Familial handedness and its relation to spatial ability following strategy instructions

INTELLIGENCE 10, 389-406 (1986)

Familial Handedness and Its Relation to Spatial Ability following

Strategy Instructions M. B~.TH CASEY

M A R Y M . B R A B E C K

LARRY H . L U D L O W

Boston College

This study compared subjects from fight-handed families with subjects from nonright- handed families in their ability to solve a mental-rotation task when instructed to use one of two different spatial strategies. All subjects completed a pretest Vandenberg. Next, one of the following procedures was presented prior to administering the Vandenberg posttest: Group 1 was given mental-rotation instructions, Group 2 was given spatial-orientation instructions, and Group 3 (control group) was given no special directions. For familial right-handers, no condition effects were found. In contrast, familial nonright-handers benefited significantly from mental-rotation instructions when compared both to their own control group and to familial right-handed subjects given the same instructions. However, with orientation instructions, the familial nonright-handers showed significantly less posttest improvement than their control group. These results suggest familial nonright-handers may be stronger in ability to use one spatial strategy, transformation of mental images, and weaker on a second, reorientation in relation to left-right cues. The educational and research implications of these findings are discussed.

The present study was designed to look at the differential effects of subjects' family-handedness patterns on two types of spatial abilities. In order to unravel the contradictory literature on handedness and spatial ability, this study examined subjects' handedness from the framework of family handedness based on Annett's (1972) genetic model. Thus, in this study, subjects were divided into two groups. A familial nonright-handed group consisted of individuals from families in which one or more immediate family members were either left- handed or ambidextrous. A familial right-handed group consisted of individuals from families with all right-handed immediate family members.

Secondly, in order to understand the relationship between specific spatial abilities and family handedness, two types of spatial abilities were analyzed.

Correspondence and requests for reprints should be sent to the above authors, Campion Hall, Boston College, Chestnut Hill, MA 02167. A shorter version of this paper was presented at the annual meeting of the American Educational Research Association, Chicago, IL, 1985. This research was partially funded by a Boston College Faculty Research Grant.

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390 CASEY, BRABECK, AND LUDLOW

Previous research has revealed two major spatial factors. First, spatial visualization requires the ability to mentally transform objects, for example, to mentally rotate a three-dimensional, pictorally represented object. Second, spatial orientation involves the ability to shift spatial perspective, often including the discrimination of left-right cues. In the present study, experimental subjects were instructed to use either a spatial-orientation strategy or a spatial-visualization strategy when solving the Vandenberg Test of Mental Rotation (Vandenberg & Kruse, 1978). Control subjects were given no special instructions. This design is based on the theoretical rationale (Annett, 1972) that family-handedness patterns reflect hereditary influences on cerebral organization. These influences affect the ability to utilize different types of spatial strategies.

Finally, both the subjects' spontaneous and instructed performance were examined. This was done to determine whether subjects have the capacity to show improvement in their performance when given instruction to use a particular strategy (spatial visualization or spatial orientation). Generally, in most studies of spatial ability no direct instructions are given to use a particular strategy. Consequently, it is often not clear whether subjects are incapable of using a strategy and performing well on a task or whether they simply have not considered using such a strategy.

The three issues summarized here and examined in this study----(1) familial right-handedness versus familial nonright-handedness, (2) spatial visualization versus orientation ability, and (3) spontaneous performance versus instructed capacity to use a strategy--are discussed in depth in the next three sections.

Familial Handedness A growing number of researchers attest to the importance of including family handedness in definitions of subject handedness. In fact, a number of important recent studies (Bemporad & Kinsbourne, 1983; Carter-Saltzman, 1978; Gesch- wind & Behan, 1982) have shown that right-handed relatives of left-handed individuals are more similar in many respects to left-handed individuals than they are to other right-handers. This has been found to be true in such diverse areas as spatial ability, reading, learning disabilities, and autoimmune diseases.

Consequently, it is important to approach the literature on spatial ability and handedness from the prevailing theoretical framework for family handedness. A major theoretician in this field (Annett, 1972) has accounted for behavioral manifestations of family handedness in terms of cerebral specialization for spatial ability. According to Annett's model, the human tendency for right-handedness is the result of a single gene, the right-shift+ gene. When this gene is present on one or both chromosomes, rs+ - or rs+ +, speech is lateralized to the left hemisphere with an accompanying preference for the right hand. However, not all right-handers have this genotype.

Left-handers and an equivalent number of right-handers make up the r s - - genotype. According to Annett, these individuals are missing the right-shift gene, and, thus, chance factors determine whether speech is lateralized to the left

FAMILIAL HANDEDNESS AND SPATIAL ABILITY 391

or right hemisphere. Many of these subjects are ambidextrous to some extent and the right-left dexterity skills of this group show no strong bias to either side (Annett, 1983).

It is proposed in this study that individuals with this genotype would encounter difficulty with those spatial orientation tasks which require the discrimination of left-right cues. Their lack of consistent bias to the right-side may result in a more confused internal sense of left and right. The literature (discussed in the section below on spatial-orientation tasks) supports this prediction.

According to Annett, the most advantageous and common genotype is the rs+ - heterozygous genotype. However, a third genotype, those individuals with a double dose of the right-shift gene's effects (the rs + + homozygous genotype) would be at a disadvantage for some types of spatial tasks. This problem would arise because their language skills would be developed at the expense of vi~uo- spatial skills.

It is proposed in this study that the rs+ + individuals would have particular difficulty with the most abstract and least verbal of the spatial factors, the spatial visualization factor. Therefore, individuals with this genotype would have problems when attempting to make mental transformations on spatial stimuli.

Identifying Individuals with a Particular Genotype. One probelm with An- nett's model is that it is difficult to pinpoint which individuals have which genotype, especially among right-handers. Left-handed and ambidextrous individuals are likely to be r s - - genotypes. However, r s - - right-handers are frequently misclassified and included among the rs+ genotypes. This may ac- count for the considerable confusion in the literature on handedness and spatial ability. Nevertheless, clear predictions can be made about the two family-handedness types. Those individuals from families with nonright-handed members have the r s - - genotype within their families. Therefore, they would have a higher probability of being an r s - - genotype and a lower probability of being an rs+ + genotype than those individuals who come from families without any left-handed or ambidextrous family members.

It is proposed here that the least confounded general predictions would come from identifying an individual's family handedness rather than through identifying his or her own handedness. It is predicted in this study that the familial nonright-handed subjects would have the most difficulty with a spatial orientation approach requiring discrimination of left-right cues, because there would be more r s - ~ genotypes among this group, whereas the familial right-handed subjects would have the most difficulty with a spatial visualization approach, because there would be more rs+ + genotypes among this group. (Note: Individ- uals with the rs+ - genotype would be found among both types of families and would have no particular difficulty with either type of spatial strategy.)

Measurement of Handedness and Family Handedness. There are differences in the literature with respect to how handedness is defined and mea-


sured. Some researchers use self-report, others use a standardized test such as the Annett Handedness Scale or the Edinburgh Handedness Inventory. Some researchers define handedness through these standardized tests, but include only the extreme right- and left-handed subjects; others include a middle group la- beled ambidextrous. These problems clearly lead to discrepancies in findings.

Consistent with Annett's model, subjects' handedness in this study was identified as right, left, or ambidextrous. This was defined by the Edinburgh Handed- ness Inventory (Oldfield, 1971). Familial handedness was then examined and subjects who were right-handed and had neither left-handed nor ambidextrous immediate family members were classified as familial right-handed subjects. Subjects who were left-handed or ambidextrous, or were from families with one or more nonright-handed (left-handed or ambidextrous) immediate family members were classified as familial nonright-handed subjects.

This approach is unique in including ambidextrous family members in the definition of familial nonright-handed subjects. In most studies, only left-handed relatives have been identified and, by default, ambidextrous family members have usually been classified with right-handed family members. However, An- nett's model suggests that both left-handed and ambidextrous relatives would be likely to have the r s - - genotype. Therefore, in this study, subjects were classified as familial nonright-handers based on both their left-handed and ambidextrous family members.

Definition of Types of Spatial Abilities and Their Relationship to Handedness As indicated earlier, two factors generally are considered to be measured by tests of spatial ability. Although there is not complete agreement on terminology, these factors are usually called spatial visualization and spatial orientation. Spa- tial visualization refers to the ability to mentally manipulate or rotate two- and three-dimensional pictorially represented objects. Embedded in this skill is the ability to visualize or mentally imagine an object that must be then mentally rotated or transformed in some manner. Spatial visualization is associated with the ability for abstract mathematical reasoning.

The second spatial factor, usually called spatial orientation, is the ability to remain unconfused by the changing orientation of self in relation to an object. This ability is associated with map reading and navigating skills. Embedded in spatial orientation is the ability to discriminate left from right and to then orient oneself in relation to these directional cues.

The Vandenberg Test of Mental Rotation (Vandenberg & Kruse, 1978) has been identified as a test of spatial visualization, because it is generally assumed that subjects solve the test through mental rotation. However, researchers have begun to explore individual differences relating to mental rotation tasks (Egan, 1978; Mumaw, Pellegrino, Kail, & Carter, 1984). Componential analyses of spatial abilities suggest that subjects solve items in different ways (Kyllonen,


Woltz, & Lohman, 1981). Some individuals report using a mental-rotation strategy, other subjects report using strategies involving verbal coding of stimuli features (e.g., counting blocks, moving hands, using verbal left/right cues.) (Allen, 1974; Allen & Hogeland, 1978; Freedman & Rovegno, 1981).

Just and Carpenter (1985) have recently noted that the Vandenberg test has the interesting characteristic that it can be solved by either a spatial visualization strategy, for example, mental rotation, or by an orientation strategy, for example, reorientation of self in terms of left and right. This flexibility means that the Vandenberg can be solved either through a strategy which allows subjects to avoid making left-right discriminations (by using a mental-rotation strategy) or through a strategy which allows them to avoid making mental transformations of images (by using a spatial-orientation strategy). In the past, these two spatial factors have been examined on separate tasks. Thus, the Vandenberg test is a useful instrument for evaluating the respective patterns of spatial abilities among familial right-handed and nonright-handed subjects.

Literature Review. The prevailing view has been that nonright-handed subjects do poorly on both spatial-visualization and orientation tasks (see review by Bemporad & Kinsbourne, 1983). However, it is proposed here that the deficit in performance of nonright-handed subjects may be restricted to those spatial- orientation tasks which require left-right discrimination. Researchers have con- sistently found inferior spatial performance by left-handed subjects on these types of tasks (Harris & Gitterman, 1978; James, Mefferd, & Wirland, 1967; Silverman, Adevai, & McGough, 1966). As mentioned earlier, Annett (1983) found evidence that the left-right dexterity skills of left-handed and ambidextrous individuals are not biased strongly favoring either hand. In contrast, many right- handers show a right-side advantage. Therefore, nonright-handedness in these individuals may be reflecting a weak left-right internal sense, leading to difficulties in discriminating left and right. Porac and Coren (1981) make an argument for this position in their book on lateral preference and behavior.

The literature on spatial visualization tasks and handedness is less consistent than the findings on spatial orientation tasks. Freedman and Rovegno (1981) found that right-handed subjects did better than left-handed subjects on the Vandenberg Test of Mental Rotation. However, one problem in this study is that ambidextrous subjects may have been included in the sample of right-handers, given Freedman and Rovegno's lenient criterion for defining right-handedness. Johnson and Harley (1980) found no differences between ambidextrous and right-handed subjects on a mental-rotation task, but left-handers did worse than right-handers. One study using the National Institute of Industrial Psychology Form Relations Group Test (Miller, 1971) found that ambidextrous subjects performed worse than right-handed subjects on a shape-matching task. However, this task fits the criterion as a spatial-visualization task only for the more difficult items where some of the choices involved two- and three-dimensional rotations.


In contrast to the above findings, there are five studies which found nonright- handed subjects outperforming right-handed subjects on spatial-visualization tasks. Two studies showed ambidextrous subjects outperforming right-handers (Burnett, Lane, & Dratt, 1982; McGee, 1978), and two showed left-handers outperforming right-handers (Hermann & Van Dyke, 1978; Porac & Coren, 1981). Finally, Porac and Coren (1981) found individuals with left-sided, mixed responses and crossed patterns in hand and foot preference outperforming the other groups.Furthermore, there is consistent evidence in the literature of a higher proportion of nonright-handed individuals in majors and careers which depend on good spatial visualization, for example, artists, architects, and mathematicians (Mebert & Michel, 1980; Peterson & Lansky, 1974; Porac & Coren, 1981).

The research on handedness and spatial ability suggest two patterns of results. First, left-handed subjects do not perform as well as right-handed subjects on tests requiring left-right discrimination. Second, despite inconsistencies, the weight of evidence indicates that right-handed subjects do more poorly than left- handed and ambidextrous subjects on tasks requiring mental rotation.

The issue of familial handedness has not been addressed in the above studies on spatial ability. This may have led to two problems. First, if Annett's model is correct, the right-handed groups included subjects of all three genotypes, rs+ +, rs + - , and r s - - . Depending on the proportion of these different genotypes, the method of classification would lead to the confounded findings described earlier. Also, a number of researchers (Carter-Saltzman, 1978; Satz, 1972) have argued that left-handers with no other left-handed family members may be individuals who switched to their left hand because of brain damage to the left hemisphere. Thus, the findings may be distorted further when this subpopulation of left- handers is included with the familial left-handers.

The Effect of Instructions on Spatial Strategies A final issue to consider is that the spatial-ability studies usually provide no instructions on the strategy to use in solving, these tsts. Thus, the tests are tapping spontaneous use of spatial strategies, a metacognitive choice (Katz, 1983), rather than the capacity to use such a strategy. It has been documented that subjects spontaneously use a wide range of strategies on the Vandenberg test, in addition to the mental rotation and orientation strategies. For example, Freedman and Rovegno (1981) found some subjects counted blocks, others thought in words, used their hands as aids, and, of course, sometimes subjects simply guessed. Furthermore, subjects frequently shifted their strategies from trial to trial, depending on the item. Thus, in order to assess a subject's capacity to solve the Vandenberg using a mental-rotation or an orientation strategy, it is necessary to clearly instruct them on what they are to do. Even if differences do not always show up between familial-handedness groups when assessing spontaneous use of a strategy, there may be a difference in the actual capacity to put different spatial


strategies into practice, which would be revealed only after instructions to use a particular strategy (Campione, Brown, & Bryant, 1984; Casey, 1984; Casey, 1986).

This study was designed to first test the spontaneous pretest performance of familial right-handed and familial nonright-handed groups on the Vandenberg Test. Next, following the pretest, different groups were given simple instructions for the posttest to use either: (1) a mental-rotation strategy, (2) an orientation strategy or (3) no special directions (control group). Familial nonright-handed subjects may have better potential to use spatial-visualization strategies than the familial right-handers, but they may not use this capacity spontaneously. There- fore, they would not be expected to differ from familial right-handed subjects on the pretest, but to improve relative to the familial right-handers and to their own control group when instructed to use a mental-rotation strategy. However if the familial nonright-handed subjects have poor spatial-orientation abilities as suggested by the literature, they should do worse than the controls and the familial right-handed subjects when given the orientation instructions. Thus, the study was designed to tease out the capacity of familial right-handed and nonright- handed subjects to use two types of spatial strategies on the same spatial task.

METHODS

Subjects One hundred and nineteen paid volunteers (47 males and 72 females) were randomly assigned to two experimental and one control group. A total sample size of 108 was suggested by a pilot-study power analysis (Cohen, 1977; d - 1.0, al, = .05, 1 - 13 - .8). There was a total of 55 familial right-handed subjects and 64 familial nonright-handed subjects. Among the familial right-handers were 15 in the mental-rotation group (8 females, 7 males), 15 in the orientation group (9 females, 6 males), and 25 in the control group (13 females, 12 males). Among the familial nonright-handed subjects were 16 in the mental-rotation group (11 females, 5 males), 26 in the orientation group (18 females, 8 males), and 22 in the control group (13 females, 9 males). Subjects were randomly assigned to conditions. Unequal sample sizes resulted from the uneven number of subjects available during each group testing session.

Measures The Edinburgh Handedness Inventory (Oldfield, 1971) consists of 10 questions about the hand preferred for writing, drawing, throwing, scissors, toothbrush, knife (without fork), spoon, broom (upper hand), striking match (match), and opening lid (lid). Subjects are asked to indicate whether they have a strong preference, ( + + in the appropriate column) moderate preference (+ in the appropriate column), or no preference at all (+ in each of the two columns) regarding which hand they use for the 10 activities. Scores are obtained by


subtracting the number of pluses in the left column from the number of pluses in the fight, dividing by the total pluses and multiplying by 100. Scores can range from -100 (strongly left-handed) to +100 (strongly fight-handed). Based on findings by Burnett et al. (1982), a score of above +40 was used to identify right- handed subjects. A score of below - 4 0 was used to identify left-handed subjects, and ambidextrous subjects were designated from +40 to - 4 0 . A self- report measure of handedness was used in place of a behavioral measure, since it has been shown that overall concordance on the two types of assessment measures tends to be above 90% (Porac & Coren, 1981).

In the Family Handedness Questionnaire subjects were asked to categorize members of their immediate family in terms of left-handed members, right- handed members, and ambidextrous members (able to use both hands with ease). They were asked to base their responses on the tasks presented in the Edinburgh questionnaire which they had completed.

The Vandenberg Test of Mental Rotation requires the correct matching of two out of four choices to a standard similar to the figures originally designed by Shepard and Metzler (1971). The standard is presented on the left with the four choices on the right. The distractor stimuli vary from the standard either as mirror images or in terms of slight variations in the features of the standard (see Figure 1 for two examples). The test is divided into two parts with 10 items in each part. The two parts do not vary greatly and are simply different versions of the same test. The test is timed with a total of 5 min allowed to complete each

Item type A: Items with feature dlstractors

@ @@ Correct answers: The second and fourth

Item type B:

@ Correct answers:

Items with mirror-image distractors

The first and fourth

FIG. 1. Sample Items from the Vandenberg Test


part. In the present study, a linear transformation of the original scoring procedure was employed. That is, no points were taken off for the incorrect choices, whereas each correct choice received one point for a total of 40 possible points. Also, in this study, 10 versions of the test were constructed with the order of the 20 items randomly ordered to control for order effects resulting from the timed nature of the administrations. Subjects were randomly assigned each version and were given different versions for the pretest and posttest.

Procedure The one-hour experimental session consisted of four parts:

Part I: All subjects completed the Edinburgh Test of Handedness followed by a Family Handedness Questionnaire.

Part H: All subjects in each group were given the Vandenberg Test of Mental Rotation.

Part III: Each of the groups was given different instructions and then readministered a different version of the Vandenberg test. (Note: Subjects in the experimental groups were not given extra training in the use of the strategies presented in the instructions. They were simply directed to apply the strategy on the readministration of the Vandenberg).

Part IV: All subjects completed a report of the strategies they used and a biographical questionnaire.

Instructions

In the mental-rotation instruction group, the subjects were shown models of a standard and of choice stimuli and instructed on the items in the test, with a careful demonstration of this (mental-rotation) strategy. They .were given the following directions:

In a few minutes we will be asking you to do the task again. Our purpose is to see if performance can be improved by using a particular strategy. Look at the standard and in your mind imagine it rotated (or turned) to a new position, similar to the position of the choice object. Compare this rotated image of the standard against the choice object and check whether all blocks of both objects are in the same location. Let me show you on this model. (Note: During the demonstration the blocks were physically rotated.) I 'm looking at the standard and imagining it rotated to a new position, similar to that of the first choice object. Since all blocks are in the same location, the first choice object is identical to the standard. Now I will imagine the standard rotated to a new position, similar to that of the second choice object. Since some of the blocks for the two objects are in different locations, the second choice object is different from the standards. Let me do it one more time. Please use this mental rotation strategy when I ask you to take the test again.


In the orientation instructions group, instructions on the orientation approach were given, using the same models as in Group 1. A careful demonstration of the orientation strategy was given. Subjects were given the following directions:

In a few minutes we will be asking you to do the task again. Our purpose is to see if performance can be improved by using a particular strategy. Look at the standard and imagine yourself standing in front of some point on the object such as this (pointing at the end of the last block on the top). Then look at the first choice object and imagine yourself standing at the equivalent location. Compare the choice object against the standard and check whether all blocks of both objects are in the same location. Let me show you on this model. I am picking this point here on the standard (use first pointer) and I notice that the arm goes back 3 blocks, down 4 blocks, back 3 blocks, and over to my right three blocks (use the second pointer). Now I place myself at the equivalent point on the first choice object. From this point the object goes back 3 blocks, down 4 blocks, back 3 blocks and over to my right three blocks. Since all blocks are in the same location, the first choice object is identical to the standard. Now I place myself at the equivalent location on the second choice object, (first pointer). From this point the object goes back 3 blocks, down 4 blocks, back 3 blocks, but instead of going to my right, it goes to my left 3 blocks (use second pointer). Since some of the blocks are in different locations, the second choice object is different from the standard. Let me do it one more time. Please use this orientation strategy when I ask you to take the test again.

In the control group, no instructions on strategies were given; the subjects were shown the same models and the prior standardized instructions from the Vandenberg test were simply repeated as in Part I. They were given the following repeated instructions:

This is a test of your ability to look at a drawing of a given object (the standard) and find the same object within a set of dissimilar objects. The only diference between the standard object and the correct choices will be that they are presented at different angles. In a few minutes we will ask you to do the task again. Our purpose is to see if performance can be improved from practice. Let me re-read with you the directions at the bottom of page 2 as you look at this standard and two choice objects typical of the objects on the task.

R E S U L T S

Subgroup Comparisons Because the familial nonright-handed group was composed of several different types of subjects (left-handers [N -- 7], ambidextrous subjects [N = 16], and right-handers with nonright-handed relatives), the first step in the analyses was to determine whether these subgroups differed from one another. A one-way ANOVA comparison was made of these three groups for both the pretest Van- denberg scores and for the adjusted posttest scores with pretest performance


partialled out. No significant or borderline significant differences were obtained for either the pretest or adjusted posttest scores. Therefore, in the rest of the analyses, these subgroups were combined together to form the familial nonright- handed group, a procedure that was consistent with the original theoretical formulation.

Vandenberg Pretest Analysis A 2 × 2 x 3 ANOVA was performed on the first administration of the Vanden- berg test comparing familial handedness (familial right-handers vs. familial nonright-handers), sex (males vs. females), and condition factors (mental-rotation instructions group vs. orientation instructions group vs. control group). A significant main effect of sex was obtained, F (1,107) = 7.44, p = .007, with males scoring a mean of 26.32 and females scoring 22.68. A significant main effect of condition also was obtained, F (2,107) = 3.88, p --- .024. This result was surprising, because the subjects were randomly assigned to groups and were not treated differently on the first administration of the Vandenberg. Using the Neuman-Keuls multiple comparison test for post hoc analyses, it was found that the mental-rotation instructions group (M score = 21.23) performed significantly worse than both the control group (M score = 25.47) and the orientation instructions group (M score = 24.56). No significant difference was found between the control group and the orientation instruction group. In the ANOVA, the family-handedness main effect was neither significant nor borderline. In addition there was no significant or borderline three-way or two-way interaction.

Vandenberg Adjusted Posttest Analyses On the second administration of the Vandenberg, pretest performance on the first Vandenberg administration was used as a covariate to control for the observed differences in performance level prior to instruction. A nonorthogonal Analysis of Covariance adjusted the posttest scores for the initial unequal performance levels and weighted the adjusted posttest mean scores based on the different sample sizes within the individual cells of the design. Thus, these adjusted posttest scores reflected improvement on the posttest relative to the pretest scores. The differences between the adjusted and nonadjusted posttest scores were small. The effect of applying the ANCOVA design was strongest in its reduction of the mean square error term.

A series of a priori planned comparisons (Winer, 1971) was performed using the adjusted means from the 2 x 2 × 3 ANCOVA (sex [males vs. females] by familial handedness [nonright-handed vs. right-handed] by condition [mental- rotation instructions group vs. orientation instructions group vs. control group]. In the context of the present research, the seven planned comparisons were more important than the results of the global ANCOVA. That is, these directional comparisons were proposed to test comparisons previously derived from the


literature and from Annet t ' s model. The sole function for A N C O V A was, therefore, to obtain posttest performance scores adjusted for initial unequal performance levels. As expected, the effect of the pretest covariate was significant

(F = 161.78, p < .001) and homogeneity of regression was observed. In the A N C O V A , the sex difference was no longer significant. This occured

because the initial sex differences on the pretest were partialled out. However , the difference between familial r ight-handed subjects (M = 28.63) and familial nonright-handed subjects (M = 30.58) was significant, F (1,106) = 4.48, p

(one-tailed) = .025. The main effect of condition was significant as well, F (1,106) = 9.18, p (one-tailed) < .001, with the following adjusted means for the mental rotation, control and orientation instruction groups respectively (M = 31.85, M = 29.96, M = 26.97).

As stated, the major focus of the posttest analyses consisted of a series o f seven planned comparisons of the effect of instructions on the two familial handedness groups (see Table ' l ) . Given the number of comparisons performed, the modified Bonferroni test (Keppel, 1982) was used to set the per comparison one-tailed error rate. For the familial ly right-handed subjects, no significant difference was obtained between either the mental-rotation instructions group and the control group or between the orientation instructions group and the control group.

For the familial nonright-handed subjects, however, a significant difference was obtained between the mental-rotation instructions group and the control group, F (1,106) = 5.45, p (one-tailed) < .05. The adjusted mean score of the

TABLE 1 Adjusted Posttest Mean Scores on the Vandenberg Test of Mental Rotation as a Function of

Instructions and Family Handedness

Group Condition

Mental-rotation Orientation Instruction Control Instruction

Familial Handedness M SD M SD M SD

Familial right- handers 29.23" 4.67 29.27 5.59 27.39 5.33

Familial nonright- handers 34.45 '~,c 4.43 30.62 ".b 3.68 26.63 b 5.30

Note: A modified Bonferroni test was used to adjust significance levels, given the number of analyses performed.

"F (1,106) = 5.45, p (one-tailed) <.05 bF (1,106) -~ 5.97, p (one-tailed) <.05 cF (1,106) = 10.18, p (one-tailed) <.01


mental rotation group was 34.45, while the control group mean was 30.62. In contrast, the adjusted mean for the orientation instruction group ( M = 26.63) was significantly lower than the control group, (M = 30.62), F (1,106) = 5.97, p (one-tailed) < .05.

Next, the two types of familial-handedness subjects were compared separately for each instruction group on the adjusted posttest Vandenberg scores. The familial right-handers did not differ from the nonfight-handers for either the control group, or the orientation group. In contrast, for the mental-rotation group the familial nonright-handed subjects showed a higher adjusted mean score (M = 34.45) than the familial right-handed subjects (M = 29.23), F (1,106) = 10.18, p (one-tailed) < .01.

The literature suggests that there may be critical differences between left- handed subjects and other familial nonright-handers, and between left-handed subjects with only right-handed family members versus left-handed subjects with other nonright-handed family members. T o provide more information about these types of subjects within the familial nonright-handed group, the above series of seven planned comparison analyses were repeated on the adjusted posttest Vandenberg scores with different subsets of subjects removed. First, the 7 left-handed subjects were omitted from the familial nonright-handed group. When these left-handed subjects were eliminated from the analyses, no change in results occurred. Thus, given the small number of left-handers, the present findings are most relevant to ambidextrous individuals and right-handed individuals with nonright-handed relatives.

Next, the seven planned comparisons were repeated, this time omitting the 4 subjects who were left-handed and had no left-handed nor ambidextrous relatives. As indicated earlier, this type of subject has been singled out as having a possible pathological basis for his or her left-handedness. Interestingly enough, when these subjects were removed from the familial nonright-handed group, the planned comparison between the mental rotation (M = 34.93) and the control group (M = 30.73) for the familial nonright-handers showed a slight increase in significance level, as did the planned comparison between the orientation group (M = 26.53) and the control group. No other changes occurred for the other planned comparisons.

Self-Report on Use of Instructions There is evidence that the instructions were effectively presented. At the end of the testing, subjects were asked whether they were able to use the suggested strategy on all items, or had to use another strategy on some of the times, on most of the items, or on all of the items. With the exception of the familial nonright- handed orientation group, the majority of subjects in the experimental groups (80% to 87%) reported that they were able to follow instructions most or all of the time. In contrast, 38% of the familial nonright-handed orientation group


reported that they reverted back to using another strategy most or all of the time. These strategies included rotating the whole or part of the object, counting blocks, and scanning the choices for shape differences.

DISCUSSION

The present results indicate that familial nonright-handed subjects benefitted most from mental-rotation instructions as compared with simple practice (the control group). However, orientation instructions were even less effective for this type of subject than practice alone. For the familial right-handed subjects, no effect of instructions was found. The special sensitix;ity of the familial nonright- handed subjects given these two types of spatial instructions may help to clarify the confusion in the literature on the spatial abilities of nonright-handers. Thus, this particular group of individuals may be especially strong in one spatial capacity, the ability to transform mental images, and weak in their ability to reorient themselves in relation to left-right cues.

However, it should be noted that, despite the relatively poor performance of those familial nonright-handers given orientation instructions, they did not do significantly worse than the familial right-handed orientation group. There may be a number of explanations for this. One obvious one is that some familial nonright-handed subjects may have reverted back to using a previous strategy when they encountered difficulty in trying to use an orientation strategy. The self-report data on use of instructions suggests this may have been a factor. The question still remains, why did the familial right-handed subjects not benefit from orientation instructions? It seems that the orientation spatial strategy was no more effective than the strategies which they were using spontaneously. Perhaps no spatial strategy would be completely effective for a sample which contains the highly verbally dependent rs+ + subjects.

The present findings are consistent with Annett's model of family handedness. The group which should contain a higher proportion of individuals with the rs+ + genotype did not benefit from mental-rotation instructions, whereas the group which should contain a higher proportion of individuals with the r s - - genotype did benefit from these instructions. Yet the latter group had difficulties with the orientation instructions which require a left-right discrimination.

If individuals with the rs+ + genotype have spatial problems, as Annett suggests, then they should be at a disadvantage when required to use spatial strategies. Annett (Annett & Kilshaw, 1982) also found evidence for fewer rs+ + genotypes among mathematicians. Mathematics has been associated with good spatial-visualization ability (Sherman, 1983). Individuals with the r s - - genotype may simply have problems with left and right. In the present study, familial nonright-handers definitely experienced problems when trying to imple- ment a strategy requiring a left-right discrimination.

It should n0t be ignored that, in the present study, these family-handedness


differences occurred only after instructions. This suggests that, although familial nonright-handed individuals have the potential to do mental transformations on visual images, they do not necessarily spontaneously use this ability when performing a spatial task. Thus, in understanding the relationship among cerebral lateralization, handedness, and spatial abilities it is necessary not only to ask what type of ability is being tapped, that is, visualization or orientation, but also to ask what kind of strategy or strategies are spontaneously applied to a task. The distinction between metacognitive choices and the potential or capacity to use a particular strategy needs to be addressed in greater depth in future research. As a case in point, Ashton, McFarland, Walsh and White (1978) found that high imagers did better on a mental-rotation task only after instructions to mentally rotate. Such findings suggest that in doing all research of this type, the critical importance of instructions must be considered carefully.

It is also of interest to note that the sex differences and familial-handedness differences on the Vandenberg appear to relate to very different aspects of abilities on the test. Sex differences showed up on spontaneous, noninstructed performance on the Vandenberg, but when these initial differences in abilities were controlled for, there were no sex differences in improvement in scores on the posttest. Also, sex differences were not influenced by the type of instructions given either in terms of an increase in a male advantage or reducing this advantage. In sharp contrast, the family-handedness differences did not emerge until the posttest when subjects were differentially primed to use a particular strategy. This is especially relevant because a connection has been made between women's poor performance on spatial tasks and the alleged poor performance of left-handed individuals on these tasks. The proposed similarity was explained in terms of a reduction in cerebral lateralization among these two groups (Levy & Reid, 1978). The present results suggest that the pattern of responses on spatial tasks are sharply different when considering sex differences from the pattern of responses when familial-handedness differences are considered.

Finally, it is important to address a problem in the pretest data. A condition effect was obtained despite the random assignment of subjects to conditions on the first Vandenberg test, prior to instructions. It might be argued that the significant improvement by the posttest mental-rotation group was due to a ceiling effect for the control group. In other words, the control group may have had lower adjusted posttest scores because they started at a higher level initially and therefore did not have as much room for improvement. If that were true, a restricted range of scores would be expected for this group. However, homogeneity of variance was tested and obtained for the familial nonright-handers when comparing the mental-rotation group with the control group.

A second possibility is that the familial nonright-handed mental-rotation subjects were simply rising to some optimal level of performance which would be reached by all subjects given practice. However, this is unlikely, because the two family-handedness mental-rotation groups did not differ on the initial pretest and


yet the familial nonright-handed group showed more improvement than the familial right-handed group.

Implications of This Research There has been the perception that left-handed individuals are different from other people in their spatial abilities, but the research findings demonstrating this have been difficult to replicate. It is conceivable that part of this difficulty may occur because left-handed individuals represent only one member of a family type which includes ambidextrous and some right-handed members as well. Thus, prior research has often compared left-handers with others who may be similar in thinking patterns and abilities and who should have been classified with them rather than being grouped with right-handers from right-handed families.

The present results suggest nonright-handed family members may have both strengths and weaknesses within the domain of spatial abilities. The educational implications of such findings are numerous. First of all, problems with left and right would predispose familially nonright-handed children to encounter difficulties at the outset of reading instruction (e.g., confusion of d vs. b and reversal of letters and numbers). There is, in fact, evidence that familial left-handedness, though not a characteristic of all dyslexic conditions, may characterize one subtype of this disorder (Bemporad & Kinsbourne, 1983). A closer evaluation of ambidextrous and right-handed children from nonright-handed families might help to clarify the conflicting results in the field of dyslexia research.

Conversely, the analysis of strengths is also useful. If familially nonright- handed children do have greater spatial-visualization abilities, this clearly should be attended to by teachers. As a case in point, it is clear that, in this study, the subgroup of individuals from nonright-handed families did not spontaneously use their mental-rotation abilities under normal testing conditions in the pretest. Instead, they required specific instructions to actually make use of their ability to use this strategy on the test. This group was by no means a small minority. They made up 54% (64 out of 119) of the subject population. In conclusion, there may be many ways in which this subgroup can develop their potential once researchers have identified those components which are unique in their thinking processes.

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Familial handedness and its relation to spatial ability following strategy instructions