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Mean growth of wealth (or GDP) is generally dened as the growth rate of average wealth, but
it can alternatively be dened as the average growth rate of wealth. While this raises an important
conceptual issue, we argue that stochastic stability should be used as a guide to safely discriminate
between the two notions of mean growth. Our discussion is illustrated on the class of continuous time
AK-type models subject to geometric Brownian motions. First, stability concepts are introduced
and applied to stochastic linear homogenous differential equations. Second, a preliminary application
to the canonical AK model is provided. It is readily shown that in this case exponential balanced
paths are not robust to uncertainty. Third, and more importantly, we evaluate the quantitative
implications of the alternative denition of mean growth on the seminal global diversication model
due to Obstfeld (1994).
