The National Merit Twin Studyl

This chapter will be concerned with some of the methodological problems and some early results of the National Merit Twin Study which is still in progress and when finished will be reported in detail elsewhere. Analyses completed so far have been primarily concerned with the relative effects of heredity and environment on ability measures, which will therefore be the major focus of this chapter. Of course, both heredity and environment are necessary for any behavior to occur. If heredity had not produced a behaving organism there would be no behavior, and if environment had not provided the appropriate situation for the development of the organism there would also be no behavior. Both are necessary and completely -interdependent influences, and it is not possible to say that one is more important than the other. If, in an experimental situation, either heredity or environment were completely controlled, variation in any behavior could be produced by variation of the other. However, ,this is not the question we are asking. We are interested in accounting for variance in human behavior as it is observed in the natural social setting, and it is a perfectly legitimate question to ask how much of this variance is due to variation in heredity and how muc4 is due to variation in the environment. The question ':4:1£ everyone had the same heredity, how much would the variance in behavior be reduced?" is a convenient simplification of the question we are asking. It is a simplification because there are undoubtedly many instances in human behavior of a phenomenon which has been demonstrated in animals that the effect of environ-