  The Born probability measure describes the statistics of measurements in
which observers self-locate themselves in some region of reality. In
$\psi$-ontic quantum theories, reality is directly represented by the
wavefunction. We show that quantum probabilities may be identified with
fractions of a universal multiple-time wavefunction containing both causal and
retrocausal temporal parts. This wavefunction is defined in an appropriately
generalized history space on the Keldysh time contour. Our deterministic
formulation of quantum mechanics replaces the initial condition of standard
Schr\"odinger dynamics with a network of `fixed points' defining quantum
histories on the contour. The Born measure is derived by summing up the
wavefunction along these histories. We then apply the same technique to the
derivation of the statistics of measurements with pre- and post-selection.
