Nonshared environmental influences and personality differences in adult twins

The twin design was used to examine the importance of different experiences of siblings within the family and to identify relations between twins' personality differences and their differential experiences. A sample of 161 monozygotic and 74 dizygotic twin individuals between the ages of 18 and 75 years retrospectively reported on their different experiences when growing up. The Sibling Inventory of Differential Experience (SIDE) was used for the first time with a sample of twin siblings. In addition, the twins provided self-report measures of affect and personality. In contrast to results from a sibling adoption design, this study of twins showed greater evidence for genetic variance in the SIDE scales. Nevertheless, the SIDE showed significant associations with differences in personality and affect for monozygotic twins, which reflect pure environment-behavior relations.