Bell Curve

276 The National Context Ethnic Differences in Cognitive Ability 277matching by socioeconomic status is a good idea if one wants to knowan estimate of the overall difference between East Asians and whites(we will return to the question of matching by socioeconomic statuswhen we discuss comparisons between blacks and whites). In our judgment,the balance of the evidence supports the proposition that theoverall East Asian mean is higher than the white mean. If we had to puta number on it, three IQ points currently most resembles a consensus,tentative though it still is. East Asians have a greater advantage thanthat in a particular kind of nonverbal intelligence, described later in thechapter.Do Blacks Score Differently from Whites on Standardized Tests ofCognitive Ability?If the samples are chosen to be representative of the American population,the answer has been yes for every known test of cognitive abilitythat meets hasic psychometric standards of reliability and ~alidit~.'~'Theanswer is also yes for almost all of the studies in which the black andwhite samples are matched on some special characteristics-samples ofjuvenile delinquents, for example, or of graduate students-but thereare exceptions. The implication of this effect of selecting the groups tobe compared is discussed later in the chapter. Since hlack-white differencesare the ones that strain discourse most severely, we will probedeeply into the evidence and its meaning.How Large 1s the Black-White Difference?The usual answer to this question is one standard deviation." In discussingIQ tests, for example, the black mean is commonly given as 85,the white mean as 100, and the standard deviation as 15. But the differencesobserved in any given study seldom conform exactly to onestandard deviation. The figure below shows the distribution of theblack-white difference (subsequently abbreviated as the "R/W difference")expressed in standard deviations, in the American studies conductedin this century that have reported the IQ means of a black sampleand a white sample and meet basic requirements of interpretability asdescribed in the note.12" A total of 156 studies are represented in theplot, and the mean B/W difference is 1.08 standard deviations, or aboutsixteen IQ The spread of results is substantial, however, reflectingthe diversity of the age of the subjects, their geographic loca-Overview of studies of reporting black-white differences incognitive test scores, 19 18-1990Number of studies20 -0.0 0.5 1 .O 1.5 2.0Magnitude of the black-white difference in test scores,in standard deviationsSour~~s: Shuey 1966; Oshorrle and McGurk 1982; Sattler 1988; Vincent 1991; Jensen 1985,1993h.tion, their background characteristics, the tests themselves, and samplingerror.When we focus on the studies that meet stricter criteria, the range ofvalues for the R/W difference narrows accord~ngly. The range of resultsis considerably reduced, for example, for studies that have taken placesince 1940 (after testing's most formative period), outside the South(where the largest B/W differences are found), with subjects older thanage 6 (after scores have become more stable), using full test batteriesfrom one of the major IQ tests, and with standard deviations reportedfor that specific test administration. Of the forty-five studies meetingthese criteria, all but nine of the B/W differences are clustered between.5 and 1.5 standard deviations. The mean difference was 1.06 standarddeviations, and all but eight of the thirty-one reported a R/W differencegreater than .8 standard deviation.Still more rigorous selection criteria do not diminish the size of thegap. For example, with tests given outside the South only after 1960,when people were increasingly sensitized to racial issues, the number ofstudies is reduced to twenty-four, but the mean difference is 1.10 standarddeviations. The NLSY, administered in 1980 to by far the largest