  The triggering mechanism for the most luminous, quasar-like active galactic
nuclei (AGN) remains a source of debate, with some studies favouring triggering
via galaxy mergers, but others finding little evidence to support this
mechanism. Here, we present deep Isaac Newton Telescope/Wide Field Camera
imaging observations of a complete sample of 48 optically-selected type 2
quasars $-$ the QSOFEED sample (L$_{\rm [OIII]}>$10$^{8.5}$$L_{\odot}$; $z <
0.14$). Based on visual inspection by eight classifiers, we find clear evidence
that galaxy interactions are the dominant triggering mechanism for quasar
activity in the local universe, with 65$^{+6}_{-7}$ per cent of the type 2
quasar hosts showing morphological features consistent with galaxy mergers or
encounters, compared with only 22$^{+5}_{-4}$ per cent of a stellar-mass- and
redshift-matched comparison sample of non-AGN galaxies $-$ a 5$\sigma$
difference. The type 2 quasar hosts are a factor 3.0$^{+0.5}_{-0.8}$ more
likely to be morphologically disturbed than their matched non-AGN counterparts,
similar to our previous results for powerful 3CR radio AGN of comparable [OIII]
emission-line luminosity and redshift. In contrast to the idea that quasars are
triggered at the peaks of galaxy mergers as the two nuclei coalesce, and only
become visible post-coalescence, the majority of morphologically-disturbed type
2 quasar sources in our sample are observed in the pre-coalescence phase
(61$^{+8}_{-9}$ per cent). We argue that much of the apparent ambiguity that
surrounds observational results in this field is a result of differences in the
surface brightness depths of the observations, combined with the effects of
cosmological surface brightness dimming.
