Background. Social support may reduce the risk of psychiatric illness. Though perceived as an environmental measure, genetic factors may influence levels of social support. A relationship between social roles and personality with social support suggests possible sex effects on the sources of individual differences in social support. Method. We used the responses of MZ and DZ same and opposite sex twins to 16 questions regarding their social life. Six factors - friend support, relative support, friend problem, relative problem, confidants and social integration were used for structural equation modelling. Factor derived scales were analysed for genetic, shared and unique environmental influences. Quantitative and qualitative gender differences were analysed using the software package Mx. Results. Except for relative support and confidants, no qualitative sex differences were seen. Genetic and individual specific environmental influences accounted for the variance for friend support, friend problems, relative problems and social integration and no quantitative gender differences were seen. For relative support genetic factors were detected in females but not males, while for confidants, the shared environment was important in females but not males. Conclusions. Except for relative support in males, genetic factors influence variation in all dimensions of social support. Shared environmental factors influence relative support and relative problems in both sexes. Sex differences were detected for confidants and relative support.