We call the measurement aspect of this phenomenon the Productivity J-Curve. As firms adopt a new GPT, total factor productivity growth will initially be underestimated because capital and labor are used to accumulate unmeasured intangible capital stocks. Later, measured productivity growth overestimates true productivity growth because the capital service flows from those hidden intangible stocks generates measurable output. The error in measured total factor productivity growth therefore follows a J-curve shape, initially dipping while the investment rate in unmeasured capital is larger than the investment rate in other types of capital, then rising as growing intangible stocks begin to contribute to measured production. In the long run, as intangible investments and capital stocks reach their steady-state growth rates, the return-adjusted value of the unmeasured intangible capital stock service flows (in expectation) approaches the value of the initial unmeasured investment. This means that some of the mismeasurement effects on productivity growth can persist even in the long run.