Review abstract:

Background:
There is evidence for and against functional somatic syndromes being either lumped together or separated apart. The aim of this presentation is to review the evidence regarding the heterogeneity of chronic fatigue syndrome (CFS) and to provide new data addressing this issue… [not freely available?]

Is chronic fatigue syndrome heterogeneous? A review of the literature and new study of the lumping versus splitting debate for functional somatic syndromes, by T.E. Williams, L. Pangiotopoulou, T. Chalder, M. Sharpe, P.D. White in J Psychsomatic Research, June 2016 Volume 85, Pages 88–89 [Available online 19 May 2016]