Comparative studies of magnetoelectric susceptibility (α), magnetization (M), and magnetostriction (u) in TbMn_2O_5 reveal that the increment of M owing to the field-induced Tb^3+ spin alignment coins a field-asymmetric line shape in the α(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

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... icant role of Tb magnetoelastic effects on the low field magnetoelectric phenomena of TbMn_2O_5. An effective free energy capturing the observed experimental features is also suggested.