  Flat-spectrum radio-loud Narrow-Line Seyfert 1 galaxies (NLS1s) are a
recently discovered class of $\gamma$-ray emitting Active Galactic Nuclei
(AGN), that exhibit some blazar-like properties which are explained with the
presence of a relativistic jet viewed at small angles. When blazars are
observed at larger angles they appear as radio-galaxies, and we expect to
observe an analogue parent population for beamed NLS1s. However, the number of
known NLS1s with the jet viewed at large angles is not enough. Therefore, we
tried to understand the origin of this deficit. Current hypotheses about the
nature of parent sources are steep-spectrum radio-loud NLS1s, radio-quiet NLS1s
and disk-hosted radio-galaxies. To test these hypotheses we built three samples
of candidate sources plus a control sample, and calculated their black hole
mass and Eddington ratio using their optical spectra. We then performed a
Kolmogorov-Smirnov statistical test to investigate the compatibility of our
different samples with a beamed population. Our results indicate that, when the
inclination angle increases, a beamed source appears as a steep-spectrum
radio-loud NLS1, or possibly even as a disk-hosted radio-galaxy with low black
hole mass and high Eddington ratio. Further investigations, involving larger
complete samples and observations at radio frequency, are needed to understand
the incidence of disk-hosted radio-galaxies in the parent population, and to
assess whether radio-quiet NLS1s can play a role, as well.
