Temporal evolution of the average stellar mass of the four most massive (brightest) galaxies identified in each of our galaxy groups. From top to bottom, panels correspond to all groups: combined, single-BGG groups, double-BGG groups, and non-BGG groups. Data points represent the mean stellar mass of m₁ (magenta), m₂ (grey), m₃ (brown), and m₄ (cyan) galaxies, with the associated shaded regions indicating the standard error.