The Jevons paradox
The evidence is real and the argument about it is still running: how strong it is, how far it travels, or whether it repeats. Trust the direction, and hold the numbers loosely.
In 1865 a young English economist named William Stanley Jevons published a warning that sounded backwards. Britain was celebrating a generation of engineering triumph. James Watt's improved steam engine used far less coal for the same work, and everyone assumed efficiency would stretch the nation's coal reserves for centuries. Jevons looked at the numbers and said the opposite was happening. Every time engines became more efficient, Britain burned more coal, not less. His explanation is now known as the Jevons paradox, or more cautiously as the rebound effect. Efficiency makes each use cheaper. Cheaper uses attract more users, and create new uses nobody bothered with before. Total consumption rises. Modern economists broadly accept that rebound is real. They argue, sometimes sharply, about how large it is and about whether total consumption genuinely ends up higher than it started, which is the strong version of the claim.
The measured sizes separate two different claims. Economists distinguish rebound, where some of the saving gets eaten by extra use, from backfire, where consumption ends up higher than before anything was improved. Backfire is what Jevons actually described. For household energy the reviews put direct rebound below thirty percent, with lighting studies finding people leave efficient lights on only about five to twelve percent longer, and more efficient cars producing perhaps ten to thirty percent more driving over the long run. So efficient lighting, electric cars and heat pumps all deliver real savings, just smaller savings than the brochure promised. Backfire at the level Jevons claimed for coal is rare in these settings and remains contested even at the level of a whole economy.
When something becomes cheaper or faster to produce, the saving usually gets spent rather than banked. Make a report cheap to produce and you do not get fewer reports, you get far more of them. This is worth holding onto while every organisation adopts tools that promise to save time, because the freed capacity tends to be consumed by expanded demand rather than returned to anyone. The useful move is to decide in advance what you will do with the time a tool saves, and write it down, because if you do not, the answer will be more of whatever the tool made cheap. On whether the tool saves the time in the first place, see the productivity paradox.
Which version applies comes down to one question. Does demand for the thing have a ceiling? Nobody wants their office lit like a stadium or their house at forty degrees, so illumination and heating are bounded needs, and efficiency there produces real savings with some of it eaten back. Coal in 1865 had no ceiling, because cheaper power opened uses that did not previously exist rather than more of the same use. That is the condition the strong version needs. It is also why the question is live for anything general purpose: when a tool makes an entirely new category of work possible rather than making existing work cheaper, there is no natural point at which demand stops.
Solow says a tool accelerates whatever process it lands on, so where the work was not redesigned around it the company has bought faster versions of its existing habits and the productivity numbers do not move. Jevons says that where a tool does save time, the saving gets spent rather than banked, because making something cheap creates more demand for it. They look like the same complaint about technology and they are two questions in sequence. First, did the tool save anything, which is Solow. Second, where did the saving go, which is Jevons. A company can fail either one. The one that passes Solow by redesigning the work then meets Jevons, and the only defence against the second is to decide in advance what the freed time is for.
Source: Jevons, The Coal Question, 1865. The modern literature on rebound effects agrees the effect exists and disputes its magnitude.
Vaclav Smil, 2017
On how energy has actually been used across human history, by someone unusually resistant to comfortable narratives.
Draw your own card. It does not take long, and it rewards taking your time.