Scores on intelligence tests such as the WAIS-R have been correlated with a host of demographic variables—age, sex, education, ethnicity, occupation, socioeconomic status, culture, nationality, urban—rural residence, family size, birth order, and others. Intelligence test scores have also been related to diet, drugs, hormones, exposure to other chemical substances, genetic disorders, neural anatomy and physiology, climate, season of birth, and many other biological, environmental, behavioral, and personality variables. For example, comparisons of the correlation between the IQs of identical twins with the correlations between the IQs of individuals with other familial relationships provide support for a strong genetic basis for whatever is measured by intelligence tests (Bouchard, Lykken, McGue, Segal, & Tellengen, 1990; Bouchard & McGue, 1981; Plomin, 1988, 1989). Another important research finding, the significant relationship between lead exposure and intelligence test scores, indicates that exposure to high levels of lead during early childhood can have adverse effects on intellectual development (Fulton et al., 1987; McMichael et al., 1988; Needleman, Schell, Bellinger, Leviton, & Allred, 1990). In addition, research has documented the effects of fetal and infant malnutrition on low intelligence and the persistence of such effects throughout childhood and adolescence (Stoch & Smythe, 1963; Zeskind & Ramey, 1981).
Sex Differences. Sex differences in overall scores on the Wechsler tests and other measures of general mental ability are usually nonsignificant, but the differences in specific cognitive and psychomotor abilities are noteworthy. Females usually score higher than males on measures of verbal fluency, reading comprehension, finger dexterity, and clerical skills, whereas males typically score higher than females on tests of mathematical reasoning, visuo-spatial ability, and speed and coordination of large bodily movements (Minton & Schneider, 1980). The superiority of women to men in verbal ability and the superiority of men to women in performance abilities tend to persist over the life span. It has also been found that, as a group, women's verbal abilities increase more than those of men during adulthood, but the reverse is true for performance abilities (Eichorn, Hunt, & Honzik, 1981). Whether these sex differences are due mainly to biological differences between the sexes or to differences in social expectations and experiences has been the subject of debate. Admittedly, social expectations are greater for girls to become more adept in linguistic and social skills and for boys to perform better in mathematical, mechanical, and spatial reasoning. Still, findings concerning the role of hormones and other biological variables in promoting sex differences in abilities are provocative (Christiansen & Knussman, 1987; Hier & Crowley, 1982; Kimura & Hampson, 1993).
Ethnicity. Even more controversial than the results of research on sex differences in mental abilities are those obtained from studies of racial differences in intelligence and specific cognitive abilities. For example, whites outscore blacks by approximately one standard deviation on both the WAIS-R (Reynolds, Chastain, Kaufman, & McLean, 1987) and the Stanford-Binet: Fourth Edition (Thorndike, Hagen, & Sattler, 1986). These findings have been interpreted in various ways. Jensen (1980) maintained that blacks have lower mean scores than whites on measures of abstract reasoning and problem solving but equivalent mean scores on measures of rote learning ability because the frequency of genes carrying higher intelligence is lower in the black population than in the white. Supporters of an experiential explanation for black—white differences in intelligence test scores have taken issue with Jensen's conclusion. They interpret racial differences in scores on tests of mental abilities as the consequence of variations in educational and cultural experiences rather than genetics. However, an environmental explanation must also account for the fact that Native Americans, Hispanics, and Asians living under poor socioeconomic conditions score higher than blacks. In fact, Asians score higher than whites on tests of quantitative ability and, at least in the case of Japanese tested in their own country, in many instances on measures of general intelligence (Lynn, 1982, 1987).
Education, Socioeconomic Status, and Occupational Status. The fact that groups of people with less education and of lower socioeconomic status tend to make lower intelligence test scores than people of higher educationai and socioeconomic levels is not surprising. Both educational attainment and socioeconomic status are negatively correlated with scores on intelligence tests. Educational attainment, socioeconomic status, and IQ scores are also related to occupational status. Individuals with more education, higher socioeconomic status, and higher IQs tend to enter higher-status occupations. Although the reasons for these relationships are not entirely clear, Cronin, Daniels, Hurley, Kroch, and Webber (1975) concluded that intelligence is related to occupational status because both variables are associated with socioeconomic status. According to these investigators, parents of middle- or upper-class backgrounds are more likely than those of lower-class backgrounds to prepare their children for entry into higher-status occupations by encouraging them to do well in schoolwork and on tests.
Education and socioeconomic status are also related to ethnicity, so it is possible that ethnic-group differences in measured intelligence stem from educational and/or social-class differences rather than ethnicity per se. The results of one research study suggest, however, that intelligence rather than education or social class is the pivotal variable (Thomas, Alexander, & Eckland, 1979). These investigators found that the positive correlation between IQ and educational attainment remained significant even when socioeconomic status was statistically controlled. On the other hand, when IQ was statistically controlled, the correlation between socioeconomic status and educational attainment was slightly negative. These findings were interpreted as demonstrating that intellectual ability affects both socioeconomic status and educational level.
Place of Residence and Family Structure. Three other demographic variables that are related to intelligence test scores, as well as to education and socioeconomic status, are urban versus rural residence, family size, and birth order. Studies conducted earlier in this century found significantly lower IQs in individuals living in rural than in urban areas (McNemar, 1942). Advances in information transmission and transportation during the past several decades, however, have broadened the educational and social experiences of children arid improved the living conditions of Americans in all geographical areas. Television, better schools, and other sources of intellectual stimulation have undoubtedly contributed to the improved intelligence test scores of persons living in rural areas of the United States and other countries as well (Cronbach & Drenth, 1972; Scribner & Cole, 1973).
It has been noted by scientists for over a century that mentally duller individuals tend to come from larger families than mentally brighter individuals. Studies supporting this observation are plentiful (e.g., Belmont & Ma-rolla, 1973; Kellaghan & MacNamara, 1972; Zajonc, 1976). The negative correlation between family size and IQ scores is not entirely due to socioeconomic status, because the correlation remains significant when differences between the socioeconomic status of large and small families are statistically controlled.
Not only do children from larger families tend to have lower IQs than those from smaller families, but IQ also varies with the birth order of children within families. Firstborn children, on the whole, make higher intelligence test scores than subsequently born children. Whether this is due to the greater attention, encouragement, and assistance provided by parents to firstborn than to later-born children or to some other factor is debatable (Kilbride, Johnson, & Streissguth, 1977; MacPhee, Ramey, & Yeats, 1984). Whatever the cause, firstborn children are, as a group, more serious, responsible, studious, and competitive than later-borns, whereas later-born children are more outgoing, relaxed, imaginative, and athletic than firstborns. These are characteristics that would seem to be shaped to a large extent by differential treatment of children within families rather than by biological variables.
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