The properties of a complete sample of flat-spectrum radio quasars are analyzed and compared to those of radio-selected BL Lacs, studied in a previous paper, to look for any relationship between the two classes. It is shown that microlensing of quasars by stars in foreground galaxies can probably be ruled out as an explanation for the BL Lac phenomenon, while an evolutionary connection between flat-spectrum radio quasars and BL Lacs does not seem to be supported by the present data. The intrinsic properties of the two classes, that is extended radio emission and line luminosities, are significantly different. BL Lacs are not quasars with emission lines swamped by an enhanced optical continuum, but their line luminosities are intrinsically weak. The most probable scenario is one where BL Lacs and flat-spectrum radio quasars represent separate instances of relativistic beaming in low- and high-luminosity radio galaxies respectively.
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P. Padovani (1992) studied this question.