  The production of top-antitop pairs at the Fermilab Tevatron shows a
forward-backward asymmetry in which the top quark tends to follow the proton
direction, while the antitop tends to follow the antiproton direction. The
effect grows with increasing effective mass $m_{t \bar t}$ of the top-antitop
pair, and with increasing rapidity difference between the top and antitop. The
observed effect is about three times as large as predicted by
next-to-leading-order QCD, but with the same sign. An estimate of
non-factorizable effects based on a QCD string picture finds they are
negligible, but that small distortions of the $m_t$ spectrum are possible.
Tests for such effects, both at and above the level of this estimate, are
suggested.
