Empirical models based on neoclassical theory predict that if investment is sensitive to current financial performance, this is a sign that something is 'wrong' and is to be regarded as a problem worthy of a policy intervention. Evolutionary theory, however, refers to the principle of 'growth of the fitter' to interpret investment-cash flow sensitivities as the workings of a healthy economy. In particular, I attack the neoclassical assumption of rational profit-maximizing firms. Such an assumption is not a helpful starting point for empirical studies into firm growth. One caricature of neoclassical theory could be "Assume firms are perfectly efficient. Why aren't they getting enough funding?", whereas evolutionary theory considers that firms are heterogeneous and that not all firms should grow. This essay highlights how interpretations and policy interventions can be framed by the initial modelling assumptions, even though these latter are often chosen with analytical tractability in mind rather than realism. © 2010 Elsevier B.V.