  The detection rate of gamma-ray burst (GRB) afterglows is only ~30% at radio
wavelengths, much lower than in the X-ray (~95%) or optical (~70%) bands. The
cause of this low radio detection rate has previously been attributed to
limited observing sensitivity. We use visibility stacking to test this idea,
and conclude that the low detection rate is instead due to two intrinsically
different populations of GRBs, radio bright and radio faint. We calculate that
no more than 70% of GRB afterglows are truly radio bright, leaving a
significant population of GRBs that lack a radio afterglow. These radio bright
GRBs have higher gamma-ray fluence, isotropic energies, X-ray fluxes and
optical fluxes than the radio faint GRBs, confirming the existence of two
physically distinct populations. We suggest that the gamma-ray efficiency of
the prompt emission is responsible for the difference between the two
populations. We also discuss the implications for future radio and optical
surveys.
