The

relative stability and magnitude of genetic andenvironmental effects underlying major dimen-sions of adolescent personality across time were investigated. The Junior Eysenck Personality Questionnaire was administered to over 540 twin pairs at ages 12, 14 and 16 years. Their personality scores were analyzed using genetic simplex model-ing which explicitly took into account the longitudinal nature of the data. With the exception of the dimen-sion lie, multivariate model fitting results revealed that familial aggregation was entirely explained by additive genetic effects. Results from simplex model fitting suggest that large proportions of the additive genetic variance observed at ages 14 and 16 years could be explained by genetic effects present at the age of 12 years. There was also evidence for smaller but significant genetic innovations at 14 and 16 years of age for male and female neuroticism, at 14 years for male extraversion, at 14 and 16 years for female psychoticism, and at 14 years for male psychoticism. Eysenck has argued extensively for a trait theory of personality in which personality dimensions are best explained as stable constructs across the life span (Eysenck & Eysenck, 1980; Eysenck & Eysenck, 1985). An important question is the degree to which the observed phenotypic stability in personality is determined by genetic and environmental effects. McCrae and Costa (1990) found that beyond the age of 30, the means of cross-sectional and longitudi-nal measures of personality show little or no change, with retest correlations ranging from.55 to.85. Their conclusion is supported by Conley (1984) in their extensive review of longitudinal studies of intelligence, personality and social attitudes. They demonstrated that personality can be regarded as a relatively stable construct across the adult lifespan. Watson and Clark (1984) in a review of 55 studies examining measures of negative affect, similar to Eysenck’s Neuroticism Scale, found that retest correlations were typically highest (>.80) in the first five months but then declined and lev-elled off (.50–.60) over the next 5 to 6 years. More recently, Kirk and colleagues (2000) analyzed data from the Australian Twin Registry to examine the retest correlations between the EPQ (Eysenck &amp