Nature or Nurture? A Note on the Misinterpreted Twin Decomposition

The classical twin model has often been used to determine whether variation in outcomes such IQ, schooling and other behavioral traits, originate from genetic endowments or environmental factors. Despite some heavy criticism from prominent scholars, the model has recently reappeared in highly ranked economics journals to perhaps spark off a revival of the method. This article seeks to specify the assumptions which generate the apparently profound divide in viewpoints. A general problem is that most authors do not properly discuss the underlying assumptions of the twin model. It has partly led to a disarray of thoughts, concrete examples are provided, since the interpretation of the results and the risk for misleading interpretations are not spelled out. Therefore, perhaps surprisingly, a brief account of the theories behind the main assumptions of the twin model provides a useful contribution.