Integrating experimental and observational personality research – the contributions of Hans Eysenck William Revelle and Katherine Oehlberg Northwestern University 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. 50 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 com- bining experimental with multivariate correlational techniques? We review the progress made since Cronbach 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. Introduction research could make to the seemingly unrelated re- search questions of experimental psychology1 (H. J. A central theme of Hans Eysenck’s research and Eysenck, 1966, 1983, 1997). In this article we eval- writings was the integration of the scientific study of uate the degree to which current work in personal- personality into the field of psychology as a whole, ity theory and research has aimed at and reached as well as the rest of the natural sciences (H. J. Eysenck’s lofty goals of the integration of these two Eysenck, 1966, 1997; H. J. Eysenck & Eysenck, fields. 1985). Genetic and physiological questions were as much a part of Eysenck’s theoretical framework Personality and as were basic findings in learning and motivation Experimental Psychology (H. J. Eysenck & Eysenck, 1985). He pioneered the use of the most recent developments in psycho- Ever since Wundt introduced experiments into logical measurement and psychometrics and the ap- psychology (Wundt, 1874/1999, 1904) and Gal- plication of these techniques to self-report and be- ton (1892) studied individual differences in ge- havioral observations. Unsatisfied with merely try- nius, there has been a persistent tension between ing to utilize classic experimental psychology as a the experimental and correlational methodological guide for personality theory, Eysenck also empha- and statistical approaches taken by experimental sized the contribution that personality theory and and personality psychology, respectively. Cronbach (1957, 1975), H. J. Eysenck (1966, 1997) and Vale and Vale (1969), however, highlighted the strengths and weaknesses of the alternative approaches and Prepared for a special issue of the Journal of Person- argued for the reunification of the two disciplines. ality on Eysenckian Themes, edited by Kate Walton and They believed that the field of psychology would Robert Krueger. We gratefully acknowledge the help of Josh Wilt, Katherina Hauner, three anonymous reviewers 1 The terms used by Cronbach (1957) and H. J. and the editors for the suggestions they made to improve Eysenck (1966) seem somewhat quaint in that now most this manuscript. Revised version submitted September psychologists refer to cognitive psychology or cognitive- 28, 2007. neuro psychology for what used to be the domain of “ex- contact: William Revelle: [email protected] perimental” psychology. 2 W. REVELLE & K. OEHLBERG be improved if experimentalists and correlational- the best of observational analysis. To Eysenck, fac- ists could share methods, theories, and findings. tor analysis and structural equation modeling were Eysenck’s most impressive statement of the need tools to describe structure, but not tools to explain to combine the two disciplines was his (posthu- structure or process. For explanation, experiments mous) 1997 paper contending that personality re- were required. searchers should adapt a paradigmatic approach (H. J. Eysenck, 1997) in order to make progress. The current state of Following Kuhn (1970), he used paradigm to refer integration of to a coherent theoretical and methodological model experimental methods within which a scientific field conducts its work. and personality research He suggested that personality psychology, insofar as it resisted the integration of experimental meth- In order to evaluate the current level of usage ods, remained pre-paradigmatic; that is, it lacked of experimental techniques in research on individ- an explicit framework that related constructs via ual differences, we analyzed all 2005 and 2006 vol- causal mechanisms, and moreover lacked the ability umes of the five major personality journals: Euro- to test hypothesized causal relationships. Most im- pean Journal of Personality (EJP), Journal of Per- sonality (JoP), the Journal of Personality and So- portantly, he suggested that a research agenda com- 2 bining experimental and correlational techniques to cial Psychology , Journal of Research in Personal- develop and test causal theories of personality was ity (JRP), and the Personality and Individual Differ- crucial for the field to develop a paradigm within ences (PaID), by performing computer searches for which progress can be made. the use of the words “random, experiment, experi- mental, condition, or assigned” (Table 1). As many readers will recognize, dichotomiz- Perhaps the most obvious finding from this clas- ing research approaches into the experimental and sification is the infrequency of experimental work correlational confounds research design with the published in the last two years in any of the jour- method of data analysis. The traditional statistical nals. 0% of the articles in the EJP, <6% of the tool for the experimentalist has been the comparison articles in JoP, ≈ 12% in the journal that Eysenck of means using the t-test or its generalization, the edited for 20 years (PaID) and 16% of the articles in analysis of variance (ANOVA). This is in contrast to JRP contained some experimental study; the journal the analysis of variability and covariance using the with the highest percentage of experimental studies correlation coefficient and multivariate procedures. of personality was the personality section of JPSP However, because ANOVA and the correlation co- with 28%. efficient are both special cases of the general linear In the journal that had the most experimental model, it is better to consider the distinction to be studies (PaID), the plurality were tests of hypothe- between experimental and observational methods ses derived from Reinforcement Sensitivity Theory rather than experimental and correlational analysis. (J. A. Gray & McNaughton, 2000; Corr, 2007). In Perhaps Eysenck’s greatest strength was his com- addition to studies where there were actual experi- mitment to developing personality psychology into mental manipulations there were a few studies using a mature scientific field of inquiry. By that, he tasks more typically seen in experimental psychol- meant one in which we have gone beyond obser- ogy (e.g., the wholistic-analytic or the “forest-trees” vations and hunches to the development and test- perceptual task developed by Navon (1977)). ing of causal models. He observed that scientific The unfortunate conclusion from this brief re- inquiry in general, and personality theory in partic- view of publication practices is that the use of ex- ular, ranges from inspired hunch to formal theory perimental techniques is uncommon in current re- and hoped that it was possible to develop formal search. This suggests that the desired unification of theory that was subject to rigorous test. In addi- tion to his concern with developing good measures 2 For JPSP we included only those articles that also of personality traits, he was an advocate of experi- had the word “personality” either as a keyword or in the mental and physiological techniques to tease apart abstract. In the analysis of JPSP we report both the total the intricacies of personality, for he recognized that of articles published as well as that subset having to do it was impossible to test causal theories from even with personality. EXPERIMENTAL AND OBSERVATIONAL 3 Table 1 ple size. This increases statistical power because Frequency of experimental research in personality the standard errors have been reduced to allow for published in 2005 or 2006. * For JPSP, we have “statistical significance” for the particular popula- included all articles and then just the ones with per- tion effect size of interest (see Harlow, Mulaik, and sonality in the abstract or as a keyword. Steiger (1997) for a critique of this approach of conventional null hypothesis testing). Given the Journal Total Experimental % Experimental size limitations of undergraduate subject pools, it is Personality Personality more typical to use within-subject designs that ef- EJP 68 0 0 fectively remove the between individual effects. If JoP 125 7 6 one is concerned with measuring reaction time (RT) JPSP 280 26 9 differences associated with semantic priming or per- JPSP∗ 92 26 28 ceptual interference in a global-local task, that par- JRP 102 16 16 ticipants differ in ability, age, arousal, and motiva- PaID 586 73 12 tion, all large sources of variance in reaction time, is Total* 1161 122 11 irrelevant. RT paradigms are particularly sensitive to the power of within-subject designs: the between conditions effects might be of the order of 10-20 ms and the within subject standard deviations are of