Integrating experimental and observational personality research--the contributions of Hans Eysenck

A fundamental aspect of Hans Eysenck's research was his emphasis upon using all the tools available to the researcher to study personality. This included correlational, experimental, physiological, and genetic approaches. Fifty years after Cronbach's call for the reunification of the two disciplines of psychology (Cronbach, 1957) and 40 years after Eysenck's plea for experimental approaches to personality research (H. J. Eysenck, 1966), what is the status of the unification? Should personality researchers use experimental techniques? Do experimental techniques allow us to tease out causality, and are we communicating the advantages of combining experimental with multivariate correlational techniques? We review the progress made since Cronbach's and Eysenck's original papers and suggest that although it is still uncommon to find experimental studies of personality, psychology would benefit from the joint use of correlational and experimental approaches.