Star formation rate (SFR, in M_⊙ yr^-1) as function of total stellar mass (M_⋆, in M_⊙) in two galaxy samples. The data points are estimates of the star formation rate and stellar mass for samples of galaxies at z ~ 1 (blue circles; Elbaz et al. 2007) and z ~ 2 (red circles; Daddi et al. 2007). The green and purple solid lines indicate the best fits to the data sets at z ~ 1 and z ~ 2 from the original papers. The two red lines indicate our simple star formation model with the best scaling parameters adopted (see text for details).