menter wishes so that the experimenter can accurately determine that genotype's score. The same sources of variation still contribute to each subject's score but these variations are assumed to vary equally in a positive and negative direction and therefore not bias the final determination of the true mean associated with the particular genotype of interest. This issue is discussed in more detail in Blizard and Bailey ( 1979).
Genetic Methods Permit Examination of the Prenatal Environment. Our review of research on the Maudsley strains has also shown that prenatal influences that are under genetic control contribute to phenotypic variation in open-field activity. This demonstration shows that prenatal influences can contribute to behavior within the normal range and opens up this period of development for empirical study. The special experimental designs that were described earlier that permit examination of pre-and postnatal influences illustrate the power of genetic manipulations to study the sources of individual variation. Only additional experimentation will reveal whether prenatal influences contribute to other phenotypic differences between the Maudsley strains or individual differences in behavior in man.
Preparation of this chapter was supported in part by NIMH grant by MH-39210 to the author.
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