This paper is one of a series reporting studies we have made of differences in implicit presuppositions and of how such differences affect the ways people reason. In the study reported here 26 students (14 at Caltech; 12 at Claremont) read and rated four letters which had appeared in the correspondence columns of Science. Two of the letters defended the guidelines governing DNA research; two criticized them. The students rated the letters on six scales, or "dimensions," each of which represents a contrasting pair of implicit presuppositions, which we have identified and defined. For two of the six dimensions all four of the letters were rated in the predicted direction, and all are statistically significant. On a third dimension all four of the letters were rated in the predicted direction, but only three of the four are statistically significant. For the other three dimensions there was no consistent pattern, though some of the results on some of the dimensions were in the predicted direction and are statistically significant. Thus this study shows that in certain important respects the presuppositions of the proponents and the presupposition of the opponents of the guidelines are not only different but diametrically opposed
