• Depression;
  • environmental influences;
  • children;
  • genetic influences

This research note considers whether normal and abnormal depressive symptoms are caused by the same or differing etiologies. The comparison of the heritability of individual differences with extreme group heritability, along with the use of multiple cut-offs to define abnormal groups, can be used to answer this question and to bridge the gap between dimensional and categorical approaches to developmental psychopathology. Depressive symptoms from 395 same-sex child twin pairs were analysed in this way. Genetic factors contributed to a similar extent both to individual differences (h2= .48) and to extreme group membership using multiple cut-offs (hg2= 2.0–2.3). The common environment did not contribute significantly in any of the analyses but showed a trend for a greater role in extreme group membership. These results are in good agreement with previous data.