The relationship between twins has been described as one of the most unique and close of interpersonal bonds. Many twins maintain that being a twin is a significant influencing factor in their lives. A broad selection of psychological twin studies has been carried out with twins as methodological tools, but few empirical studies has focused at the co-twin relationship at its own right and how twins influence each other. This study seeks to examine the degree and quality of contact between twins and how it is associated with their mental health. Does a close co-twin relationship protect against poor mental health? The independent variables are six questions concerning contact, and the dependent variable is different measures of mental health (both symptoms of anxiety and depression (SCL-5 index) and personality dimensions (six personality factor scores)). The study is based on data from a national sample of adult twins (the Norwegian Institute of Public Health Twin study, N = 6662). Based on the Norwegian Birth Registry, all twins aged 18 to 31 was mailed a questionnaire. The statistical analysis used were factor analyses (PCA) and multiple regression analyses. The predictions, derived from psychological theory and evolutionary theory, were partly supported. There were significant effects, which indicates that there is an inverse correlation between co-twin contact and mental health – when contact increase, symptoms of poor mental health decrease. Perhaps contrary to evolutionary theory, few significant group differences between MZ and DZ were found.
The present study also seeks to explore whether the variables zygosity and gender moderate the effects. Are there differences between monoygotic and dizygotic twins, same-sex or opposite-sex twins, or male or female twins? The issue of causality is important to consider in relation to some of the findings, as well as practical significance versus statistical significance. The data was compared with excisting research and the theories. One practical implication of the study support for the for the notion that twins should be given opportunity to attend same class in school.