Evolution of the AGN fraction in clusters or protoclusters and the field. Cluster or protocluster measurements are solid symbols, and the field measurements are open symbols. The cluster AGN fraction is lower than the field fraction at z < 1, comparable at 1 < z < 1.5, and higher at z > 2. USS 1558−003 is the filled black circle at z = 2.53, and the corresponding field fraction is the open black circle. Measurements selected with emission lines are shown as red circles, and measurements selected via broadband colors are shown as green squares. The fractions and luminosity limits at z > 1.5 are summarized in Table 3. The blue and magenta symbols at z < 1.5 represent averages for large samples of clusters, not individual overdensities (see Section 4).