  Given the anisotropic emission from quasar accretion discs, their viewing
angle affects estimates of the quasar luminosity, black-hole mass and Eddington
ratio. Discs appear overluminous when viewed pole-on and underluminous when
viewed at high inclination. In radio-quiet quasars, the viewing angle is
usually unknown, although spectroscopic indicators have been proposed. Here, we
use a recently discovered universality in the variability structure function
(SF) of quasar light curves (LCs), where all quasars show the same SF when
clocks run in units of orbital timescale. As an offset from the mean relation
can be caused by incorrect orbital timescales and thus incorrect luminosities,
we correlate these offsets with suggested inclination indicators. We derive SFs
from NASA/ATLAS LCs spanning $\sim 6$ years of observation, using a sample of
183 luminous quasars with measured H$\beta$ lines as well as 753 quasars with
CIV and MgII lines. Starting from the proposed orientation indicators, we
expect quasars with narrower H$\beta$ lines and with more blueshifted CIV lines
to be viewed more pole-on and thus appear overluminous. In contrast, our SF
analysis finds that presumed pole-on discs appear underluminous, consistently
for both line indicators. We discuss possible explanations for the behaviour of
quasars with highly blueshifted CIV lines irrespective of inclination angle,
including dusty outflows that might render the accretion disc underluminous and
flatter disc temperature profiles with longer orbital timescales than in
thin-disc models but reach no satisfying conclusion.
