Top-left panel: [N II] BPT diagram for 1.4 ≤ z ≤ 2.7 MOSDEF galaxies. Green [blue] points indicate z ∼ 2.3 [z ∼ 1.5] MOSDEF galaxies with ≥3σ detections of all four BPT emission lines. Median MOSDEF error bars are indicated in the lower right-hand corner of each panel. The grayscale histogram and orange curve correspond, respectively, to the distribution and running median of local SDSS galaxies. The running median line ratios are calculated in closely spaced bins of stellar mass. Large red [gold] stars indicate measurements from composite spectra, binned by stellar mass, of all MOSDEF z ∼ 2.3 [z ∼ 1.5] galaxies with coverage of the relevant emission lines and ≥3σ Hα detections. Stacks of increasing stellar mass have lower [O III]λ 5007/Hβ. The black dotted curve is the “maximum starburst” line from Kewley et al. (2001), while the black solid curve is an empirical AGN/star formation threshold from Kauffmann et al. (2003). Although plotted here for completeness, SDSS galaxies falling above the Kauffmann et al. (2003) curve are not included in our analysis. Top-right panel: [S II] BPT diagram. Symbols are the same as in the top-left panel. Bottom panel: O₃₂ vs. R₂₃ diagram, corrected for dust. Stacked points are not shown for the z ∼ 1.5 sample in this panel, given the small size of the sample with [O II]λλ3726, 3729 coverage.