A decline exists in children's ability at ages 4 and 5 to accurately respond to the difference between polar adjectives such as "big" and "tall.""Taller" and "bigger" are both taken to mean "having a higher top point," rather than "bigger" meaning "greater overall mass." Two hypotheses are put forth to explain this. The "strong cognitive hypothesis" bases the negative change in children's understanding of "big" and "tall" on changes in the child's cognitive processing of the world. Confusion of referents occurs when certain perceptual categories of one adjective become more salient and are overgeneralized to other referents. The vertical spatial dimension of "tall" may be generalized to be the critical dimension of "big" as well. The alternative hypothesis, the "strong semantic hypothesis," is similar but states that overgeneralization is of semantic, rather than perceptual qualities, these qualities having been acquired at an earlier age. This could cause confusion between "tall,""high," and "big," with overextension of the features (+vertical) and (+positional). The latter hypothesis may be made more or less plausible by researching data on the acquisiton of spatial adjectives in other languages. (PJM)