  Quasar pairs are either physically distinct binary quasars or the result of
gravitational lensing. The majority of known pairs are in fact lenses, with a
few confirmed as binaries, leaving a population of objects that have not yet
been successfully classified. Building on the arguments of Kochanek, Falco &
Munoz (1999), it is shown that there are no objective reasons to reject the
binary interpretation for most of these. In particular, the similarity of the
spectra of the quasar pairs appears to be an artifact of the generic nature of
quasar spectra. The two ambiguous pairs discovered as part of the Large Bright
Quasar Survey (Q 1429-053 and Q 2153-0256) are analysed using principle
components analysis, which shows that their spectral similarities are not
greater than expected for a randomly chosen pair of quasars from the survey.
The assumption of the binary hypothesis allows the dynamics, time-scales and
separation distribution of binary quasars to be investigated and constrained.
The most plausible model is that the quasars' activity is triggered by tidal
interactions in a galatic merger, but that the (re-)activation of the galactic
nuclei occurs quite late in the interaction, when the nuclei are within 80+/-30
kpc of each other. A simple dynamical friction model for the decaying orbits
reproduces the observed distribution of projected separations, but the decay
time inferred is comparable to a Hubble time. Hence it is predicted that binary
quasars are only observable as such in the early stages of galactic collisions,
after which the quiescent super-massive black holes orbit in the merger remnant
for some time.
