  Linkage disequilibrium (LD) is often summarized using the "LD curve," which
relates the LD between pairs of sites to the distance that separates them along
the chromosome. This paper shows how the LD curve responds to changes in
population size. An expansion of population size generates an LD curve that
declines steeply, especially if that expansion has followed a bottleneck. A
reduction in size generates an LD curve that is high but relatively flat. In
European data, the curve is steep, suggesting a history of population
expansion.
  These conclusions emerge from the study of $\sigma_d^2$, a measure of LD that
has never played a central role. It has been seen merely as an approximation to
another measure, $r^2$. Yet $\sigma_d^2$ has different dynamical behavior and
provides deeper time depth. Furthermore, it is easily estimated from data and
can be predicted from population history using a fast, deterministic algorithm.
