Turkheimer et al.'s (1993) analysis lent more support to the Inglis/Lawson hypothesis than to Mc-Glone's hypothesis, but Turkheimer et al. also recognized that more complex studies with larger samples are necessary for clarification. The results of empirical analyses discussed in earlier sections of this chapter and shown in Tables 8.13, 8.16, 8.17, and 8.18 generally contradict the Inglis/ Lawson hypothesis and give more support to McGlone's (1977, 1978, 1980) notion of less asymmetry in the cerebral organization of female versus male brains. Anatomically, Witelson (1989) notes that right-handed males have greater cortical asymmetry than females in the temporal and parietal regions of the brain, and the part of the corpus callosum that connects these asymmetric regions is larger in females than males. Furthermore, the corpus callosum gets smaller for males with increasing age (between 25 and 68), but not for females (Lezak, 1995), which may make it less likely for older men than older women to use both sides of the brain to compensate for an injury to one hemisphere.
One reason for the failure of researchers to consistently find gender differences in cerebral organization may be that the difference resides in the potential for females to compensate for damage or dysfunction. Perhaps women tend to solve problems in a manner similar to men under ordinary circumstances. However, when the cir cumstances are extraordinary, such as having the type of brain dysfunction that might cause a learning disability or having a lateralized brain lesion associated with depressed Verbal or Performance IQ, females may be better equipped to spontaneously compensate for the deficit.
Many more boys than girls are learning disabled. Possibly girls respond better to the brain dysfunction; conceivably, girls may be able to compensate for a kind of brain dysfunction that in boys would lead to a learning disability. Even LD females may have better compensatory strategies than LD males. Indirect evidence for this hypothesis comes from an examination of subtest profiles of LD children, adolescents, and young adults. Some research suggests that the so-called ACID profile (see Chapter 9) associated with learning disabilities holds only for males; females perform poorly on three of the four ACID subtests, but tend to do quite well on Coding/Digit Symbol. The latter task has elements of both left-hemisphere and right-hemisphere processing and may be amenable to compensation from an intact part of the brain. Similarly, females with left-hemisphere lesions may show a smaller deficit in V-IQ than males with left lesions (and, analogously, females with right lesions may show a smaller deficit in P-IQ) because, subsequent to the damage, they may use their capacity for compensation when solving verbal (or nonverbal) problems.
This greater flexibility and capacity for compensation may result from a superior ability of females to demonstrate interhemispheric integration. Denckla (1974) has speculated that dyslexics may have faulty interhemispheric integration, and Witelson (1976, 1977) has also stressed the value of good integration for success in school-oriented activities. Because girls are far less likely than boys to be dyslexic, and women with lateralized lesions evidence less predictable V-P profiles than men with right versus left lesions, it may be that females surpass males in the ability to integrate the two cerebral hemispheres. Certainly cerebral integration is an important aspect of a complex psychomotor task like Digit Symbol.
Greater interhemispheric integration would be a plausible mechanism to explain McGlone's (1977/1978, 1980) suggestion that females have more lateralization of speech functions than males, and it also may relate to Inglis and Lawson's (1982) claim that women have a greater capacity than men to solve nonverbal problems via verbal mediation. Certainly, if females really do have a more efficient system for integrating strategies from the two hemispheres, one would expect women to demonstrate less cerebral asymmetry than men, at least under a circumstance (like an acute brain lesion) that prevents the application of an optimal problem-solving strategy.
The evidence presented here does not provide clear support for the Inglis/Lawson hypothesis in preference to McGlone's position or vice versa. Most of the evidence supporting McGlone's hypothesis is indirect, and it is unwise to discount either the criticisms of McGlone's (1980) conclusions (Hier & Kaplan, 1980; Inglis & Lawson, 1982; Kinsbourne, 1980) or the model-fitting evidence (Turkheimer et al., 1993) that was contrary to McGlone's hypothesis. The "verbal strategy" hypothesis advanced by Inglis and Lawson and by others (Bryden, 1980; Harris, 1980; Sherman, 1978) cannot be ignored, especially in light of the support recently advanced by Turkheimer et al. (1993). What is needed is a series of investigations of normal and brain-damaged males and females that evaluate directly and systematically possible gender differences in the application of verbal mediation to nonverbal problem solving (Kaufman, 1990).
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