  Comparative studies of magnetoelectric susceptibility ($\alpha$),
magnetization ($M$), and magnetostriction ($u$) in TbMn$_{2}$O$_{5}$ reveal
that the increment of $M$ owing to the field-induced Tb$^{3+}$ spin alignment
coins a field-asymmetric line shape in the $\alpha(H)$ curve, being conspicuous
in a low temperature incommensurate phase but persistently subsisting in the
entire ferroelectric phase. Correlations among electric polarization, $u$, and
$M^{2}$ variation represent linear relationships, unambiguously showing the
significant role of Tb magnetoelastic effects on the low field magnetoelectric
phenomena of TbMn$_{2}$O$_{5}$. An effective free energy capturing the observed
experimental features is also suggested.
