I'm not sure about 'non-physical labor' but an IMF report [1] found that in many industrialized countries, such as the US, Germany, Japan, etc., depressed wages correlate strongly with low labor union participation rates. Surprisingly, other factors such as globalization and automation where found to be of little significance.

[1] https://www.imf.org/external/pubs/ft/fandd/2015/03/pdf/jaumo...