A quantitative theory of spectral lags for \gamma -ray bursts ( GRBs ) is given .
The underlying hypothesis is that GRB subpulses are photons that are scattered into our line of sight by local concentrations of baryons that are accelerated by radiation pressure .
For primary spectra that are power laws with exponential cutoffs , the width of the pulse and its fast rise , slow decay asymmetry is found to increase with decreasing photon energy , and the width near the exponential cutoff scales approximately as E _ { ph } ^ { - \eta } , with \eta \sim 0.4 , as observed .
The spectral lag time is naturally inversely proportional to luminosity , all else being equal , also as observed .