Decrease of stress level at a given strain with increase of strain rate is discussed in terms of three theories: the Johnston‐Gilman theory of yield and flow, diffusional aging mechanisms, and stress induced structural instability. None of these theories explain inverse rate effects. However, a modified concept of stress‐induced structural instability in conjunction with the Johnston‐Gilman theory provides a physical mechanism whereby a material is expected to display ordinary rate effects at high or low strain rates, but may exhibit inverse effects over some intermediate range of strain rates, depending upon the degree of structural instability.