ences in profile contour for sibs and unrelated pairs; as noted above, this does not furnish a measure of con-cordance for either group or a means of testing whether the concordance level is (i) greater than zero or (ii) significantly different for the two groups. Therefore, the methodsand data of the unpublished study will need careful examination before the results can be interpreted.

On balance, none of the results or conclusions from the original report are changed by McCall's comments. Wachs in his first paragraph substitutes the terms intellectual development, intelligence, and IQ for the original term infant mental development, which was specifically chosen to designate the capabilities being manifested during in-fancy. The point was made clearly that there are pronounced changes in de-velopmental precocity from one age to the next, which reflect the changing capabilities being measured by the Bayley mental scale plus the idiosyncratic spurts and lags in development ex-hibited by each infant. The strength of the twin data is in showing that the changes in precocity occur concordant-ly for genetically related pairs. But there is no direct correspondence be-tween precocity in infancy and later measures of intelligence, and Wachs' substitution ofterms serves to confuse an essential distinction between the capabilities being measured at different ages.