The figure of $400 annually (as expressed in 1990 international dollars) is commonly is used as a measure of “bare bones subsistence” and was previously believed to be the average income in England in the middle ages.
However the University of Warwick led researchers found that English per capita incomes in the late Middle Ages were actually of the order of $1,000 (again as expressed in 1990 dollars). Even on the eve of the Black Death, which first struck in 1348/49, the researchers found per capita incomes in England of more than $800 using the same 1990 dollar measure. Their estimates for other European countries also suggest late medieval living standards well above $400.
This new figure of $1,000 is not only significantly higher than previous estimates for that period in England — it also indicates that on average medieval England was better off than some of the world’s poorest nations today including the following (again average annual income as expressed in 1990 dollars).
Here’s a chart of the wages of unskilled English workers:
The increase after 1300 is usually attributed to the population collapse induced by the Black Plague. England’s population remained stagnant until ~1500. At that point higher productivity started to get eaten up by population growth. An “iron law” of human history. Table 24 on page 61 of the working paper has estimates for various nations. I plotted them on a chart for 1300 to 1700:
I don’t have time to reading the working paper right now, but I think these results do suggest some limitations of GDP calculations. Even the poor in most nations have watched television, or known someone with access to a mobile phone. Affluence isn’t just a number. On the other hand, from what I have read the English peasant of 1450 was rather healthy and hearty because of the large surplus of land over labor. So in that way I do think the GDP measure is telling us something real. But then the translation of currency into food is more straightforward than currency into computational processing power. The latter category just didn’t exist in 1300.