I nfor mati on on per sonal i ty, on anxi ety and depr essi on and on sever al aspects of r el i gi on w as col l ected i n 1974 Dutch fami l i es consi sti ng of adol escent and young adul t tw i ns and thei r par ents. A nal yses of these data show ed that di ffer ences betw een i ndi vi dual s i n r el i gi ous upbr i ngi ng, i n religious affiliation and in participation in church activities are not influenced by genetic factors. The familial resembl ance for di ffer ent aspects of r el i gi on i s hi gh, but can be expl ai ned enti r el y by environmental influences common to family members. Shared genes do not contribute to familial r esembl ances i n r el i gi on. The absence of geneti c i nfl uences on var i ati on i n sever al di mensi ons of religion is in contrast to findings of genetic influences on a large number of other traits that were studi ed i n these tw i n fami l i es. Di ffer ences i n r el i gi ous back gr ound ar e associ ated w i th di ffer ences in personality, especially in Sensation Seeking. Subjects with a religious upbringing, who are currently religious and who engage in church activities score lower on the scales of the Sensation Seek i ng Questi onnai r e. The most pr onounced effect i s on the Di si nhi bi ti on scal e. The r esembl ances betw een tw i ns for the Di si nhi bi ti on scal e di ffer accor di ng to thei r r el i gi ous upbr i ngi ng. Recei vi ng a r el i gi ous upbr i ngi ng seems to r educe the i nfl uence of geneti c factor s on Di si nhi bi ti on, especially in males.Keyw or ds: genotype ϫ envi ronment (G ϫ E) interaction, twins, religion, personality, sensation seeki ng I ntr oducti on A frequentl y observed fi ndi ng from behavi oural geneti cs research i s that resembl ances betw een fami l y members for a w i de range of human characteri sti cs are mai nl y due to shared genes and not to shared envi ronment. We have studi ed vari abl es rel ated to personal i ty, psychopathol ogy, l i fe styl e, cardi ovascul ar di sease, brai n functi on, cogni ti on and i ntel l i gence i n tw i ns of di fferent ages. These tw i ns, and someti mes thei r fami l y members such as parents and si bl i ngs, parti ci pate i n research projects that requi re them to come to the l aboratory for el ectrophysi ol ogi cal assessment of brai n functi on, for exampl e, or they are vi si ted at home for assessment of IQ or for 24-hour ambul ant regi strati on of heart rate and bl ood pressure. Other tw i n families parti ci pate i n l arge scal e surveys that are conducted by mailed questionnaires.In Fi gures 1 and 2 a summary i s presented, separatel y for mal es and femal es, of the mai n resul ts from these studi es i n Dutch tw i ns, w ho are regi stered w i th the Netherl ands Tw i n Regi stry. 30 The dark bars represent heritabilities, the shaded bars give esti mates for the i nfl uence of shared envi ronment and the l i ght bars show the i nfl uence of uni que envi ronmental effects (i ncl udi ng any effect of age). In these fi gures, the vari abl es have been grouped i nto four ...