
Mostly Economics 
