Background. Subjective well-being (SWB) can be partitioned into the components life satisfaction and affect. Research on factors influencing these components of well-being has mainly focused on environmental characteristics. The aim of this study was to investigate the relative contribution of genes and environment to individual differences in life satisfaction in a large sample of Dutch twins and their singleton siblings. Method. Life satisfaction of 5668 subjects registered with The Netherlands Twin Registry (NTR) was measured with a Dutch version of the self-reported Satisfaction with Life Scale. An extended twin design was used to obtain correlations in life satisfaction scores for monozygotic twins, dizygotic twins and sibling pairs and to estimate the contribution of genes and environment to the variation in life satisfaction. Results. No differences between males and females were found in the mean level of life satisfaction. Broad-sense heritability was 38%. Non-additive genetic factors explained all or most of the genetic influences. The remaining 62% of the variance in life satisfaction could be attributed to unique environmental factors, both persistent and transitory, plus measurement error. Conclusions. Individual differences in life satisfaction are determined in part by genetic factors that are largely or entirely non-additive in nature. © 2005 Cambridge University Press.