  Various methods (or recipes) have been proposed to predict future solar
activity levels - with mixed success. Among these, some precursor methods based
upon quantities determined around or a few years before solar minimum have
provided rather high correlations with the strength of the following cycles.
Recently, data assimilation with an advection-dominated (flux-transport) dynamo
model has been proposed as a predictive tool, yielding remarkably high
correlation coefficients. After discussing the potential implications of these
results and the criticism that has been raised, we study the possible physical
origin(s) of the predictive skill provided by precursor and other methods. It
is found that the combination of the overlap of solar cycles and their
amplitude-dependent rise time (Waldmeier's rule) introduces correlations in the
sunspot number (or area) record, which account for the predictive skill of many
precursor methods. This explanation requires no direct physical relation
between the precursor quantity and the dynamo mechanism (in the sense of the
Babcock-Leighton scheme or otherwise).
