  The positron fraction observed by PAMELA and other experiments up to ~ 100
GeV is analyzed in terms of models of cosmic-ray propagation. It is shown that
generically we expect the positron fraction to reach ~ 0.6 at energies of
several TeV, and its energy dependence bears an intimate but subtle connection
with that of the boron to carbon ratio in cosmic rays. The observed positron
fraction can be fitted in a model that assumes a significant fraction of the
boron below ~ 10 GeV is generated through spallation of cosmic-ray nuclei in a
cocoonlike region surrounding the sources, and the positrons of energy higher
than a few GeV are almost exclusively generated through cosmic-ray interactions
in the general interstellar medium. Such a model is consistent with the bounds
on cosmic-ray anisotropies and other observations.
