  The local enhancement of antiferromagnetic correlations near vacancies
observed in a variety of spin systems is analyzed in a single framework.
Variational calculations suggest that the resonating-valence-bond character of
the spin correlations at short distances is responsible for the enhancement.
Numerical results for uniform spin chains, with and without frustration,
dimerized chains, ladders, and two dimensional clusters are in agreement with
our conjecture. This short distance phenomenon occurs independently of the long
distance behavior of the spin correlations in the undoped system. Experimental
predictions for a variety of compounds are briefly discussed.
