It Takes Two (Non-Motown Version) 

 The New York Times recently published an  obituary for David Lykken , who was a pioneer of twin studies.  His “Minnesota Twin Studies” suggested the importance of genetic factors in life outcomes.  But his work with twins also spurred empirical research in many fields, not just genetics – and for good reason.   

 The idea of using twins for social science studies is very appealing:  some twins are genetically identical, and also grow up in the same family and environment.  So from a statistical perspective, comparing outcomes such as earnings between pairs of twins is like having a “perfect match."  This idea made the rounds in many fields, such as labor economics.  By using the argument that all unobserved characteristics (e.g. “genetic ability”) should be equal and can thus be differenced away, twin studies were used to estimate the returns to education – the effect of education on wages. 


 Alas there are potential problems with using twin data.  For example, measurement error in a difference estimation can lead to severe attenuation bias precisely because twins are so similar.  If there is little variation in educational attainment, even small measurement errors can strongly affect the estimate.  Researchers have been ingenious about this (e.g. by instrumenting one persons’ education with the level that her twin reported, as in  Ashenfelter and Krueger ).  While this may reduce the attenuation bias it can magnify the omitted variables bias which motivated the use of twins in the first place.  Because there are only small differences in schooling, small unobserved differences in ability can lead to a large bias.  The culprits can be details such as differences in birth weight ( Rosenzweig and Wolpin  have a great discussion of such factors).  In addition, twins who participate in such studies are a selected group: they are getting along well enough to participate, and many of them get recruited at “twin events.”  But not all twins party in  Twinsburg, Ohio . 

 Of course none of this is to belittle the contribution of Dr Lykken, who besides helping to start this flurry of work also was also a major contributor to happiness research.