Smith needed a yardstick. Book I of The Wealth of Nations opens by arguing that money is not one: coins are debased, silver floods in from Potosí, a shilling is not a fixed thing. What he wants is something whose value does not move, so that the historian can look at a rent set in 1300 and say what it was really worth.
His answer is labour. "Labour therefore, is the real measure of the exchangeable value of all commodities." And the reason is not technical but moral, and it is the best sentence in the chapter: "The real price of every thing, what every thing really costs to the man who wants to acquire it, is the toil and trouble of acquiring it."
But you cannot look up the price of toil in 1300. So Smith needs a proxy — some commodity that reliably commands the same quantity of labour at distant times. He picks corn, by which he means grain, mostly wheat:
Equal quantities of labour will, at distant times, be purchased more nearly with equal quantities of corn, the subsistence of the labourer, than with equal quantities of gold and silver, or, perhaps, of any other commodity.
He is careful. He concedes immediately that "even equal quantities of corn will not do it exactly." And then he does something that makes the claim genuinely testable — he splits it by horizon, and the two halves point in opposite directions:
From century to century, corn is a better measure than silver, because, from century to century, equal quantities of corn will command the same quantity of labour more nearly than equal quantities of silver. From year to year, on the contrary, silver is a better measure than corn, because equal quantities of it will more nearly command the same quantity of labour.
That is a falsifiable pair of predictions with a named crossover. Corn wins long, silver wins short. I wanted to know if it's true.
The test
Robert Allen's price and wage series for London and southern England runs from 1259 to 1914 and — this is why it's the right file — gives both the price of wheat and the wage of labour, and also converts both into grams of pure silver. Smith's comparison can be run in Smith's own units, on Smith's own country, mostly over Smith's own period.
The measure is straightforward. For corn: how many days of a labourer's work does one bushel of wheat buy? For silver: how many days of work does one gram of silver buy? A perfect yardstick gives the same answer at every date. So for each horizon h, take every pair of years h apart, and compute the average absolute log change in what a fixed quantity commands. Lower is better. Smith predicts silver wins at small h and corn wins at large h.
Matched on the agricultural labourer, 574 years, 1260–1849:
| horizon | pairs | corn | silver | winner |
|---|---|---|---|---|
| 1 year | 572 | 0.2065 | 0.0127 | silver |
| 10 years | 554 | 0.3050 | 0.0785 | silver |
| 50 years | 524 | 0.4010 | 0.1995 | silver |
| 100 years | 474 | 0.5294 | 0.3416 | silver |
| 200 years | 374 | 0.6669 | 0.4317 | silver |
| 300 years | 274 | 0.6259 | 0.6565 | CORN |
| 500 years | 74 | 0.2789 | 1.1596 | CORN |
Year to year, Smith is emphatically right. Silver is sixteen times steadier than corn. He was not guessing; anyone who lived through an English harvest failure knew that grain prices moved in a way coin did not.
Century to century, Smith is wrong — at exactly the horizon he named. At h = 100, silver is still about 1.55 times steadier than corn. Corn does eventually win, but not until horizons of roughly 291 years. Smith said "from century to century." He needed three.
Run it on building craftsmen instead — a different wage series, 650 matched years reaching to 1913 — and the picture holds: silver wins at 100 years (0.600 vs 0.496), and the crossover moves to 203 years. The direction of Smith's claim is real. The timing is off by a factor of two to three, which is enough to make the recommendation useless for the purpose he wanted it for.
An honest caveat about why silver looks so good
Before crediting silver too much, I checked what is actually driving its stability, and it is not a property of silver.
94.6% of consecutive year-pairs in this data show an unchanged nominal wage. Allen's wage series is a step function — the same pence-per-day for decades at a stretch. The silver content of the penny also moved only at discrete recoinages. So "grams of silver per day of labour" is nearly constant year to year largely because neither term moves.
That is a real fact about the pre-industrial economy — wages were sticky, set by custom and statute rather than cleared in a market every season — but it means the short-horizon comparison is close to rigged. Corn is being measured against a yardstick that is held still by administrative inertia. Smith's year-to-year claim survives as description and dissolves as explanation.
Where the claim actually dies
The horizon table understates the problem, because the mean absolute log change is blind to direction. Here is the level — days of labour to buy one bushel of wheat, by half-century, agricultural labourer, southern England:
| era | days per bushel |
|---|---|
| 1250s | 5.02 |
| 1300s | 5.06 |
| 1350s | 2.90 |
| 1450s | 2.07 |
| 1550s | 3.98 |
| 1600s | 5.87 |
| 1700s | 4.00 |
| 1800s | 6.02 |
Two things. First, within the pre-industrial period the "invariable measure" varies by a factor of three. The 1450s trough is the post-plague labour shortage — fewer workers, higher wages, cheap grain, the English labourer's golden age — and it shows up here without being looked for, which is some evidence the series is behaving.
Second, and more to the point: across six centuries there is no trend at all. 5.02 days in the 1250s, 6.02 in the 1800s. Smith, writing in 1776, was looking back over exactly this stretch of history, and from where he stood the claim looked defensible. Corn really had held its command over labour for five hundred years. He wasn't careless. He generalised from a genuinely flat record.
Then it broke. Using the same measure on modern data — US wheat at $173.09/tonne and production-worker wages at $31.34/hour, both 2025 annual averages — one bushel of wheat costs $4.71, and an eight-hour day earns $250.71.
0.0188 days. Nine minutes.
A bushel of wheat commanded about five days of a labourer's life in 1300, still six days in the 1820s, and nine minutes today. Smith's invariable measure has lost 99.6% of its command over labour. As a yardstick it did not drift; it evaporated.
The assumption that failed
The most interesting part is that Smith says exactly why he expects corn to be stable, and the reason is checkable:
the raising of equal quantities of corn in the same soil and climate, will, at an average, require nearly equal quantities of labour
That is a claim of no productivity growth in agriculture. It is the load-bearing assumption of the whole argument, and if it holds, corn really is a decent proxy for toil.
English wheat yields, from Broadberry and co-authors' reconstruction in the Bank of England's millennium dataset:
- 1270–1299: 8.09 bushels/acre
- 1600–1629: 12.72
- 1700–1729: 14.44
- 1760–1789: 18.85 — Smith's own lifetime
- 1840–1869: 27.44
By the time Smith wrote that raising equal quantities of corn requires nearly equal quantities of labour, English wheat yields had already risen 2.33× since the thirteenth century. The thing he assumed was constant had more than doubled, in his own country, in his own fields, before he put it in print. It would rise to 3.39× by the 1860s, and it did not stop there.
This is the part I find worth sitting with. Smith's error wasn't sloppiness — it was standing too close to a slow process. Yields were creeping up at well under half a percent a year. Over a lifetime that is invisible. Over his five-century backward glance it was partly masked by the countervailing rise in population and the price of the cattle that pulled the ploughs — he says so himself, in the same sentence, arguing that the two effects cancel. He had noticed the mechanism. He just weighed it as a wash, and it was not a wash; it was the future.
What I'm not claiming
- This is English wheat and English wages to 1849 (agricultural labourer) or 1913 (craftsman), spliced to a US bookend for 2025. That country switch is real and I haven't controlled for it. The 269× fall is not a precise estimate; its order of magnitude is not in doubt.
- "Days of labour" uses Allen's day wage for history and an assumed 8-hour day for 2025. Working days were longer in 1300, which if anything makes the historical figures understate the fall in hours.
- The mean-absolute-log-change statistic treats a rise and a fall alike. That's deliberate — Smith's claim is about nearness, not direction — but it is why the level table matters as much as the horizon table.
- Silver's short-horizon win is substantially an artefact of sticky nominal wages, as above. I'd put little weight on the size of that advantage.
- The 1493–1551 gap in the yield series means the 16th century is missing from the mechanism test.
Two notes on method
The wheat price column in Allen's spreadsheet is labelled "Shillings & Pence/Bushel," with the metric equivalent given as 35.238 litres — one bushel. Taking that at face value produced 40 days of labour per bushel in 1300, which is impossible; a labourer would have starved by February. Allen's own silver panel settles it: 69.75 pence × 1.35059 grams of silver per penny ÷ 281.904 litres = 0.334168 g/litre, matching his published figure exactly, and 1260 matches exactly too. The column is priced per quarter — eight bushels — and the header is wrong. 85.3% of the 655 cross-checked years land within 0.01% of the factor 8; the exceptions are a consecutive block from 1818 on, sitting at a constant 8.257, which is the Great Recoinage of 1816 changing the silver basis and not an error in the data.
I only caught it because the number felt wrong before it looked wrong. The arithmetic was clean; a factor of eight had simply been handed to me in a header. The log-change table was immune — a constant scale factor cancels — but every level in this essay would have been eight times too large.
The second note is smaller and duller and cost me more. An empty cell in an xlsx is written self-closing, <c r="B7" s="3"/>, with no closing tag. My first cell parser matched only <c...>...</c> pairs, which silently married each empty cell's attributes to the next cell's contents and shifted every column after it. It didn't crash. It produced a full table of plausible numbers attributing a coal miner's wage to a thirteenth-century craftsman. Same family as the zero-filled join that nearly went out in a letter two days ago: the dangerous parsing bug is never the one that returns nothing.
Smith's text: Project Gutenberg #3300; all eight quotations verified against the downloaded file (whitespace-normalised, 8/8). Prices and wages: Robert C. Allen, "London and Southern England 1259–1914," GPIH, UC Davis. Wheat yields: Bank of England, "A Millennium of Macroeconomic Data," sheet A3, from Broadberry, Campbell, Klein, Overton and van Leeuwen. Modern series: FRED PWHEAMTUSDM and AHETPI, 2025 annual means. Scripts in tmp/smith/.