> Throughout human history, starting with the spread of agriculture, increased labor efficiency has always led to people consuming more, not to them working less.
Can you give a single example where that happened?
During the industrial revolution it was definitely not what happened. In the late 1700s laborers typically averaged around 80 hours per week. In the 1880s this had decreased to around 60 hours per week. In the 1920s the average was closer to 48 hours per week. By the time Keynes was writing, the 40 hour work week had become standard. Average workweek bottomed out in the mid 1980s in the US and UK at about 37 hours before starting to increase again.
That never was the case (except for short periods after salary increases).
And this is not a question where there could be any speculation: in those days there were already people collecting such statistics, and we have a bunch of diaries describing the actual state of affairs, both from the workers themselves and from those who organized their labor - and everything shows that few people worked more than 50 hours a week on average.
Most likely, the myth about 80 hours a week stems from the fact that such weeks really were common, but it was work in the format of "work a week or two or a month for 80 hours, then a week or two or a month you don't work, spend money, arrange your life"
There is also agriculture, which employed a significant part of the population in the past. There, on average, there was usually even less than 40 hours of productive work, it's just that timing is of great importance there, and there are bottlenecks, and when necessary, you have to work 20 hours a day, which is compensated by periods when the workload is much less than 6 hours a day.
It most certainly was the case. As you correctly point out, people were collecting such statistics at the time, we know how much they worked and they worked a lot. In London from 1750 to 1800 the average male laborer worked over 4000 hours per year, and the typical year had 307 workdays. We have records of employment that list who worked which days at particular businesses, and court cases where witnesses testified about their work schedules, and we know of complaints from people at this time about the excessive amount of time they worked.
Take the Philadelphia carpenters' strike in 1791, where they were on strike demanding a reduction in hours to a 60 hour work week. The strike was unsuccessful. In the 1820s there was a so called "10 Hour Day" labor movement in New York City (note that at this time people worked 6 days a week). In the 1840s mill workers in Massachusetts attempted to get the state legislature to intervene and reduce their 74 hour workweeks. This was also unsuccessful. Martin Van Buren signed an executive order limiting workdays for federal employees to 10 hours per day. The first enforceable labor law in the US came in 1874, which set a limit of 60 hours in a workweek for women in Massachusetts.
Can you give a single example where that happened?
During the industrial revolution it was definitely not what happened. In the late 1700s laborers typically averaged around 80 hours per week. In the 1880s this had decreased to around 60 hours per week. In the 1920s the average was closer to 48 hours per week. By the time Keynes was writing, the 40 hour work week had become standard. Average workweek bottomed out in the mid 1980s in the US and UK at about 37 hours before starting to increase again.