The FortyHourWeekHistory in American history comes from Henry Ford in the early 20th century. Workers in the new factories, sweat shops, and other industry related fields of the late 19th / early 20th centuries worked up to as many as 60 hours a week under poor conditions and with little in the way of wages. As the workers gained strength in unions, they started demanding a reversal of this poor state of affairs. Eventually, the union members in their various fields of labor won out against their employers through a variety of tactics (organizing, striking, etc.) and managed to get the FortyHourWeek. This was put into action Federally in 1938 with the Fair Labor Standards Act, which created an implicit maximum 40 hour work week by requiring time and a half for any hours in excess of 40 or 8 hours in one day, established a minimum wage, and forbade labor by children under the age of 16. That's the story in a nutshell as far as I understand it; feel free to correct / add more detail.

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Countervailing forces: 

The high cost of benefits, especially health insurance premiums, provides an incentive to demand overtime because it is cheaper to pay 1.5 times for the extra hours than to pay a second worker's premiums.

Paradoxically, it encourages part time employment or hiring of temporary employees, because many companies do not pay benefits to part time employees, and most temporary agencies do not provide health insurance.

Also, it encourages the classification of employees as "exempt," that is exempt from timekeeping and without any overtime pay at all. Almost all companies treat their programmers as exempt.

In the last 15 years or so, the laws have changed so that "highly paid" hourly employees do not have to be paid time and a half, but only straight time for hours above 40.

-- RobertField

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The EightHourDay ("8 hours work, 8 hours play, 8 hours sleep, 8 bob a day") began to be promoted in New Zealand in 1840; it became a legislated maximum for non-overtime in November 1935. Parts of it were anticipated (viz. limitation of women's working hours) in 1873, but no-one really paid much attention.

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It is my understanding that Henry Ford, tired of the high turnover of his employees, decided to double their wages, and to reduce their hours to 8 per day (48 per week, until he decided to give them Saturday off too).  In addition to reducing turnover, he also attracted the best mechanics to his plant.  For what it's worth, according to this Wikipedia article (http://en.wikipedia.org/wiki/Henry_Ford), Ford even hated unions!

There is an economic reason for encouraging people to have high wages and lots of time off, although sometimes management is too dense to recognize that!  -- Alpheus