I ntr i nsi c and extr i nsi c r el i gi ousness: geneti c and envi r onmental i nfl uences and per sonal i ty cor r el ates Thomas J Bouchard Jr, M att M cGue, Davi d Lykken and A uke Tel l egen University of Minnesota, Minneapolis, USA Thi s r epor t pr esents fi ndi ngs for the I ntr i nsi c (I R) and Extr i nsi c (ER) r el i gi ousness scal es fr om the M i nnesota Study of Tw i ns Rear ed A par t. The scal es w er e show n to be i nter nal l y consi stent, suffi ci entl y di sti nct fr om the scal es of the Cal i for ni a Psychol ogi cal I nventor y and the M ul tidi mensi onal Per sonal i ty Questi onnai r e and unr el ated to a number of measur es of r esponse styl e to justi fy tr eati ng them as di sti nct tr ai ts. The I scal es al so show ed consi der abl e evi dence of constr uct val i di ty i n i ts cor r el ati ons w i th r el i gi ous fundamental i sm and author i tar i ani sm as assessed by the M M PI and A l temeyer 's Ri ght-Wi ng A uthor i tar i ani sm scal e. Data on I R and ER fr om 35 pai r s of monozygoti c tw i ns r ear ed apar t (M ZA ) and 37 pai r s of di zygoti c tw i ns r ear ed apart (DZA) were fitted to a biometric model and demonstrated significant heritability (0.43 and 0.39), w i th a model contai ni ng geneti c pl us envi r onmental factor s fi tti ng si gni fi cantl y better than a model contai ni ng onl y an envi r onmental component. Tw i n si mi l ar i ty coul d not be expl ai ned by pl acement on a sel f-r epor ted measur e of fami l y M or al Rel i gi ous Emphasi s as measur ed by the Family Environment Scale. Keyw or ds: tw i ns, rel i gi ousness, personal i ty I ntr oducti on A ccordi ng to EO Wi l son, 'The predi sposi ti on to rel i gi ous bel i ef i s the most compl ex and pow erful force in the human mind and in all possibility an i nerradi cal part of human nature.' 1 Thi s evol uti onary theme has now been expl ored by a number of schol ars, 2,3 al though i t appears to have been l argel y i gnored by evol uti onary psychol ogi sts. 4,5 Wilson's cl ai m i s, of course, a rei terati on of si mi l ar asserti ons made over the millennia by thoughtful peopl e. Neverthel ess, i n spi te of the recogni ti on of i ts pervasi veness, compl exi ty and i mportance, rel i gi ousness as a psychol ogi cal trai t has been negl ected by modern psychol ogi sts rel ati ve to most other psychol ogi cal constructs. It has been rel egated, perhaps not to the fri nges, but to the si del i nes. Thi s i s surpri si ng gi ven the i mportance attached to rel i gi ousness by both William James, 6 a foundi ng father of modern psychol ogy and the author of one of the di sci pl i ne's most famous textbooks, 7 and Gordon Allport, 8 the father of modern personal i ty theory. The most recent personal i ty textbooks to cross our desks 9,10 do not contai n the terms rel i gi on or rel i gi ousness i n thei r i ndexes. Thomas and Carver 11 report si mi l ar fi ndi ngs based on thei r exami nati on of 60 textbooks devoted to chi l d and adol escent development. The fai l ure to ci te rel i gi ousnes...