The influence of gender, handedness, and performance level on specialized cognitive functioning

BRAIN

AND

COGNITION

15,

37-61 (1991)

The Influence of Gender, Handedness, and Performance Level on Specialized Cognitive Functioning W. GORDON

HAROLD University

of Pittsburgh School of Medicine AND

SHLOMO KRAVETZ Bar Ilan University,

Ramat Can, Israel

Individual differences in performance on neuropsychological tests were analyzed across age (prepubertal to adult), gender, and handedness groups and examined for performance level as a moderating variable. No differences were observed for the factor structure of these tests across ages and between genders, suggesting similar cognitive structures among these groups. Significant differences in performance were observed between males and females and, to a lesser extent, between right and left handers. Of interest, were significant Gender x Handedness and Gender x Handedness x Level (of performance) interactions seen especially in the older (postpubertal and adult) subjects. High performing, right handed males and left handed females performed better on visuospatial tasks while left handed males and right handed females performed better on verbosequential tasks. o 1991 Academic press, IK.

Females have generally been found to outperform males on neuropsychological tests of verbal output, whereas males have generally been found to outperform females on tests of spatial orientation and rotation (Maccoby & Jacklin, 1974; Vandenberg & Kuse, 1978). Since these verbal and spatial skills are associated, respectively, with the cerebral functioning of the left and right hemispheres, gender differences in the proficient use of these skills may be related to sexually dimorphic brain organization. However, difficulties immediately arise with this explaThe advice and encouragement from Professor Sheri Berenbaum for the analyses in this paper, and her suggestions for improvement of earlier drafts of the manuscript are gratefully acknowledged. Address all correspondence and requests for reprints to Harold W. Gordon, Western Psychiatric Institute and Clinic, 3811 O’Hara Street, Pittsburgh, PA 15213. 37 0278-2626191$3.OO Copyright 0 1991 by Academic Press, Inc. All rights of reproduction in any form reserved.

38

GORDON

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nation of gender differences in cognitive performance since some specialized skills, which are theoretically associated with the left or right hemisphere, fail to produce gender differences in performance. Females do not outperform males in perception and memory of sequences of verbal stimuli, nor do males outperform females in closure and other tasks of visual perception (Gordon, 1986). In addition, environmental and societal pressures may underlie the cognitive tasks that differentiate females from males. Attitudes toward vocations created by parental role models may induce gender differences in concentration and effort in carrying out the cognitive tasks ascribed to typically male and female vocations (Luchins & Luchins, 1979). Before a theory of sexual dimorphism in right and left hemispheric organization can be used to explain differences in cognitive performance, the simple dichotomy of visuospatial versus verbosequential function associated with right and left hemisphere processes must be supplemented with a specification of the nature and extent of the cognitive tasks that consistently differentiate between their performance by female and male subjects. At present, although there is a consensus among theoreticians and researchers as to the existence of gender differences in cognition, there is considerable controversy concerning the nature and source of these differences. To resolve some of this controversy, the tasks that assess the mental rotation and perceptual skills associated with the right hemiisphere and the tasks that require verbal output and sequential ability associated with the left hemisphere must be clearly delineated. Once these tasks have been identified, especially those tasks that consistently differentiate females from males, then sexual dimorphism of the brain can more readily be examined. One of the reasons for the dispute over the nature, extent, and source of gender differences in cognitive functioning is that many studies do not demonstrate a gender difference, or if they do, the differences are very small. For many of the studies, for example, there is a failure to find gender differences in Performance IQ of the Wechsler Adult Intelligence Scale (WAIS). Not only is there failure to find male superiority on the spatial tests, but female superiority on verbal tests is also absent (Inglis & Lawson, 1984; Johnson & Harley, 1980). Other so-called spatial skills have failed to demonstrate gender differences in performance level. The best demonstration of the specificity of gender differences in cognition was produced by a study where the Spatial Relations subtest from the Differential Aptitude Tests demonstrated a male superiority while the diagrammatic section of a general intelligence test (AH4), as well as the Revised Minnesota Paper Form Board Test showed no gender differences (Birkett, 1980). These results reinforce the idea that only specific functions, especially those involving mental rotation in three dimensions, actuate male superiority. There were no gender differences on a Verbal

GENDER,

HANDEDNESS,

PERFORMANCE,

AND COGNITION

39

IQ subtest (WAIS Similarities) nor on a closure test specifically chosen to be measures of left and right hemisphere function, respectively (TenHouten, 1980). As it turned out, however, there was an influence of race and socioeconomic level on these tests which was difficult to reconcile with a biological theory. In another study where letters were flashed in random locations on a 5 x 5 matrix, females performed just as well as males in remembering the locations; males performed just as well as females in remembering the letters regardless of location (Nagae, 1985). Interestingly, there were differences in ability between the subjects on the location task according to hand preference. These seemingly inconsistent results may have been a consequence of the lack of an empirically derived specification of the characteristics of the cognitive tasks which differentiate males from females. Early observations of gender differences in cognitive function were studied in terms of global psychological concepts such as field dependency or Piaget’s schema of sensorimotor coordination. These concepts are related to “orientation” ability which involves comprehension of the relative elements of a stimulus pattern regardless of its spatial configuration or orientation (McGee, 1979). On tests of field dependence, notably the Rod and Frame Test or the Embedded Figures Test, adult males are consistently found to be more field independent than females (Parasnis & Long, 1979; Witkin, Goodenough, & Karp, 1967). Unfortunately, these differences usually account for less than 15% of the variance (Allen & Cholet, 1978). Furthermore, since the concepts are vague and the tasks that measure them are complex, the source of these differences may be just as easily attributed to learned strategies in problem solving than to neurobiological dimorphism. In other words, females perform less well on these tasks because they may have learned strategies that are less conducive for the type of problem solving required (Allen & Hogeland, 1978). In support of this view, gender differences in field dependence were not demonstrated in children under 12 years old, where strategies may not yet have been learned (Berlin & Languis, 1980). By contrast, these same children did demonstrate a male superiority on the Block Design subtest of the Wechsler Intelligence Scale for Children. The probability that gender differences in cognition are related to biological differences between males and females is increased by the observation that these differences in cognition are sometimes observed between different handedness groups. It is also true, however, that the data on differences in cognitive performance between handedness groups are perhaps more controversial than the gender differences. The oft-cited early studies where right handers outperformed nonright handers on spatial but not verbal tests of intellectual capacity (Levy, 1969; Miller, 1971) have not been replicated by investigations of large, unselected samples of both children (Hardyck, Petrinovich, & Goldman, 1976) and

40

GORDON

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KRAVETZ

adults (Briggs, Nebes, & Kinsbourne, 1976; Inglis & Lawson, 1984). Here too, one of the difficulties that perpetuate the controversy is that performance differences between the handedness groups may be demonstrated on some subtests of the Performance IQ and for some, but not all, of the tests that are covered by the rubric “spatial” (Nebes, 1971). Interactions between gender and self-reported handedness and sometimes family history of handedness may also wash out both gender and handedness main effects. For example, in one sample of university students the Verbal IQ exceeded the Performance IQ for females in every subgroup comparison except for the comparison between male and female familial left handers (Bradshaw, Nettleton, & Taylor, 1981). For subgroups who had familial left handers, males had a Verbal IQ that was much larger than their particularly low Performance IQ score. For the Block Design subtest alone, right handed males were better than left handed males, while right handed females were worse on this test (Johnson & Harley, 1980). This pattern was reversed for the Arithmetic subtest where the left handed males and right handed females were the superior subgroups. Another example of this interaction between gender and handedness is a reduced ability for left handed males relative to right handed males to recall letter positions on a 5 x 5 matrix, whereas there was no difference between right and left handed females (Negae, 1985). Right handed males were superior to left handed males on the first principal component of six spatial tests for which there were also highly significant gender differences (Yen, 1975). There was no difference in performance between the female left and right handers. Unfortunately, matters are complicated by one report in which left handed males outperformed both ambilateral and right handed males on a composite spatial score while left handed females fared much worse than ambilateral and right handed females (Sanders, Wilson, & Vendenberg, 1982). However, all male subgroups outperformed all female subgroups. If the Gender x Handedness interactions can be shown to be stable, then they may provide one of the stronger arguments for a neurobiological basis of cognitive gender differences. Environmental arguments that take both factors into account can be constructed, but these explanations would be even more speculative. In addition, there are interstudy discrepancies that need to be resolved. One suggestion to this end has been made with regard to intellectual level and the possibility of other moderator variables (Harshman, Hampson, & Berenbaum, 1983). The suggestion was made as the result of a post hoc analysis of data collected from a number of studies in three main university locations. The studies included tests of spatial function, verbal function, and speeded skills. In concert with a majority of studies, males outperformed females on most types of spatial tests. There were virtually no differences on tests class-

GENDER, HANDEDNESS,

PERFORMANCE,

AND COGNITION

41

ified as verbal, while females usually outperformed males on tests requiring speeded output. Of interest was the report of Gender x Handedness interactions for some of the spatial and perceptual speed tests where right handed males outperformed left handed males; for females, the pattern was reversed. More significantly, there was a three-way interaction where a measure of reasoning ability interacted with gender and handedness. More recent research also suggests that reasoning ability might moderate the relation between gender, laterality as measured by Right Ear Advantage, and perceptual skills (Bryden & Vrbancic, 1988). The possibility that gender differences can be modified by other variables in consistent ways could help explain the discrepant data among reported studies and gives credence to a neurobiological basis for gender differences in performance on cognitive tasks. Despite an increase in the theoretical and empirical interest in gender and handedness differences in cognition over the last 20 years, these differences have remained elusive and resistant to critical investigation. In an attempt to resolve the controversy regarding these differences, the present study was intended to be innovative in three ways. First, the test instrument was a multidimensional measure constructed especially to be sensitive to information processing associated with specialized brain function. As mentioned above, past research may have produced ambiguous results because measurement instruments were selected from such diverse domains as cognitive style assessment (the Rod and Frame Test) and intelligence testing (the WAIS Similarities, Block Design subtests). In addition, the use of a multidimensional measure permits both quantitative and structural analyses of gender and handedness differences in cognitive functioning. The second innovation of these analyses is that they were carried out on data gathered from moderately sized samples of male and female subjects from four age groups. In this way, conclusions regarding the generalizability of the results and the influence of puberty could be drawn. The third advantage of this study is that large numbers of subjects were tested on the same test instrument which not only produces a measure of specialized cognitive functioning, but also a measure of the level of cognitive functioning. These two kinds of measurement allow a clear examination of the suggestion (Harshman et al., 1983) that treating the level of cognitive functioning as a moderator variable might reveal more generalizable and consistent relations between gender, handedness, and lateralized information processing. METHODS

Subjects The subjects included in this analysis were primarily those who made up the normative samples for the standardization of the test instrument used in this study, the Cognitive

42

GORDON AND KRAVETZ

Laterality Battery (Gordon, 1986). Other subjects had taken part in various projects for which volunteers were recruited. There were 866 children who were selected as entire classrooms from primary and secondary schools representing a random sampling of subjects within a public school system. The sample of school children was subdivided according to the following age groups: 9 to II, 12 to 14, 15 to 18. This grouping corresponded to grade divisions in the districts elementary, middle, and high schools. These groups included approximately equal numbers of males and females, differing in the subgroups between 5 and 10%. Thirteen percent were nonright handers (by self-report) with younger subjects reporting more nonright handedness. Adults were selected from a number of different samples who had volunteered for other studies or for their own curiosity. There were 530 subjects, most of whom were enrolled in college classes, primarily in the School of Education. Others were selected from the general public and may not have had a college education. The age range of the adult sample was 18-73, two-thirds of whom were under the age of 40. There were 284 males and 246 females. While it is believed that relatively little bias occurred in adult selection, it is certainly true that they were not randomly selected as had been the school children. Consequently, the average adult performance is likely to be higher than would be expected from a randomly selected sample. It should be pointed out, however, that the adult sample is probably more representative of the general population than the vast majority of published psychological studies in the literature which almost exclusively use college samples.

Materials All subjects were administered the Cognitive Laterality Battery (CLB) which has been described in detail elsewhere (Gordon, 1986). In brief, this battery consists of eight tests which have been selected to assess specialized cognitive functions associated with the left and right cerebral hemispheres of the brain. These include two tests of sequencing, two tests of written word production (fluency), two tests of a mental rotation and/or imagination in two or three dimensions, a test of point localization, and a test of visual closure. Factor analyses on the normative sample (Gordon, 1986), as well as on several other samples including the present one, confirmed a two-factor structure with sequential and word production tests loading on one factor, and the four visuospatial tests loading on the other factor. Sequential and word production tests. One test of sequential processing is called Serial Sounds. The subject is first presented with nine familiar sounds (dog, baby, bugle, doorbell, cuckoo clock, telephone, bird, rooster, and horse) for the purpose of familiarization. For this test, the sounds are presented in a series of two to five for children or two to seven for older adolescents and adults. Subjects must recall each series in the correct serial order. However, the scoring allows partial credit for incomplete sequences. More points are given for longer sequences that are recalled. Serial Numbers is the second test of sequencing ability. The subject has to write sequences of two to nine digits presented at the rate of one per second. Subjects of all ages took the same Serial Numbers Test; the scoring was the same as for Serial Sounds. The Word Production, Letters Test requires the subject to write as many words as possible in I min that start with a given letter of the alphabet. The letters, C, F, and L are used and the score is the sum of all words written for the three letters. Word Production, Categories is a similar test except that the subject writes as many words as possible in 1 min for a given category. Two categories were used (foods and animals); the score was the sum of the two. Mental rotation, localization, and closure tests. In the Orientation Test there are line drawings of three geometrical shapes, two of which are the same but rotated on the plane of the page; the third is a rotated mirror image. The subject selects the two that are alike. Two-dimensional forms (Thurstone & Jeffrey, 1956) are used for children and young adolescents; three-dimensional cube constructions (Shepard & Metzler, 1971) are used for

GENDER, HANDEDNESS,

PERFORMANCE,

AND COGNITION

43

the older adolescents and adults. Touching Blocks also requires three-dimensional imagination. The stimulus item is a “stack” of line-drawn blocks (MacQuarrie, 1953). The subject’s task is to determine how many blocks are touching a designated block in the stack. In the Localization Test, the subject observes an X marked in a frame on a screen and has to mark an X on a similar frame drawn on the answer sheet. The score is the linear error for all 24 items. The Form Completion task requires the identification of whiteon-blue silhouetted figures (French, Ekstrom, & Price, 1962; Thurstone & Jeffrey, 1966).

Procedure The CLB is presented on a Bell & Howell Ringmaster Model 798 simul/sync projector. The projector plays an audio cassette tape recording, displays 35-mm slides either on its own screen or on a wall screen, and advances the slides according to a cue track on the audio tape. Instructions for all tests are prerecorded on the audio cassette tapes, as are the audible stimuli (e.g., the sounds for Serial Sounds, the numbers for Serial Numbers, etc.). Visual items and titles are presented by 35-mm slides, according to the strict timing provided by the cue track on the tape which advances the slides. Each test takes from 7 to I5 min to administer, including instructions. The entire testing could be accomplished in 80 to 90 min, including rests. The tests were administered in a fixed order, but when there were scheduling difficulties or fatigue, the tests were administered over two sessions. Subjects were tested in groups in classroom or seminar room settings. Sufficient proctors were available to answer questions and to avoid cheating. As each test was completed, it was placed in the subject’s own manila envelope and later collected for scoring. Tests were hand-scored and cross-checked by trained scorers. The data were then entered into computer files and cross-checked again before analysis.

Data Analysis Two forms of analyses were carried out on the data collected by the CLB. First, gender differences in cognitive processing were examined by comparing the factor structures of the CLB performance of male and female subjects. For these comparisons, principal component analyses with varimax orthogonal rotation were used. Second, in a three-way analysis of variance, gender, and handedness were examined as independent variables along with performance level as a possible moderator variable. To control for the effect of age and to check the stability of the results, both forms of analysis were performed separately on each of the four samples of subjects, grouped by age, from preadolescent through early and late adolescent to adult.

RESULTS Factor Analysis

The confirmation of the two-factor structure and the stability of this structure in a variety of samples of individuals has served as a major source of validity for the CLB (Gordon, 1986). This study focused on whether this stability that has been demonstrated across age levels would be maintained for both genders at each age level (see Tables 1 to 4). In particular, the question was whether the pattern of factor loadings for the eight cognitive tasks that comprise the CLB differs among groups that were subdivided by gender and age group. Gender differences in the performance of the CLB could stem either from differences in the extent to which males and females make use of components of the CLB

44

GORDON AND KRAVETZ TABLE 1 PRINCIPAL COMPONENT FACTOR ANALYSES AFTER VARIMAX ORTH~CONAL ROTATION OF SUBTEST SCORES ON THE COGNITIVE LATERALITY BATTERY FOR 9 TO 11 YEAR OLDS

Factor loadings” Total sample (N = 272) CLB tasks Serial Sounds Serial Numbers Letters Categories Orientation Form Completion Touching Blocks Localization’ Percentage of total variance

-

1

Males (N = 117)

Females (N = 155)

2

1

2

I

2

S46 .526 L824 ,825 .351 ,094 ,303 -.I20

.441 .230 ,071 ,017 .694 7471 .719 773 -

.616 .664 .781 .825 .369 .I67 .I42 .012

.376 .244 .068 .033 JlJ L592 &3 L715

.482 .408 .847

.492 .212 .091 ,017

.835 ,350 .067 .42.5 -.183

.331 .700 .812

27.16

25.88

28.70

26.99

26.93

25.02

33

” Listwise deletion of subjects; loadings greater than .400 are underscored. ’ The scores for this test were transformed by multiplying by - 1 because a high score means a lower performance for this test.

TABLE 2 PRINCIPAL COMFQNENT FACTOR ANALYSES AFTER VARIMAX ORTHOGONAL ROTATION OF SUBTEST SCORES ON THE COGNITIVE LATERALITY BATTERY FOR 12 TO 14 YEAR OLDS

Factor loadings” Total sample (N = 192) CLB tasks Serial Sounds Serial Numbers Letters Categories Orientation Form Completion Touching Blocks Localization6 Percentage of total variance

1

2

.286 .I23 ,162 - .019 .737 .543 .623 737 L

&l ,714 .768 7% 3% ,262 ,394 .1.51

,464 .083 .179 ,001 J&9 J&l .594 L707

,795 .662 .763 .781 -.155 .259 .369 ,185

.lOO .047 .314 .165 .776 .552 7% 724 -

23.65

30.12

24.60

31.56

24.80

I

2

L705 .629 L799 .812 -.169 .310 .384 .169 31.15

u See Footnote a, Table 1. ’ See Footnote ‘, Table 1.

Females (N = 87)

Males (N = 105) 1

2

GENDER,

HANDEDNESS,

PERFORMANCE,

45

AND COGNITION

TABLE 3 PRINCIPAL COMWNENT FACTOR ANALYSES AFTER VARIMAX ORTHOGONAL ROTATION OF SUBTEST SCORESON THE COGNITIVE LATERALITY BATTERY FOR 15 TO 18 YEAR OLDS Factor loadings” Males (N = 92)

Total sample (N = 199) CL9 tasks Serial Sounds Serial Numbers Letters Categories Orientation Form Completion Touching Blocks Localizationb Percentage of total variance * See Footnote ’ See Footnote

1

2

.650 J8J L806 L--838 .042 - .041 ,216 ,308

,305 .096 - ,018 ,068 &7 L575 ,785 573 L

28.20

26.26

Females (N = 107)

1

2

&9 ,038 ,137 .173 .869 #J J&l

4 L757 .732 L748 ,065 .133 ,092 ,074

.722 L520 Jg .840 .I06 -.I39 ,337 &7

,107 ,049 ,044 ,105 L759 &O L731 L454

30.92

24.68

31.52

21.75

.472

1

2

“, Table I. ‘, Table 1.

TABLE 4 FACTOR SOLUTIONS FOR PRINCIPAL COMWNENT FACTOR ANALYSES AFTER VARIMAX ORTHOGONAL ROTATION OF THE PERFORMANCEON THE COGNITIVE LATERALITY BATTERY OF 18 TO 65 YEAR 0~0s Factor loadings” Total sample (N = 485)

Males (N = 262)

Females (N = 223)

CL9 tasks

I

2

1

2

I

2

Serial Sounds Serial Numbers Letters Categories Orientation Form Completion Touching Blocks Localization’ Percentage of total variance

J.iJ L671 .823 .814 ,047 ,236 .060 .095

.305 .I85 ,002 .020 JilJ .589 .808 642 L

J4J L717 .815 .830 ,174 ,262 ,062 .Oll

,225 ,188 .042 ,049 &8 $& L761 .650

,240 ,061 ,073 .247 .800 L628 I--.733 .---593

&&3 668 L--.779 J&t3 ,062 ,090 .209 .258

28.88

27.48

31.62

26.19

26.91

26.45

“ See Footnote ’ See Footnote

“, Table I. ‘, Table 1.

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GORDON

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or from differences between the genders in the CLB factor structure. Therefore, the factor analyses were a preliminary step to investigating the impact of gender and handedness on CLB performance. Stability across ages. The use of the CLB to assess specialized cognitive function is based upon the central assumption that the component cognitive tasks require verbosequential processing (usually associated with the left hemisphere) and visuospatial processing (usually associated with the right hemisphere). If this assumption is valid, then factor analysis of the CLB should consistently produce two factors that account for a large part of the relationship among the eight chosen tasks. Furthermore, the association between these two factors and the CLB tasks should resemble a simple structure with each task having a high factor loading on only one of the factors. Finally, those tasks that require the sequential processing of verbal information should have high factor loadings on one and the same factor, whereas those tasks that require the spatial integration and/or manipulation of visual information should have high factor loadings on the other factor. Although neither the factor structure per se nor the relative performance on the tests for any one factor offer direct evidence of where in the brain these tasks are being performed, past research and theory has attributed verbal and sequential task performances to the left hemisphere and spatial task performances to the right hemisphere. Inspection of the factor structures for each age (see Tables 1 to 4) not only provides support for the assumption on which the CLB was based, but also indicates that the factor structure is relatively uninfluenced by age. For the total sample at each age level, the first two factors explained slightly more than 50% of the total CLB variance; none of the eigenvalues associated with these factors was less than 1. The pattern of factor loadings indicates that the factor structure tended to conform to a simple structure in which four cognitive tasks loaded predominantly on one factor and the other four tasks loaded predominantly on the other factor. Specifically, the verbosequential tests-Serial Sounds, Serial Numbers, Word Production, Letters and Word Production, Categories-loaded on the same factor whereas the visuospatial tests-Orientation, Form Completion, Touching Blocks, and Localization-loaded on the other factor. The only deviation from this picture in a total sample is the two-factor loading of the Serial Sounds Test for the youngest subjects (Table 1). The relative saliency of the two factors remained the same across age groups. The verbosequential information processing dimension was more stable and consistent than the visuospatial information processing dimension. Both the two word production tasks and the two serial memory tasks each had equal loadings on the same factor, but the production tasks had consistently higher loadings on the first factor (verbosequential) than did the serial memory tasks. The pattern of factor loadings for the

GENDER, HANDEDNESS,

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AND COGNITION

47

tasks indexing visuospatial processing tended to shift across the samples. The Localization Test had the highest loading for the younger groups while the mental rotation tasks (Orientation and Touching Blocks) had higher factor loadings for the older groups. Stability between genders. For each of the four age groups, the separate factor structures for the male and female subjects do not differ greatly from the total sample or from each other. In all instances, the two-factor solution accounted for more than 50% of the variance. One exception was that the eigenvalue of a third factor for 9- to 11-year-old girls exceeded 1.0 but only the two-factor solution is presented here for comparison. As for the total sample, the verbosequential tests tended to load on one factor while the visuospatial tests tended to load on the other. There were a few areas of instability where the factor loading of one test differed between males and females. For most samples the verbosequential Serial Sounds task loaded with the other verbosequential tasks. However, for the 9- to 11-year-old girls and the 15 to l&yearold boys, the Serial Sounds task had factor loadings between .46 and .56 on both the verbosequential and visuospatial factors. The Localization task typically loaded on the visuospatial factor, but for the 15 to 18-year-old adolescent males, this task was related to both factors to almost the same degree. No explanation can be offered for these three deviations from the expected factor structure. The relative saliency of the two factor dimensions changed somewhat between the genders for the factor analyses of the different age groups. For the combined genders (total sample) and for most of the factor analyses for each gender, the first factor extracted by principal component analysis (and therefore the factor that explained more variance) was the one representing verbosequential information processing. However, for the 15- to 18-year-old males and (marginally) for the adult females, the visuospatial factor explained more variance. The relative strength of the factor loadings among certain tests also varied between the genders, but not consistently across ages. For the 9- to 1l-year-old girls and the 12- to 14-year-old boys, the Localization Test had the highest factor loading on the visuospatial factor. But for the 9- to II-year-old boys and the 12- to 14-year-old girls, the Orientation Test and the Touching Blocks Test, respectively, had the strongest loadings on this factor. For each of the older groups the Orientation Test had the highest factor loading for both males and females. For both genders and across all age groups, the word production tasks usually had the higher loadings on the verbosequential factor than did the serial tasks. It will be recalled that the Orientation Test and the word production tests are those that past research has shown to consistently produce gender differences in performance level.

48

GORDON AND KRAVETZ

TABLE 5 GENDERDIFFERENCES FORSUBTESTS OFTHE C~CNITIVELATERALITYBATTERYFOR9 TO 11 YEAR OLDS Mean (SD)

Males (N = 146)

Serial Sounds

79.8 (33.7)

Serial Numbers

131.2

Letters

(54.9) 24.0

(6.6) Categories

18.4 (4.9)

Females (N = 183) 76.8 (32.3) 131.9 (56.2) 25.2 (7.2) 19.2

18.5

17.3

(5.0)

Form Completion

(5.0) 7.9 (3.9)

Touching Blocks

11.6

Localization’

107.6 (22.0)

P

0.31

NS

0.74

NS

3.17

NS

3.12

NS

3.99

c.05

0.35

NS

1.50

NS

0.64

NS

(5.1)

Orientation

(6.2)

F-Ratiob

7.8 (4.1) 10.3 (5.3) 109.7 (23.0)

* The number of subjects varies by 1 to 3 in each of the subtest comparisons due to missing data, except for the Localization Test where the Ns for males and females are 123 and 157, respectively. The Ns represent the maximum in any one comparison. ’ This is the main effect for gender in the three-way, Gender x Handedness x Level, analysis of variance; &ranges from 1,503 to 1,522; F,,& < .lO) = 2.71. ’ For this test alone, the lower scores are the better scores.

Analysis

of Variance

To examine the influence of gender and handedness on the information processes measured by the CLB, analyses of variance were carried out on the subtests and composite scores of the CLB for each age sample. The independent variables were gender, handedness, and level of performance. All three independent variables were bivariate. Gender and handedness were determined by self-report: male/female, right handed/nonright handed. Performance level was determined empirically by above- or below-average performance on a composite of all the CLB tests. The dependent variables for these analyses were the scores on the eight subtests that make up the CLB. In addition, dependent variables were created from the two composite scores, “visuospatial” and “verbosequential” made up of the four largest coding subtests from each factor, plus “profile” defined as the difference between the composite scores. Although the ANOVAs were run with all three independent variables, the main effects will be discussed separately followed by the potentially more “interesting” interactions.

GENDER, HANDEDNESS,

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49

TABLE 6 GENDER DIFFERENCES FOR SUBTESTS OF THE GXNITIVE YEAR OLDS

Mean (SD) Serial Sounds Serial Numbers Letters

Males (N = 164)

Females (N = 124)”

89.7 (38.9) 169.1 (69.5) 27.4

89.4 (42.0) 159.4 (@w 30.6 (8.9) 24.2

(8.4) Categories Orientation

21.6

(6.5)

(6.6)

17.5

16.2 (5.9) 7.5 (4.1) 12.4

(5.8) Form Completion Touching Blocks Localization

LATERALITY

7.6 (4.4) 15.3 (7.0) 102.9 (23.3)

BATTERY

F-Ratioh

FOR

12 TO 14

P

1.08

NS

2.44

NS

5.16

c.025

5.86

c.025

3.21

NS

0.57

NS

15.57

<.OOl

2.38

<.05

(6.1) 108.7 (23.0)

” The number of subjects varies from 130 to 164 for males and 103 to 124 for females. ’ See Footnote ‘, Table 5. ’ See Footnote I, Table 5.

Gender differences. Gender differences have been reported for a number of neuropsychological tests such as those used in the CLB. Likewise in these samples, consistent differences in performance were seen between males and females across age groups (see Tables 5 to 8). Females always outperformed males on the two tasks of written word production. For the youngest sample, the differences were trends (i.e., p < .lO) and may not be conservatively considered significant, considering the large number of comparisons. The differences increased with age, however, suggesting that the result may be robust. In a separate two-way analysis, there was a significant Gender x Age Group interaction (F = 4.16; p < .025; df = 2,841) for the word production with letters. For this test, therefore, female superiority in the youngest age group significantly increased over time. By contrast, males always outperformed females on the Orientation (or mental rotation) test. This was true, though barely, for the two-dimensional version of the task in the 9 to 11 and 12 to 14 age groups, and for the three-dimensional version of the task in the older age groups. For all but the youngest age group there was also a significant bias in favor of males for the Touching Blocks Test. Experience with the CLB has shown that this test is the most highly correlated with and has the most equivalent factor loading to the Orientation Test.

50

GORDON AND KRAVETZ TABLE 7

GENDER DIFFERENCES FOR SUBTESTS OF THE COGNITIVE YEAR OLDS

Mean (SD) Serial Sounds Serial Numbers Letters

Males (iv = 113)

Females (N = 123)

115.8 (45.6) 214.4 (82.5) 33.0

121.7 (47.8) 207.8 (68.9) 38.2 (9.4) 28.7

(7.8) Categories Orientation

26.1

(6.7)

(6.6)

15.0 (E, 20.7

11.6 (4.3) 8.6 (4.7) 18.0

(6.3)

(6.9)

88.5 (21.3)

91.1 (19.7)

(4.8) Form Completion Touching Blocks Localization’

LATERALITY

BATTERY

FOR

15 TO 18

F-Ratio”

P

0.09

NS

0.66

NS

13.47

c.001

7.15

c.01

10.44

C.001

0.54

NS

3.84

c.005

1.80

NS

” The number of subjects varies from 100 to 113 for males and 113 to 123 for females. ’ See Footnote b, Table 5. ’ See Footnote I, Table 5.

The gender differences described for these verbal and spatial tasks are so pervasive in all testing performed with this test battery that the composite scores are always calculated using separate norms for males and females. This manipulation would remove any bias prior to application in experimental work. Accordingly, no main effect for gender was expected (nor found) for the visuospatial and verbosequential composite scores, nor for the profile (= visuospatial - verbosequential) in these analyses since the composites are calculated from standard scores from the gender- and age-specific means. Handedness differences. Main effects for handedness were both less frequent and less consistent than those for gender. However, there were a number of tests for which the handedness groups differed in the oldest (adult) age group although most were only trends (p < .lO) (see Table 9). The pattern of performance differences for right and nonright handers resembled that for males and females. Similar to the females, nonright handers outperformed right handers on the written word production tests and tended to be better on one of the serial tests. Conversely and similar to the males, right handers outperformed nonright handers on the Orientation Test and tended to be better on the Localization Test. In terms of the composite scores, and the profile score, right handers slightly

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Table 8 GENDER DIFFERENCES FOR SUBTESTS OF THE C~CNITIVE YEAR OLDS

Mean WY Serial Sounds Serial Numbers Letters Categories Orientation

Males (N = 284)

111.5 (45.5) 253.9 (71.8) 43.4 (10.2) 33.5

(6.5)

(6.2)

15.4

12.6 (4.0) 11.9 (4.4) 21.7

12.1

(5.2) Touching Blocks Localization

Females (A’ = 246)

115.5 (51.8) 248.4 (74.6) 38.6 (10.2) 29.2

(4.6) Form Completion

LATERALITY

24.8 (5.1) 76.0 (15.6)

BATTERY

FOR

20 TO 65

F-Ratiob

P

0.44

NS

0.52

NS

6.31

<.Ol

11.66

<.OOl

5.64

c.025

1.54

NS

25.73

<.ool

1.39

NS

(5.8) 80.2 (17.0)

U The number of subjects varies from 277 to 284 for males and 233 to 246 for females. ’ See Footnote b, Table 5. ’ See Footnote ‘, Table 5.

favored visuospatial performance by about 0.1 SD whereas nonright handers favored verbosequential performance by about 0.25 SD, although these trends were not statistically significant. There were only three comparisons in the younger groups that supported the hand differences seen in the adult group. Nonright handers in the youngest (9 to 11) group performed significantly more poorly on the composite visuospatial tests (F = 4.28; p < .05; df = 1,321). This result was due primarily to a significant difference for the Orientation Test (F = 6.83; p < .Ol; G!! = 1,319). The performance of the nomight handed 12 to 14 year olds was significantly better than that of the right handers for the Serial Numbers Test (F = 4.11; p < .05; u” = 1,276). The exceptions that did not fit this trend were that right handed 15 to 18 year olds significantly outperformed nonright handers on the composite verbosequential score (F = 3.53; p < .06; & = 1,227), and on the Serial Sounds Test alone (F = 8.14; p < .Ol; Q!! = 1,219). It should be noted that this age group had the smallest number of nomight handed subjects. Interactions. There were a few two-way and three-way interactions involving gender, handedness, and performance level some of which were only trends (see Table IO). Most were found in the adult group, especially

52

GORDON AND KRAVETZ TABLE 9

HANDEDNESS DIFFERENCESFOR SUBTESTSOF THE C~CNITIVE LATERALITY BATTERY FOR 20 TO 65 YEAR OLDS

Mean (SD) Serial Sounds Serial Numbers Letters Categories Orientation

Right (N = 493)’

Nonright (N = 37)”

113.8 (49.1) 250.0 (72.4) 40.6 (10.3) 31.1 (6.7) 14.3

Ill.9 (47.7) 262.9 (84. I) 43.8 (11.9) 32.3 (7.3) 12.3 (4.3) II.1 (4.2) 23.4

(4.6) Form Completion Touching Blocks Localization’

12.0 (4.9) 23.3 (5.7) 77.6 (16.0)

F-Ratioh

P

0.11

NS

2.86

NS

7.30

<.Ol

3.32

NS

5.42

c.025

0.73

NS

0.60

NS

3.07

NS

(5.6) 83.4 (20.3)

” The number of subjects varies from 476 to 493 for right handers and 35 to 37 for nonright handers. ’ This is the main effect for hand in the three-way, Gender x Handedness x Level, analysis of variance; df ranges from 1,503 to 1,522; F,,,,@ < .lO) = 2.71. ” See Footnote ‘, Table 5.

TABLE IO PROBABILITY LEVELS FOR ALL SIGNIFICANT INTERACTIONSFOR COGNITIVE PROFILE AND COMFQSITESCORESACROSSALL AGE GROUPS

Composite dependent variable Interactions Gender x Handedness Gender x Level Hand x Level Gender x Handedness x Level ” 12 to 14 year olds. ’ I5 to 18 year olds. ‘ 20 to 65 year olds.

Cognitive profile

Visuospatial

Verbosequential

.lW .Ol’ .lO” .Ol’

.I0 .OSb,’ none .05”,.01’

none .05 .05” .05

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AND COGNITION

53

Stand. Score 1.0 (

I

-1.01 Right Handers

Left Handers

FIG. 1. Gender x Handness x Level-of-Performance interaction for the visuospatial composite. Standard score values are plotted for the visuospatial composites for high and low performing right and left (nonright) handed subjects. Males are depicted by dotted lines; females by dashed lines. Close-set dots and shorter dashes represent the higher performing subjects.

when the visuospatial composite and the cognitive profile (defined as the difference between the visuospatial and verbosequential composite) were the dependent variables. In addition to these interactions, another nine interactions at the .05 level of significance were found for component tests of the CLB. These were not consistent throughout the age groups. Across the different age groups, statistically significant interactions were found between performance level and handedness, performance level and gender, and gender and handedness. Sometimes, they indicated larger differences for low functioning subjects grouped according to gender or handedness and sometimes larger differences appeared for the high functioning subjects grouped by the same variables. The Gender x Handedness interactions were also not totally consistent across the age groups. All of these interactions included such visuospatial measures as Stand. Score 1.0

0

-0.5 -1.0 Right Handers

Left Handers

FIG. 2. Gender x Handness x Level of Performance interaction for the verbosequential component. The axes and key are the same as for Fig. 1.

54

GORDON AND KRAVETZ Stand. Score 1.0 0.5

Right Handers

Left Handers

FIG. 3. Gender X Handness x Level-of-Performance interaction for the cognitive profile which is calculated by subtracting the verbosequential composite from the visuospatial composite (e.g., a difference score). A positive standard score indicat& relatively better performance on the visuospatial composite; a negative score indicates better verbosequential performance. The key is the same as for Fig. 1.

Localization, Form Completion, and Orientation as the dependent variables. They also tended to reveal a right handed superiority on these measures for male subjects. In this respect, these interactions were consistent with similar interactions reported previously (Harshman et al., 1983). However, the results for the female subjects were less clear cut, sometimes suggesting a nonright handed superiority and sometimes, no handedness differences at all. The three-way interactions uncovered for the adult sample, when the cognitive profile and composite scores served as dependent variables, were consistent with the claim that performance level is a moderator variable for gender and handedness differences in cognitive functioning (Harshman et al., 1983). Figures 1, 2, and 3 depict these interactions with the visuospatial and verbosequential composite scores, and the cognitive profile, respectively. It appears from the graphs that the high functioning right and nonright handed males are the main source of significant interactions. High cognitive functioning, nonright handed males performed verbosequential tasks more effectively and visuospatial tasks less effectively than high functioning, right handed males. Differences between right and nonright handed low cognitive functioning males were in the same direction, but tended to be considerably smaller. For high cognitive functioning females, this pattern was reversed. Nonright handed females were slightly better on the visuospatial tasks but slightly worse on verbosequential tasks than right handed females. By contrast, low cognitive functioning nonright handed females outperformed right handers on verbosequential tasks but were outperformed by right handers on visuospatial tasks. These results partially support the conclusion (Harshman et al., 1983) that the Gender x Handedness interaction pattern for high cognitive functioning persons is the reverse of the pattern

GENDER,

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55

for low cognitive functioning persons. The present study shows this reversal only for the groups of female subjects among whom, in any case, differences tended to be small. This effect of performance level as a moderator variable did not appear for any of the younger samples. The only other statistically significant three-way interaction for a composite score appeared for the 15- to 1% year-old adolescents for the composite visuospatial scores. The main source of the interaction seems to be that right handed males outperformed nonright handed males on visuospatial tasks only for the low cognitive functioning males. By contrast, the main source of significant differences in the adult group appeared in the high cognitive functioning males. DISCUSSION Structural

Analysis

of Cognitive

Function

This study examined individual differences among male and female, right and left handers in specialized cognitive tasks that are theoretically associated with the right and left hemispheres. A compilation of these tasks (the Cognitive Laterality Battery) was used to compare the structure of cognitive processes of male subjects to that of female subjects in an effort to determine whether males and females, as a group, had different modes of information processing. The interactions between gender and handedness were also related to the possible moderating influence of performance level (Harshman et al., 1983). Factor analyses of the performance of the eight subtests of the CLB carried out on male and female subjects, separately, at each of four age levels, indicated that the cognitive activity of males and that of females could be characterized by the same two independent dimensions of information processing. One dimension is indexed by tests of perception, and memory of sequences and by verbal production (termed: verbosequential); the other dimension is indexed by tests of mental rotation, point localization, and visual closure (termed visuospatial). A popular explanation for gender differences in performances on specific cognitive tasks is that for females, verbosequential information processing is less structurally lateralized in the brain than it is for males. The term, structurally lateralized, is intentionally used here to suggest that one hemisphere is neuroanatomically or biochemically organized to carry out a particular cognitive function more effectively than the opposite hemisphere. Reduced lateralization would imply that both hemispheres are structurally more equivalent. By theoretically characterizing a brain system to be “less structurally lateralized,” it could imply that the cognitive processes associated either with the left or with the right hemisphere would predominate (Levy, 1976), thus limiting the cognitive

56

GORDON

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functioning to one system. The above arguments, in turn, could lead to the hypothesis that the cognitive structure formed by these cognitive processes should be primarily a single factor for brain systems that are less structurally lateralized, and more complex (at least two factors) for brain systems that are more lateralized. Specifically, according to this hypothesis, males should have separate factors corresponding to their laterally separate brain functioning, while females should have a more unidimensional factor structure since the same cerebral structures would be employed for both visuospatial and verbosequential tasks. Without referring to the claim that the cerebral hemispheres are less structurally asymmetric for females than for males, the argument that verbosequential processing pervades the female cognitive structure has been used to explain the relatively poorer performance of spatial tasks by females. Females would carry out spatial tasks with an inefficient use of their superior verbal skills (Sherman, 1978). While support for the hypothesis that the female cognitive structure can be described by a single (verbal) factor has been provided by a relatively small study (Wormack, 1980), the results of the present study do not show a difference in factor patterns between males and females. Across the age groupsmore than 1000 subjects in all-the factor structure was stable between the genders. In fact, there is a lack of any kind of generally accepted and fully articulated structural model of cerebral laterality (Allen, 1983) that would provide a basis for the chain of assumptions necessary to link empirical data regarding gender differences in performance of cognitive tasks to a hypothetical structure of laterafization for these tasks. The data do not support it either. The reduced laterality for females relative to that for males between visual fields in tachistoscopic studies or between ears in dichotic studies is often weak and sometimes not found and therefore cannot explain the differences in performance level of cognitive tasks between males and females. In contrast, reduced laterality for nonright handers is robust, yet performance differences are considerably weaker. While a Gender x Handedness interaction might help explain this paradox, it is clear that laterality is insufficient to explain individual differences in performance. Quantitative Differences in Cognitive Functioning An alternate model must be posited that accounts for performance differences in cognitive functioning, yet does not depend on structural differences in laterality. In other words, gender differences in the performance of specific CLB tasks uncovered by this study must be consistent with this study’s finding that the CLB’s factor structure is the same for both males and females. It can be noted that the specific cognitive tasks on which the male subjects outperform the female subjects,

GENDER,

HANDEDNESS,

PERFORMANCE,

AND COGNITION

57

and the cognitive tasks on which the females outperform males, tended to have the highest loadings on the factors with which they were primarily associated. That is, for most of the age samples, the word production tasks (female superiority) had the highest loading on the verbosequential factor. And, in a slightly less consistent manner, the measures of mental rotation (male superiority) had the highest loadings on the visuospatial factor. It is significant that the two-factor structure of the CLB subtests was relatively uninfluenced by age. However, it is also true that the performance differences that did emerge between the males and females (as well as between the right handed and left handed subjects) accentuated with age. For the gender differences, this trend was especially clear cut. Two-dimensional Orientation, a CLB visuospatial subtest, was the only CLB subtest to differentiate between the performance of the prepuberty (9- to 1l-year-old) males and females at a traditional level of statistical significance. Differences between male and female subjects on the Letter and Category Word Production subtests only reached borderline significance (p < -10) at the younger age, but increased to traditional levels of significance by adolescence. In addition to the word production tests, the three-dimensional Orientation and Touching Blocks subtests significantly differentiated between male and female performance for the old adolescent (15- to 1&year-old) sample and the adult (20- to 65-year-old) sample. Thus, not only did the magnitude of differences between the genders on verbal production and mental rotation tasks increase with age, but the number of cognitive tasks that produced these statistically significant differences also increased. The finding that gender differences in verbal and spatial abilities emerge between the ages of 11 and 12 is not new (Maccoby & Jacklin, 1974). Emergence of these differences after puberty has served as evidence for neuropsychological theories that attribute gender differences to rate of maturation (Waber, 1977) or to the direct effects of sex hormones on cognition (Hampson & Kimura, 1988). However, the evidence for the biological model is not conclusive. Counterclaims associate gender-linked cognitive differences in adolescence to the internalization of stereotypes of male and female ability patterns whose expression is triggered by the process of physical maturation. Fine-grained studies of the performance of verbal production and mental rotation tasks by male and female subjects at short intervals that span the prepuberty (at about 11 years) to puberty (at 14 years) gap, and relate these cognitive performances to subjects’ physical maturation and gender stereotypes (Newcombe & Bandura, 1983), might provide grounds for weighing the relative contribution of biological and social factors to the impact of puberty on gender-linked differences in cognition.

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Interaction as a Key to Gender and Handedness Differences The independent variables, handedness and level-of-performance, failed to clarify the nature of gender differences in cognitive functioning. Inconsistent findings might be expected across studies due to sampling and methodological differences. But inconsistent findings have also been found among the present subgroups even when tested under similar conditions and with the same battery of tests. Although less pervasive, the statistically significant handedness differences paralleled the differences between male and female performance on the subtests of the CLB. Right handed subjects performed visuospatial tasks more effectively than nonright handed subjects, whereas nonright handed subjects performed verbosequential tasks more effectively than the right handed subjects. The finding that nonright handed subjects performed cognitive tasks in a pattern similar to that of female subjects has led to the same theoretical contention that both the female brain and the brain of nonright handed persons are less lateralized for information processing. But there were inconsistencies. For example, the female-nonright handed pattern was not seen in the 15- to 18-year-old subjects. In this age group, right handed subjects received higher scores on the verbosequential composite scale and in particular, on the Serial Sounds Test. The fewer and less consistent findings of the present study regarding handedness differences in cognition keep open the question as to the extent, strength, and nature of the mutual influence of gender and handedness as variables for individual differences in specialized cognitive function. Three-way interactions between gender, handedness, and level-of-performance also produced more questions than answers. The present analyses of a relatively large amount of data were motivated by research which suggested that such interactions could provide a consistent set of results upon which to base an understanding of the influence of gender and handedness on lateralized information processing (Harshman et al., 1983). On the positive side, there was a statistically significant interaction for the adult group that partially replicated past results. Nonright handed adult males outperformed right handed adult males on verbosequential tasks and did less well than right handed males on visuospatial tasks. Females reversed the trend. Two-way and three-way interactions were also found for certain of the other age groups. However, the patterns of these interactions were inconsistent with each other and with the pattern of the interaction for the adult subjects. Part of the problem is reduced statistical power due to small cell sizes. At best, it can be said that level-of-performance moderates the relation between gender, handedness, and lateralized information processing, for adult, higher functioning subjects. Questions about gender and handedness differences among younger and lower performing subjects still need to be resolved.

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Conclusion The present multidimensional investigation of specialized cognitive function has resolved some of the ambiguities regarding gender and handedness differences. Gender differences definitely occur for certain tests of cognitive functioning, but these tend to be quantitative rather than different in cognitive structure. In other words, cognitive functioning of both males and females is characterized by a verbosequential dimension and a visuospatial dimension. Word production tasks most clearly represent the verbosequential dimension in the factor structures of both genders; tasks requiring the mental rotation of two- and three-dimensional figures most clearly represent the visuospatial dimension for both genders. Furthermore, these dimensions are present in prepubertal subjects long before gender differences in performance level emerge. Only as subjects approach and pass through puberty are there consistent gender differences in level of performance. For future studies, this research has, in the first place, identified the verbosequential and visuospatial cognitive tasks that are especially sensitive to male-female and right-nonright handed cognitive differences. Future studies of these differences would increase their efficiency if they focused on tasks of this type because the tasks, themselves, derive their validation from cognitive processes usually associated with left and right hemisphere functioning. Secondly, the present study’s results suggest that the gender and handedness differences in the performance of specialized cognitive tasks tend to be developmental. Since the biological and social bases of gender and handedness differences in cognition have not been resolved, research would benefit by focussing on those biological and social parameters that are likely to influence the development of proficiency with verbosequential and visuospatial cognitive tasks. By sticking to these brain-validated tasks, models of cognition could be designed that could be used to assess social, environmental, as well as neurobiological components of cognitive processing and human development. With any luck, the basis of gender, handedness, and level-ofperformance factors in cognitive functioning could be illuminated as well. REFERENCES Allen, M. J., & Cholet, M. E. 1978. Strength of association between sex and field dependence. Perceptual and Motor Skills, 41, 419-421. Allen, M. J., & Hogeland, R. 1978. Spatial problem-solving strategies as functions of sex. Perceptual

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