Report #3, Part 2: Productivity (2012) from Dell's multi-year technology and productivity study of 8360 workers across 11 countries.
Monitor Size and Aspect Ratio, (2008) University of Utah study commissioned by NEC. Multiple monitors are recommended when employees must handle multiple documents at the same time, or multiple document types at the same time. If the work consists predominantly of spreadsheet work, one very large monitor boosts employee performance.
The Virtues of a Second Screen, (2006) New York Times article on multiple monitors.
The 30-inch Apple Cinema HD Display Productivity Benchmark , (2005) Pfeiffer Consulting study sponsored by Apple, Inc.
Two Screens Are Better Than One, (2003) Visualization and Interaction for Business and Entertainment (VIBE) study sponsored by Microsoft Research. Multiple monitors boost productivity between 9% and 50%.