The Twin Method

The classic twin method compares the similarity of monozygotic (MZ) twin pairs to dizygotic (DZ) twin pairs. The statistical index of similarity is the correlation coefficient. Consequently, when the correlation for MZ pairs (Rmz) is significantly greater than the correlation for DZ pairs (Rdz), then there is evidence that genetic individual differences contribute to the trait. (Make certain that you review the assumptions of the twin method to know the circumstances under which this inference is valid.)

The twin method estimates heritability, environmentability, and the correlation in twin environments (eta or h) using the following equations:

,

,

and

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Exercises:

(1) Enter the answers (to the third decimal place) for the problem in the Applet window using the equations given above in the appropriate text fields and then click on Submit. For another problem, click on New Problem, and if you cannot get the right answer click on I Give Up! Continue until you feel competent about solving equations for the twin method.