Perhaps, and perhaps not. Effect sizes, and a good knowledge of the substantive question, are necessary here. This question comes closer to a yes-no question. Notice, however, that such twin research is undertaken in general to determine the relative contributions of genetics and environment. In this case, it is more important to determine the size of the difference between monozygotic and dizygotic twin pairs.
Effect size figures prominently in such research.
In this situation, we are called upon to make a decision about the truth or falsehood of a practical hypothesis. Notice, however, that the hypothesis in this case is not a typical null hypothesis. This example provides a justifiable use of point hypothesis testing. We can find at least three flaws with the methodology of Greenwald et al. We examine these in turn.