Here Paul Black comments on the paper by Clark in this issue. Black first comments that the Clark paper highlights two possible factors that might effect trends of change over time. The first is that the population has become more adept at dealing with the types of challenges IQ tests present to them. Such explanations must be analyzed in light of the fact that there is no one IQ test: Three tests are commonly used and they test different aspects of intelligence. These differences seem to be important in some contexts--for example, Lynn (1998, p. 212) argued that the IQ gains on the Wechsler tests should be regarded as genuine increases of intelligence but that the larger gains obtained by 18-year old military conscripts on the Raven Progressive Matrices, documented by Flynn (1987) in several European countries, were largely spurious and were best interpreted as school effects. The second comment following on from Lynn's argument is that the tests themselves have used more sophisticated language for expressing their demands, language with which respondents have become more familiar because of its increasing use in everyday documents. The third comment is that the article takes for granted that the samples on whose responses the data are based were selected in ways that make the results equally valid over the long time spans involved.