Cognitive performance and left-handedness: comparative analyses in adults with seizures, physical, psychological and learning disorders in a rehabilitation setting.

In western societies today, about 90% of the adult population is said to be right-handed, with the remaining 10% consisting of persons variably identified as left-handed, ambidextrous, and/or ambiguously handed (McManus, 2002; Soper, Satz, Orsini, Henry, Zvi, & Schulman, 1986). Though widely cited, this 9:1 demographic ratio appears not to be as ubiquitous or generalizable as commonly thought. For example, some studies have revealed smaller numbers of left-handers with advanced age (Coren, 1992; Ellis, Ellis, & Marshall, 1988), more left-handers today than just a century ago (Fleminger, Dalton, & Standage, 1977; McManus, 2002), and lower incidences of left-handedness in several prominent nonwestern societies, like India and Japan; where left-handed rates of 5.4% and 4.8% have respectively been reported (Singh & Bryden, 1994).

The frequency of right and left-handedness is also quite different among a variety of groups of persons with disabilities. As early as 1920, Gordon reported that roughly two and one-half times as many children with mental retardation in the London school system were left-handed compared to a control group of children in schools for the non-mentally retarded (Gordon, 1920). Since then, this finding that has been echoed many times and confirms not only a relationship between the increased occurrences of left-handedness in persons with mental retardation, but also a relationship with the degree of mental retardation (e.g. Bishop, 1990; Bradshaw-McAnulty, Hicks, & Kinsbourne, 1984). Proportionally, there are greater numbers of left-handers encountered with increasing retardation severity.

Historically, many generalizations and stereotypes have been held about people who are left-handed. In English, the word "left" actually derived from the Anglo-Saxon word "lyft" meaning "weak or useless". In German the word for left-handed is "linkisch" which evidently also carries a definition of "maladroit or clumsy", a definition still listed in many American Dictionaries. In Italian, a left-hander is "mancino", derived from the term "maimed" and can also be used to refer to being "dishonest" or "deceitful" (Coren, 1992). The original Latin word for left, refers to sinister, an ominous word presaging trouble or disaster and also meaning "unlucky."

Negative attitudes and beliefs about left-handedness continue to be reflected in everyday life and can still be seen in modern language use. For example, a left-handed compliment is often less than complimentary, someone's "right-hand man" is typically a highly trusted servant or colleague, and a legal oath is taken with the right hand raised, a left-handed oath being considered non-binding (McManus, 2002). The history of education also affords a clear example of bias against left-handedness. In the United States, it was long a common practice for children to have their knuckles slapped with a ruler, arm tied behind their chair, or other harsh punishment applied if they used their left hand to write (Clark, 1957; Ellis, Ellis, Marshall, Windbridge, & Jones, 1998). Indeed, there was professional controversy that followed this practice involving whether or not forcing a child to become right-handed actually caused other disorders, such as stuttering (Fagan, 1931; Burt, 1937). In short, being left-handed has for centuries translated into significant social handicap. At the very least, persons who preferred to use their left hand have been seen as different, if not maladapted, "backward", or belonging to a potentially at-risk or abnormal group.

Empirical studies that have been done on populations of left and right-handers who did not have any known disability have generally not shown any significant differences in such things as overall intellectual test performance or in school achievement (Douglas, Ross, & Cooper, 1967; Haefer, 1929; Hardyck, Petrinovich & Goldman, 1976; Satz & Fletcher, 1987). While there have been sporadic reports in the literature implicating various types of neuropsychological deficits (e.g. visual-spatial abilities) in nondisabled persons that are left-handed, the findings have not typically been consistently replicated (Bishop, 1990).

Theories attempting to explain why left-handedness occurs in the general population have varied widely. Some of these theories are merely a smaller part of the larger exploration of why certain behavioral patterns become lateralized, a term used to describe the tendency for one side of the brain to mediate a certain behavior or function (e.g. language which tends to be principally lateralized in the left hemisphere of the brain). Theories on the genesis of handedness range from those that largely implicate processes of environmental shaping (e.g. due to social reinforcement or determined by the way a mother carries her child) to strictly genetic models (Annet, 1978). There is also a prominent theory that suggests a form of "pathological left-handedness" (Satz, 1972; Satz, Orsini, Saslow, & Henry, 1985). In this model, early life trauma to the left hemisphere of the brain is seen as cause for a resulting right to left switch in manual dominance, and a full or partial transfer of hemispheric control for language. Still other types of biological causes for left hand preference have been proposed, including those that conceive of underlying constitutional patterns for lateralization, as well as ones emphasizing transmission through single genes, abnormalities of neuronal migration, problems in the intrauterine hormonal environment, and/or a disorder of the immune system (Bishop, 1990; Geschwind & Behan, 1982; Levy & Nagylaki, 1972).

To date, the research findings on the origins of left-handedness are still quite inconclusive and have ranged from mixed, to far less than compelling. For example, in studies of monozygotic (MZ) and dizygotic twins, there is little difference between the concordance and disconcordance rates for handedness (Neale, 1988). In effect, even when the twins have exactly the same genes (MZ) a significant number of these identical brothers or sisters (about one-quarter) have different preferred hands. At the same time, those who advocate an environmental explanation for the acquisition of handedness also face challenges from the research data. For instance, environmental models have difficulty explaining the similarities of children's early hand preferences to those of their parents, or similar parent-child hand preferences when children were raised apart from their biological parents (Bishop, 1990).

Despite the ongoing controversies and competing theories concerning the development of hand dominance, two consistent facts remain and are of importance to those working with clinical and rehabilitation populations: 1) being left-handed is more often associated with developmental disorders and disabling conditions than being right-handed and this could serve as a potential marker for intervention work; and 2) the range of disorders which show this relationship is broad, but ill-defined. For example, surveying the research, it is evident that higher than expected percentages of left-handedness have been reported in groups of persons with mental retardation syndromes (Bishop, 1990; Di Nuovo & Buono, 1997; Kinsbourne, 1997), language disorders (Eisele & Aram, 1994; Geschwind & Galaburda, 1985), autism (Chandana, Behen, Juhasz, Muzik, Rothermel, Mangner, Chakraborty, Chugani, & Chugani, 2005; Soper, Satz, Orsini, Van Gorp & Green, 1987), dyslexia (Satz & Fletcher, 1987), autoimmune disorders (Crawford, Kaplan, & Kinsbourne, 1994; Tonnessen, Lokken, Hoien, & Lundberg, 1993), schizotypal behavior patterns (Shaw, Claridge, & Clark, 2001) seizures (Dellatolas, Luciani, Castresana, Remy, Jallon, Laplane & Bancaud, 1993; Sabbah, P., Chassoux, .F, Leveque, C., Landre, E., Baudoin-Chial, S., Devaux, B., Mann, M., Godon-Hardy, S., Nioche, C., Ait-Ameur, A., Sarrazin, J. L., Chodkiewicz, J. P., & Cordoliani, Y. S. (2003), congenital deafness (Gumustekin, 2002), and even post-traumatic stress disorder (Chemtob & Taylor, 2003).

Invariably these findings raise many questions. Among them is the degree to which left-handedness might be linked to various behavioral and cognitive characteristics, and the extent to which this information might be useful in better understanding the nature of disabilities wherein that was the case. To date, despite the attention given to hand preference in some aspects of the empirical literature, issues concerning laterality have infrequently been discussed in the areas of the applied clinical work and rehabilitation. To address this, the present study aimed to determine whether a raised incidence of left-handedness existed in several commonly encountered samples of adults in a rehabilitation setting. The study also sought to determine if left-handedness was possibly associated with different levels or patterns of cognitive functioning across groups. Finally, the study aimed to prudently consider how this information might be of potential use in the assessment and planning process used by clinicians and other professionals who work with these individuals.

Method

Participants

The study sample consisted of a convenience sample, a subset taken from 750 adults consecutively referred for services in Vocational Rehabilitation (VR) offices in a Middle Atlantic region of the United States. Each of these adults was individually referred and had consented for a psychological or neuropsychological evaluation. Sufficient demographic and psychometric data were available on 60.53% cases wherein a single primary disability area had been identified. Four groups were used for comparative analysis by disability population as follows: adults with seizures (n=30), learning disabilities (n=156), mental illness/psychiatric disorder (n=139), and adults with a physical or orthopedic impairment (n=129). Thus, the total sample used for analysis was N=454.

The first group principally consisted of persons with physical handicaps, other health conditions or acquired injuries that affected their physical performance in work and daily activities (e.g. insulin dependent diabetes, back injury, amputation). This group was further operationalized in such a manner as to exclude any adults with a known or suspected neurological illnesses or injury that might have directly involved the central nervous system in a way as could have affected dominant hand use. Accordingly, persons who had a stroke, multiple sclerosis or traumatic brain injury were excluded from this analysis. Subjects who had their primary disability identified as mental retardation were also excluded from participation in this analysis.

The second group consisted of participants in the study who were adults with a history of idiopathic seizures or "uncomplicated" epilepsy with first seizure onset in childhood (Commission on the Classification of Epilepsy of the International League against Epilepsy, 1989). The mean age of onset was 6.4 years (SD=2.57). Furthermore, to be included in this study, participants with seizures must have had a history of at least three distinct seizure episodes, have continued to be in treatment for seizures, and had an onset of their first seizure prior to the age of 14 years.

The criteria used for the diagnosis of learning disability (LD) were consistent with that provided in the DSM-IV (American Psychiatric Association, 1994). This set of criteria is commonly used in clinical settings and emphasizes large discrepancies between achievement and IQ, usually of 2 SD or more. However, the DSM-IV criteria allows for smaller discrepancies between 1 and 2 SD when associated disorders of cognitive processing or other general medical conditions are present. In this study, when disparities between intellectual test performance and achievement test results were less than 2 SD, that case was evaluated in light of the individual's intellectual level and consistent with a regression model. LD was also considered in cases when a participant had at least average IQ test performance, but performed below the 5th percentile on a specific achievement area (cf. Siegel, 2003). This allowed for the inclusion of persons who where a LD still presented a significant barrier to employment, consistent with the operational definition of LD used by the Rehabilitation Services Administration, and not overly exclude individuals who were low achievers and still had LD (Rehabilitation Services Administration, 1985). For example, by doing this an individual with low average IQ score (e.g. 85) could still be considered to have a LD even though the difference between a specific achievement area (e.g. reading) and their tested IQ did not reach a full 2 SD.

Persons in the group with a diagnosed mental illness most commonly had primary diagnoses of depression, anxiety disorder, schizophrenia or bipolar illness, again consistent with the criteria presented in the DSM-IV.

All participants were between the ages of 18 to 62 years. The resulting sample of 454 participants had a mean age of 34.05 years (SD = 11.16) and a mean education level of 11.56 years (SD = 1.83). Males represented 54.4% of the final sample (n=247), and there were 207 females (45.6%). Three hundred, twenty one persons in the sample were self-identified as Caucasian (70.7%), 122 identified their race as Black or African-American (26.9%), 10 Hispanic (2.2%) and 1 Asian (0.4%).

Procedure

All participants were interviewed to obtain background information, developmental histories and demographic data. Although there was some variability in the range of cognitive tests administered, only participants who had a common core battery that included the Wechsler Adult Intelligence Scale-III (Wechsler, 1997a) and the Wide Range Achievement Test (Wilkinson, 1993) were included. Most of the participants from the groups with a LD and/or seizures were given additional tests (e.g. Wechsler Memory Scale-III (Wechsler, 1997b) or the Trail Making Test as part of their DVR evaluation. However, these data were excluded from the current analysis since they were not uniformly administered to all participants.

Handedness was determined in the study by asking the person whether or not they considered themselves right handed or left handed and by observing the dominant hand used when writing and engaged in other tasks involving pencil use. In 9 cases (1.98%) there was a discrepancy between the participant's report and observation. Seven of the 9 cases were individuals that referred to themselves as "ambidextrous", even though they still wrote only with their left hand. For the purposes of the analysis, they were considered left-handed and grouped accordingly. The other two cases wrote with their right hand and reported that they were formerly left handed in childhood and had been forced to change. Their data was excluded from the study.

All demographic information and test data were compiled. Data were then entered into a data base format and analyzed with the aid of the Statistical Package for the Social Sciences (SSPS) Version 12.

Results

Basic demographic data on the comparison groups are provided in Table 1. In a preliminary one-way ANOVA, a main effect of age was found across groups, F(3,450) = 31.868, p < .0001. Post-hoc Scheffe tests (p < .05) revealed that the group with physical disabilities was significantly older than the groups with either seizures or LD. The adults with a primary mental health diagnosis did not significantly differ from any other group on mean age. The presence of this age by group difference prompted the use of age as a covariate in subsequent multivariate analyses. However, while age was significantly correlated with some dependent variables in the study (e.g. Arithmetic score, FSIQ), the correlations were of very small order, with an R2 no greater than 9% of the variance for any given case. Hence, age was not found to have significant explanatory value.

Computation of cross-tab frequencies with Chi-Square tests revealed several patterns of group differences that also can be seen in the table. Female participants were significantly more likely to have a diagnosed mental illness than males, while males were over-represented in all other groups, [X.sup.2](3) = 11.238, p < .01. Level of education did differ to some extent. The LD group (mean grade level 11.14 years) reached a lower terminal grade than either the groups of adults with seizures (12.00) or the adults with mental health disabilities (12.08). The LD group not only had the least mean number of completed years of education, F(3,446) = 8.728, p < .044), but were the least likely to have finished high school. Among adults with LD, only 56.4% had completed high school. Though this rate was not found to differ significantly from either those adults with diagnosed mental illnesses (68.35%) or physical disabilities (66.7%), it did differ from the group of adults with seizures (80%) who had the highest high school graduation rate in the study (p <.05). Adults with seizures or LD were significantly more likely to have had special education services while in their school years, and left-handedness was more prevalent among persons with seizures and LD than the other two groups.

In the total sample, 397 of the 454 participants, or 87.4%, were right handed. Fifty-seven of the remaining participants, or 12.6%, were left-handed. This percentage of left-handedness is higher than expected for the general population in the United States. Other descriptive variables showed that across groups, 305 participants (67.2%) had a history of regular education school placement while 149 participants (32.8%) had been in special education for some or all of their school years. Again, this is higher than expected for the general population.

Table 2 depicts the mean scores for the major cognitive performance measures for the full sample (N=454). It can be seen that this VR sample generally performed lower than psychometric averages on both the WAIS-III and the WRAT-3 measures. The achievement test scores were also somewhat lower than the intellectual test battery might have predicted. Despite the lower mean scores on the cognitive tests administered, variability was very near expectations and the sample was not unlike many others reported in the VR literature (e.g. Bishop, Tschopp, & Mulvhill, 2000; Dunham & Schrader, 2000).

Independent samples t-tests were performed to examine possible differences between left and right-handers on the cognitive measures in the study. Table 3 depicts these scores. (Only the WAIS-III subtest scores that were fully available on all subjects are included in this analysis). In general, most cognitive test scores for right-handed participants were slightly higher than those for left-handers. Significant differences were found on several measures including FSIQ, PIQ, and Written Arithmetic. A near significant trend was found for oral arithmetic, t(451) = 1.873, p = .062. In each of these cases, right-handed participants outperformed those who were left-handed.

In both MANOVA and ANCOVA (age as covariate) using handedness and disability group as factors, the main effects for handedness were less apparent. Often, the group differences out weighed handedness on most dependent variables. Table 4 illustrates the main effects of group found on the cognitive measures in the study. Post-hoe multiple comparison tests (Scheffe p<.05), revealed basic similarities between the groups of adults with LD and those with a history of seizures on the intellectual test data. Both of these groups performed in the low average range. In contrast, the group of participants with physical disabilities and those with mental health disorders performed significantly higher, and within the psychometrically average range. Not unexpectedly, since it is required for diagnosis, the group of adults with a LD performed the lowest of all groups on the academic achievement tests.

While handedness may have had less pronounced effects than group membership on the dependent measures in the study, several significant interactions were found. Left-handed persons with LD had the lowest FSIQ scores of all groups, F(3,446) = 3.432, p <.017. They also obtained the lowest scores for VIQ, F(3,446) = 2.891, p <.035, Vocabulary, F(3,446) = 3.327, p <.02, and Arithmetic, F(3,446) = 3.536, p <.015. Meanwhile, the lowest Block Design scores were obtained by a different group of left-handed adults, namely those with seizures F(3,446) = 3.528, p <.015. It is noteworthy that Block Design is a nonverbal problem solving test that assesses spatial reasoning, perception and visual-motor functioning. It is a strong correlate of Performance IQ tests and is commonly is thought to relate to the efficiency of the functioning of the right cerebral hemisphere.

Participation in special education was associated with left-handedness. Of all adults who had special education while in school, 22.1% of them were left-handed. This contrasted sharply with the 7.9% of adults that did not have any special education and who were left-handed, [X.sup.2](1) = 18.59, p < .0001.

Few significant sex differences were found in the study. Within the cognitive test battery, the only significant difference found was for males to outperform females on Block Design, t(451) = 3.351, p <.001. As noted, there were a greater number of males than females who were left-handed, however, the difference found was not significant on Chi Square testing.

Discussion

An initial aim of this study was to determine if there was a raised incidence of left-handedness in an outpatient clinical and rehabilitation sample. Using a consecutive series of referred adults, the study showed that there was indeed a higher than expected number of persons with left-handedness in two of four commonly encountered groups of adults with disabilities. The incidence of left-handedness in both the groups of persons with a history of seizures and those with a LD was well above rates seen among the general population.

Secondarily, the study sought to determine if left-handedness was possibly associated with different levels or patterns of cognitive functioning across groups. Considerable support was also found for this aim in the research data obtained. Left-handed adults in the study were more likely to have had special education services during their school years. They performed lower on various test measures assessing intellectual functioning than right-handed adults, and left-handedness was more commonly seen in adults with disorders typically linked to potential brain dysfunction (seizures or LD) than to the other disorders studied.

Group differences were also found in the study. Broadly speaking, in many respects, the groups of adults with physical disabilities and mental health disorders performed in the average range psychometrically. In contrast, the adults with seizures and those with a history of LD performed similarly, but in the low average cognitive range. Within these latter two groups, the left-handed adults often did not fare as well as their right-handed counterparts. In several instances, group by handedness interactions revealed the lowest scores for left-handed adults with learning disabilities or seizures. In fact, as a group, adults with a LD may have shown the poorest of outcomes to the extent that they were least likely to have completed high school, had lower average terminal grade levels, had the lowest achievement test scores (despite the greatest likelihood of special education), and the lowest FSIQ scores.

Taken together, the current results add to the body of research that has shown higher frequencies of left-handedness among persons with various disabling conditions. In addition, the results of the study also help to clarify the relationship between the increased frequency of left-handedness, the type of disability in adulthood, and cognitive test performance.

The association seen here between left-handedness and lower cognitive performance in persons with seizures or specific learning disabilities (disorders that derive from central nervous system dysfunction) is an important one. While no specific single mechanism can be inferred as causal agent in all these cases, the results obtained certainly give rise to hypotheses about early life CNS insults, when such things as basic patterns of hand dominance, motor control, and the foundations of learning first develop. Again, it raises the question of using left-handedness as a marker variable in models of disability. Definitional models of conditions such as LD and MR which place heavy reliance on psychometric criteria for eligibility might be augmented by factors pertinent to early CNS development such as potential teratogenic influences and factors such as left-handedness.

Importantly, it must be emphasized that this research was also clear in indicating that not all persons who are left-handed have developmental disorders or any other disability. Indeed, it was the thrust of this type of finding that lead Satz, Soper and Orsini (1988) to propose three subtypes of non-right-handedness: pathological left-handedness, ambiguous handedness, and natural left-handedness. In their view, the first two examples represent individuals who are genotypic right-handers who effectively "masquerade" as phenotypic non-dextrals. Their model would suggest that the link between left-handedness and any brain-based disorder is present in only two of the types of left-handedness they propose (i.e. pathological and ambiguous). The findings from the present study are consistent with that model.

From a practical perspective, professionals who work in clinical or rehabilitation settings might do well to carefully attend to the handedness of their clients. Though by itself, knowledge of handedness may add relatively little to the total understanding of a particular given case, left-handedness most certainly appears to be a significant marker variable of risk that is worthy of our attention. Against the background of the other pertinent literature on this topic, the data from this study show that when associated with disabilities involving CNS dysfunction, left-handedness may well be a strong signal of added risk for adults including an unidentified learning disability, and the consequent need for vocational or clinical accommodations.

While no one cause (e.g. early brain insult) can be isolated from a descriptive study such as this, careful history taking can often prove a valuable aid in making reasonable inferences about disability and modifying plans to best suit the needs of the client. Rehabilitation counselors and other clinical staff might be encouraged to ask about the development of handedness, the degree to which the individual has a preference for varied activities, patterns of handedness in the family, and any difficulties that the individual might have with uni-manual and bi-manual tasks. Some variability of handedness across certain activities is to be expected as hand dominance is in many ways a continuum also shaped by life experience rather than just a sharp strictly categorical variable (Bishop, 1990), but patterns of behavior are often most telling.

Professionals who work with adults in such things as VR settings might also do well to remain aware of the limited ways that the world of work is adapted for persons who are left-handed, and to make adjustments accordingly. Most people are aware of the scarcity of tools like truly left-handed scissors (with the fingers holes and the cutting edge in proper position), baseball mitts, or schoolroom desks, however, fewer are likely to think of many other instruments in common use that left-handed people must adapt for their use. Consider the employee in a food service job using a manual or electric can opener, or peeling a potato. Not surprisingly, at work left-handed people are found more likely to sustain injury (Stellman, Wynder, DeRose, & Muscat, 1997), often because the equipment that they use is designed for people who are right-handed (e.g. drill press, power saw). Even in the office setting, accommodations might be needed (e.g. the mouse on a computer is made for a right-hander who must frequently press with their index finger). Just providing left-handed desks, reasonable table placement of materials (centralized to allow equal access for right and left-handed individuals), allowing left-handers the option of sitting at the left end of a table to reduce elbow crowding or inquiring about any accommodations that would help a left-handed client feel more comfortable is typically a good starting point. Even many of the common instruments and techniques used for assessing adults for the vocational context that involve measuring such things as hand speed, fine-motor control or eye-hand coordination do not have adequate norms for left-handers but instead speak only of the use of the "nondominant" hand (Strauss, Sherman, & Spreen, 2006; Thompson, Heaton, Mathews, & Grant, 1987). Clearly, clinician sensitivity is what is called for.

It is important to address some limitations of this research. An initial one concerns the nature of the design of the study and use of a convenience sample. Nonrandom samples such as these have limited generalizability to larger populations. Even though the sample was chosen from 750 consecutively referred cases, and likely is representative of the typical group of VR individuals in the environment studied, convenience sampling is still effectively out of the researchers' control and consequently limits attempts to verify external validity.

A second limitation concerns the manner in which hand preference was assessed. While the participants used in this study reported little variability of their dominant hand preference, it can be a matter of degree (Bishop, 1990); a continuum shaped by experience rather than just a sharp categorical variable. There can also be a difference between hand preference and hand skill (McManus, 2002).

A third limitation also involves external validity. Despite the overall sample size and consistency with earlier work, there were still only 57 left-handed individuals in the research. Thus, caution must be used in generalizing the findings to other groups of persons who are left-handed.

Fourth, for anyone who works in a clinical or rehabilitation setting with adult clients, it is obvious that knowing the primary disability is not enough. Many individuals have multiple disabilities. For the purposes of empirical research and data tracking, a group analysis is a useful approach aiding in meaningful statistical comparison, but may not tell the whole story for a given individual who may have other co-morbid conditions that could affect their results.

Finally, the manner in which eligible clients were included in the research could have affected and even minimized some of the findings. Many VR centers exclude most individuals with mental retardation (MR) syndromes, and those who otherwise perform cognitively in the MR range. Consistent with this, persons with MR were excluded from this research and analysis. However, excluding these individuals may well have reduced the overall rates of left-handedness in the study and any associated effects, to the extent that higher numbers of left-handed individuals have been found among groups of individuals who perform lower on cognitive tests (Bishop, 1990; Bradshaw, et. al., 1984). Future studies should consider including lower functioning adults to get a more representative picture of the intellectual continuum.

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Michael Ferrari

University of Delaware 

Michael Ferrari, Ph.D., Department of Individual and Family Studies, 108 Alison Hall, University of Delaware, Newark, DE 19716 Email: [e-mail]

Table 1
Demographic data by referral group.

                                   Groups

                          Seizure        Learning
                                         Disabled
Variable

Mean Age (a)              31.73 (10.2)   28.24 (9.7)

Sex (b)
      % Male              56.7           57.1
      % Female            43.3           42.9
Years of educationa       12.00          11.14
Handedness (b)
      % RH                73.3           84.6
      % LH                26.7           15.4
% Special education (b)   50.0           68.6

                                   Groups

                          Mental         Physical
                          Illness        Illness
Variable

Mean Age (a)              35.94 (10.2)   39.58 (10.7)

Sex (b)
      % Male              43.2           62.8
      % Female            56.8           37.2
Years of educationa       12.08          11.40
Handedness (b)
      % RH                89.9           91.5
      % LH                10.1           8.5
% Special education (b)   16.5           3.1

Note: (a) ANOVA value was significant (p < .01.). SD listed
parenthetically.

(b) Chi-square value was significant (p < .01).

Table 2
Mean cognitive test scores for the full sample (N = 454).

Variable             Mean        SD

FSIQ                 91.17       13.3
VIQ                  92.18       13.7
PIQ                  91.37       13.4
Reading              88.89       17.6
Spelling             85.61       16.8
Written Arithmetic   85.45       14.9

Table 3
Mean cognitive test scores by handedness.

                                   Handedness

Variable                 Right (n = 397)   Left (n = 57)

FSIQ (a)                 91.63 (13.4)      87.95 (12.7)
VIQ                      92.56 (13.8)      89.53 (12.5)
PIQ (a)                  91.89 (13.4)      87.75 (12.9)
Vocabulary                9.29 (2.9)        9.07 (2.9)
Block Design              8.61 (2.7)        8.05 (2.9)
Arithmetic                7.91 (2.8)        7.18 (2.5)
Reading                  88.85 (17.8)      89.18 (16.0)
Spelling                 85.84 (17.2)      84.41 (14.6)
Written Arithmetic (a)   86.04 (14.8)      81.35 (15.3)

Note: (a) Independent samples t-test significant (p < .05).

Table 4
Cognitive test means by group.

                               Groups

                       Seizure        Learning
                                      Disabled
Variable

FSIQ (a)             84.17 (9.0)    84.69 (11.3)
VIQ                  86.87 (9.4)    84.71 (10.5)
PIQ                  82.67 (9.9)    87.33 (13.3)
Vocabulary            8.87 (2.7)     7.74 (2.4)
Block Design          6.87 (l.9)     8.01 (2.8)
Arithmetic            6.57 (2.4)     6.19 (2.1)
Reading              86.17 (16.8)   76.40 (16.7)
Spelling             87.44 (13.7)   74.40 (13.3)
Written Arithmetic   79.73 (11.3)   74.78 (12.4)

                               Groups

                        Mental        Physical
                       Illness        Illness
Variable

FSIQ (a)             94.85 (14.2)   96.65 (11.7)
VIQ                  96.78 (14.6)   97.50 (12.6)
PIQ                  93.24 (13.9)   96.27 (11.1)
Vocabulary           10.41 (3.13)    9.95 (2.6)
Block Design          8.90 (2.8)     9.19 (2.7)
Arithmetic            8.59 (2.9)     9.27 (2.4)
Reading              97.31 (13.8)   95.59 (13.1)
Spelling             95.46 (15.1)   91.84 (13.9)
Written Arithmetic   91.06 (13.5)   93.67 (11.3)

Note: (a) ANOVA value was significant (p < .01.). SD listed
parenthetically.