Income is just money supply

The average annual salary of an American worker in the year 1920 was $1,236. The '20s are generally regarded as a prosperous time for the U.S. economy.

Read more: The Average Salary in 1920 | eHow.com http://www.ehow.com/facts_7426286_average-salary-1920.html#ixzz17w0Ip3i5

 The daily salary in the 1920s was 3 dollars and 40 cents. This is because there just had not been as much money printed during that period. In 1880, the daily wage was 2 dollars, so a lot of money was printed in 1920 comparatively to 1880. Today, numbers like that are considered to be laughable. 

 The money supply has grown a lot these last 100 years, so 3.40 would never be considered acceptable today. Incomes generally track the amount of money we print or create through loans. So, the faster we print or loan new money, the faster incomes are expected to rise.