Background: There is currently little evidence concerning the impact of the inter-twin relationship on behavioral outcomes and yet the twin relationship is frequently hypothesized to be a unique source of psychopathology in twins. The current study asked whether the inter-twin relationship is a predictor of behavioral difficulties and whether there are zygosity differences in this relationship.

Method: An Australian sample of same sex twins (N = 356, 164 MZ and 192 DZ) was studied in the year prior to school (Time 1) and again in first year of school (Time 2). Associations between twin-relationship and behavior problems were examined via path-analysis, and Satorra-Bentler chi-square difference tests were used to compare twins across zygosity.

Results: Results show that both conflict and lower levels of warmth at Time 1 were predictive of hyperactivity and conduct disorder at Time 2, but they were not associated with emotional difficulties or peer problems. While DZ twins shared less warmth than MZ twins, there were no differences in behavior problems.

Conclusion: Conflict and lower levels of warmth in the inter-twin relationship are associated with hyperactivity and conduct disorder and may serve as important considerations when making decisions regarding class separation at entry to school.