Personality Characteristics of Research Volunteers JAN-ERIK LO ¨ NNQVIST 1,2 * , SAMPO PAUNONEN 3 , MARKKU VERKASALO 2 , SOINTU LEIKAS 2 , ANNAMARI TUULIO-HENRIKSSON 1 and JOUKO LO ¨ NNQVIST 1 1 National Public Health Institute, Department of Mental Health and Alcohol Research, Finland 2 Department of Psychology, University of Helsinki, Finland 3 University of Western Ontario, Canada Abstract We evaluated Big Five personality factor differences between research volunteers and nonvolunteers. In the first study, 158 military officers were asked to participate in a mail survey. The personality scores of the officers were available from an archival data set. In our second study, adult siblings from large families were invited to participate in extensive clinical epidemiological evaluations. The personality scores of volunteers (N ¼ 55) and nonvolunteers from the same families (N ¼ 29) were estimated from sibling ratings made by those who participated in the study. In both studies, respondents, compared to non- respondents, were found to be significantly lower in Neuroticism and higher in Con- scientiousness. The second study further indicated respondents as being higher in Extraversion and Agreeableness. Copyright # 2007 John Wiley & Sons, Ltd. Key words: nonrespondents; self-selection; participation; personality INTRODUCTION The purpose of this paper is to present new data bearing on the differences between people who volunteer as participants in psychological or clinical research and those who choose not to participate. The behavioural and medical sciences are clearly indebted to volunteer participants for much of their empirical data. A quick scan of published studies in those disciplines will confirm that human participants are almost always people who have wilfully agreed to assist in the research. Under ideal conditions, of course, all people asked to serve as participants would acquiesce, in order to promote the generalizability and external validity of the research results. But this is an unreasonable expectation and, especially with modern strictures on informed consent, a person might readily choose not to participate in any particular study. European Journal of Personality Eur. J. Pers. 21: 1017–1030 (2007) Published online 31 August 2007 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/per.655 *Correspondence to: Jan-Erik Lo ¨nnqvist, Department of Psychology, University of Helsinki, PO Box 9, Helsinki 00014, Finland. E-mail: jan-erik.lonnqvist@helsinki.fi Copyright # 2007 John Wiley & Sons, Ltd. Received 7 March 2007 Revised 1 July 2007 Accepted 1 July 2007