Visual Disciminability of Headings in Text
http://www.ieeexplore.ieee.org/iel1/47/3880/00144865.pdf
  peer-reviewed

Williams, Thomas R. and Jan H. Spyridakis
IEEE Transactions on Professional Communication
1992

Abstract:

Headings in text provide critical symbols that help a reader discern a writer's structural treatment of a topic.