The salary of college graduates has been grown since 1963 whereas wages for people without high-school diplomas dropped. The recent rise in earnings for skilled workers is uncommon. There were 2 declines in wage differences in the past.

In the 14th century, because the interest rates were high and life expectancy was short, few people were willing to spend time on an apprenticeship in order to become a craftsman. However, when the Black Death struck England, the interest rates plummeted, causing the increased number of skilled workers. Therefore, the wage premium dropped.

Another decline happened due to the Industrial Revolution. Many craftsmen were replaced by machines, which were operated by workers with few months’ training. Hence, the wages of skilled workers declined and not recovered until 1960.

Nowadays, companies favored skilled workers formed by the universities but this phenomenon may not continue because of the artificial intelligence. According to the projections of America’s Bureau of Labor, there are 4 out of 5 occupations without the need for bachelor’s degree. In addition, companies have a tendency to replace those employers who have a wage premium.

Source:
http://www.economist.com/news/finance-and-economics/21723403-recent-rise-earnings-skilled-workers-rare-phenomenon-what-history