We revisit the shocked shell model for the class of active galactic nu
clei known as gigahertz peak spectrum (GPS) sources, incorporating new
observational data on the radiation brightness temperatures. We argue
that in addition to free-free absorption, induced Compton scattering
will also have an important effect in forming the approximately gigahe
rtz peak and in shaping the radio spectra that characterize these sour
ces. Indeed, our arguments suggest that GPS sources may provide the fi
rst real evidence for the role of induced Compton scattering in extrag
alactic radio sources.