Complex sentence: Richard Posner, "Overcoming Law" (Cambridge: Harvard University Press, 1995), 305-311 credit employment at will as A major factor behind the strength of the U.S. economy, and behind the success of Silicon Valley as an entrepreneur-friendly environment.
Split into simpler sentences: Richard Posner, "Overcoming Law" (Cambridge: Harvard University Press, 1995), 305-311 credit employment at will as a major factor underlying the strength of the U.S. economy. At-will employment has also been stressed as a significant reason for the success of Silicon Valley as an entrepreneur-friendly environment.