  The growth rate of the large-scale structure of the universe has been
advocated as the observable par excellence for testing gravity on cosmological
scales. By considering linear-order deviations from General Relativity, we show
that corrections to the growth rate, f, can be expressed as an integral over a
`source' term, weighted by a theory-independent `response kernel'. This leads
to an efficient and accurate `plug-and-play' expression for generating growth
rates in alternative gravity theories, bypassing lengthy theory-specific
computations. We use this approach to explicitly show that f is sensitive to a
degenerate combination of modified expansion and modified clustering effects.
Hence the growth rate, when used in isolation, is not a straightforward
diagnostic of modified gravity.
