Additive Growth

That is a new and quite interesting paper by Thomas Philippon.  Here is the abstract:

Growth theory is based on the assumption of exponential total factor productivity (TFP) growth. Across countries and time periods I find that TFP growth is actually linear. Unlike the exponential model, the additive growth model provides useful medium-term forecasts of TFP. It also explains the TFP slowdown and the volatility puzzle, and predicts falling real interest rates. For the distant future the model predicts ever increasing increments in standards of living but with growth rates that converge to zero. For the distant past the model suggests that the size of TFP increments has changed in the late 1600’s, the early 1800’s, and around 1930.

Or consider this presentation:

Initial trend growth is around 2.5%. After 40 years, TFP doubles, and since increments are constant, the trend growth rate is half of what it used to be. After 60 years later, it is only one percent.


…the process of US TFP increments has only one break over the past 130 years, around 1930, following the large-scale implementation of the electricity revolution…For the UK, I find two breaks between 1600 and 1914. The first is between 1650 and 1700, when growth becomes positive. The second is around 1830. These breaks are consistent with historical research on the first and second industrial revolutions…

The author argues that linear TFP growth holds for Thailand, Taiwan, and Korea as well, and indeed for all recent countries with data for TFP.

As Philippon puts it informally “New ideas add to our stock of knowledge; they do not multiply it.”

As stated above a very interesting paper, but I do have some worries.  First, his model fits “TFP” better than gdp growth per se, which (at least until recently) does appear to be exponential in advanced economies.  If I read the author’s pp.21-22 correctly, he is suggesting (speculatively) that 20th century gdp growth received an artificial inflation from improvements in educational achievement that perhaps are unlikely to be replicated.  Maybe, but the broader predictions of the theory — including on gdp growth — require further consideration.

Second, is TFP even “a real thing”?  Or is it a meaning-poor residual, based on arbitrary distinctions between “innovation” and “investment”?  Maybe the ongoing trend is simply that more innovation is embodied in concrete investments, thus causing TFP to measure lower?  Too much of the paper takes the TFP concept for granted.

Nonetheless worth a real ponder.


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