Journal of Personality and Social Psychology 1994, Vol. 66, No. 4,722-730 Copyright 1994 by the American Psychological Association Inc 0022-3514/94/S3.00 A Developmental Genetic Analysis of Adult Personality: Extraversion and Neuroticism From 18 to 59 Years of Age Richard J. Viken, Richard J. Rose, Jaakko Kaprio, and Markku Koskenvuo Developmental genetic analyses were conducted on Extraversion (E) and Neuroticism (N) scale scores from nearly 15,000 male and female Finnish twins, ages 18-53 at baseline, who were tested on 2 occasions, 6 years apart. Significant genetic effects on both traits were found, at all ages, in men and women, on each measurement occasion. For E, heritability was invariant across sex but de- creased from late adolescence to the late 20s, with a smaller additional decrease at about 50 years of age. Heritabiiity for N also decreased from late adolescence to late 20s and remained stable thereaf- ter. For all ages after the early 20s, heritability of N was significantly higher among women. Means for E and N were sex-dependent and, apparently, influenced by cohort and time of assessment, as well as by age. There was little evidence of new genetic contributions to individual differences after age 30; in contrast, significant new environmental effects emerged at every age. No dimensions of adult personality have been subjected to greater study than Extraversion and Neuroticism, the "Big Two" in current five-factor models of personality. That fact is apparent in the research literature on twin studies: > 50,000 ad- olescent and adult twins have completed self-report measures of Extraversion and Neuroticism, and substantial consistency emerges from their data (Loehlin, 1992). There is compelling evidence of a strong genetic effect on both Extraversion and Neuroticism, from samples of both twins reared together and twins reared apart (Eaves, Eysenck, & Martin, 1989; Pedersen, Plomin, McClearn, & Friberg, 1988; Rose, 1988; Rose, Kosken- vuo, Kaprio, Sarna, & Langinvainio, 1988; Tellegen, Lykken, Bouchard, Wilcox, Segal, & Rich, 1988). The genetic effects are consistent across gender and culture and are found both in self- reports and in ratings by others (Heath, Neale, Kessler, Eaves, & Kendler, 1992). Common environmental effects on Extraver- sion and Neuroticism are less evident in twin data, although effects of shared experience can be found in contrasts of geneti- cally identical twin pairs who differ in frequency of their social contact; cotwins in more contact are more alike (Rose & Ka- prio, 1988). But effects of social contact are modest and cannot explain consistent differences in behavioral resemblance be- tween monozygotic and dizygotic cotwins (Rose et al., 1988). Richard J. Viken and Richard J. Rose, Department of Psychology, Indiana University Bloomington; Jaakko Kaprio, Department of Public Health, University of Helsinki, Helsinki, Finland; Markku Koskenvuo, De- partment of Public Health, University of Turku, Turku, Finland- Data analyses and article preparation were supported by Grants AA- 08315 and AA-07611 from the National Institute on Alcohol Abuse and Alcoholism (NIAAA). Formation of the older Finnish Twin Cohort and data collection in 1975 and 1981 were supported by the Council for Tobacco Research. Richard J. Rose was supported by NIAAA Research Scientist Award AA-00145, and Jaakko Kaprio was supported by the Finnish Academy of Science. We thank Kauko Heikkilla and Eila Voi- pio for their skilled assistance. Correspondence concerning this article should be addressed to Rich- ard J. Viken, Department of Psychology, Indiana University, Blooming- ton, Indiana 47405. Thus, twin studies of Extraversion and Neuroticism, the largest twin studies in the behavior-genetic literature, affirmatively an- swer the basic question: Do genes contribute to individual vari- ation in personality? But more important questions remain, and this study addresses several: Does the genetic contribution to Extraversion and Neuroticism vary across ages of adulthood? Do genes contribute to consistency and change in Extraversion and Neuroticism during adult development? If so, what is the relative magnitude of genetic contributions to age-to-age con- sistency? Is the genetic contribution to age-to-age stability mod- ulated by age? By gender? Is the genetic stability different for Extraversion than for Neuroticism? Despite many twin studies of Extraversion and Neuroticism, few have investigated age-related changes in genetic and envi- ronmental effects. Eaves et al. (1989) reanalyzed Swedish twin data reported by Floderus-Myrhed, Pedersen, and Rasmuson (1980), to evaluate heterogeneity of genetic effects on Extraver- sion and Neuroticism for three age cohorts; genetic variance for both traits was highest in the youngest twin pairs. In a longitu- dinal follow-up (average interval 10 years) of twins who were late adolescents or young adults at baseline, McGue, Bacon, and Lykken (1993) assessed age modulation of genetic effects on the Multidimensional Personality Questionnaire (MPQ) and found a consistent tendency for heritabilities to be greater at baseline, when the twins were younger. Information on gender differences in genetic effects on per- sonality is also very limited, in part because large samples are necessary to establish such effects. In their reanalysis of the Floderus-Myrhed et al. (1980) data from the Swedish Twin Reg- istry, Eaves et al. (1989) found evidence for greater genetic con- tributions to Extraversion and Neuroticism for female subjects. Similarly, Martin and Jardine (1986) reported significantly higher neuroticism heritability for women, but no significant genetic gender modulation for Extraversion. Thus, there may be gender differences in genetic effects on personality, particularly for Neuroticism. Many studies have established the relative stability of individ- ual differences in Extraversion and Neuroticism in adulthood 722