Sexual Dimorphism and Language; implications of existing sex differences in performance and associated neural substrates.

Tyler Mosdell 20947733

University of Western Australia, Neuroscience

Sexual dimorphism is a concept which has been known to underpin innumerable heterosocial and homosocial intra-specie interactions (Agren et al., 1999). Primary examples of this come in the form of gross morphology as is common in order Lepidoptera, or Galliformes, with wing patterns or plumage respectively (Gilbert and Schneiderman, 1961; Wiens, 2001). It has long been noted that these features largely determine the mating success of an individual and thus place a large evolutionary pressure on the continuation of such characteristics (Gilbert and Schneiderman, 1961; Wiens, 2001). With a number of such examples being present across a range of not only species but phyla, it can be inferred that such characteristics role in evolution will continue to exist. A key area of research in this field which appears to still be, somewhat, in its infancy, is the application of these well understood concepts to humans. The ubiquity of human examples is underpinned by the fact that as a species, we have moved towards a high sensory dependence, particularly in relation to communication (Hinde, 1974). As a result of this, it has been found that dimorphism exists to a great extent in both language and, still to some extent, physical characteristics, associated with social relations (Ardila et al., 2011; Baron-Cohen, 2009; Giedd et al., 1997; Weiss et al., 2003; Gouchie and Kimura, 1991; Perrett et al., 1998). Whilst the latter of these is something quite thoroughly explored, language based dimorphism appears a relatively young concept, seeming to have only been formally been explored over the last 30 years. Recently, this difference in language ability between sexes has been linked to disparity in gross cortical anatomy of language-associated regions (Harasty et al., 1997). The implications of exploration in this area are able to lend credit to the previously unsubstantiated notions to the effect of males and females learning differently.

With the primary research base of human sexual dimorphism tending towards facial and other gross morphological characteristics, subjective areas of research such as cognitive abilities have been, until recently, largely unexplored (Perrett et al., 1998; Rhodes et al., 2003). Studies have frequently found three areas of major difference; language processing, spatial processing and arithmetic ability, favouring females, males and males respectively (Halpern, 2013). It has been found that females have higher performance specifically in verbal processing tasks, fluency and vocabulary and word list-learning tasks (Ardila et al., 2011). The fact that apparent increased capacity for language processing exists in discrete areas of language implies that the dimorphism exists in such a way that it effects particular areas of language (Halpern, 2013). This is supported in, what were originally seen to be controversial, findings showing little to no statistically significant difference between male-female general verbal ability tests (Ardila et al., 2011). Whilst this was originally seen as an indication that, perhaps, sexual dimorphism does not play as key a role in language, once coupled with numerous conclusive studies showing differences in specific areas, it appears to have acted as an initial probe into the mechanisms at play. A number of subsequent studies were able to validate the statistical findings of the language tests, and as a result, great interest has been placed on correlating these findings with anatomical differences underlying such dimorphism (Gouchie and Kimura, 1991; Gur et al., 1999). Gross morphology of language-related cortical regions were analysed, specifically the superior temporal gyrus and inferior frontal gyrus. Preliminary findings indicated 17.8% (P=0.04) and 20.4% (P=0.05) larger cortical volumes respectively (as a proportion of total cerebral volume) in females (Harasty et al., 1997). Looking specifically at the superior temporal gyrus, it was found that the increased volume was attributable to a 29.8% (P=0.04) increase in planum temporale (PT) volume, an area thought to be closely involved in linguistics (Harasty et al., 1997). Though these results seemed promising, subsequent research into the exact role of the PT indicated that it played a much more upstream role in linguistics (Binder et al., 1996). The area responded equally to both tones and words, and so appeared more closely related to early auditory processing rather than being a language specific area (Binder et al., 1996). Though these anatomical findings did not explain the results of previous studies, they have indicated the presence of not only sexual dimorphism in language but of anatomical nature as well.

In exploring additional areas of cerebral dimorphism it has been found that not only are there differences in proportional sizes of specific cortical areas, there are overarching morphological differences in the cortex as a whole (Gur et al., 1999). A global difference in neural substrates would account for different areas of language being effected differently. MRI scans have found that females have a larger proportion of grey matter than males relative to their intracranial volumes (Gur et al., 1999). Though these differences are not yet attributable to any particular behavioural difference, it has been hypothesised that due to grey matters relevance to computation rather than transfer to physically distant neurons, it could account for an increased cognitive ability in particular areas (Gur et al., 1999). This is largely dependent on a number of things; whether language processing is facilitated primarily through grey matter, and whether areas of increased grey matter correlate with language regions of the cortex. As these findings have limited research backing and are based on a number of critical assumptions, a, perhaps, more pertinent sexual dimorphism, is that of relative neuron numbers. It has been found that females have, on average 1.13 times more neuronal processes than males (Rabinowicz et al., 1999). This concept serves as a viable explanation as to how areas of language processing in the female brain could have an increased processing capacity without having a larger measurable volume. Though a large number of areas were sampled in this study, to further investigate this concept in terms of language based areas of interest, more acute testing of these areas would be required, particularly cortical areas corresponding to tasks in which females had higher performance.

Whilst these inquiries into the anatomical bases for this language are understandably important, cognitive performance is not always reflected in morphology of cortical substrates but rather through interaction with neural plasticity (Ungerleider, 2002). A key example of this is in musically inclined individuals, where areas of music-related brain activity are largely plastic (Rauschecker, 2001). Johnson and Bouchard (2007) provided some of the most pertinent as well as recent findings in this area. The rationale behind their study was to demonstrate that a female language advantage existed independently of general intelligence. Scores in verbal ability, adjusted to remove the effects of general intelligence, favoured females, whilst full scores showed little to no sex differences. This indicated that as general intelligence was able to mask the effects of sex differences in specialised abilities. With processes involved in general intelligence, able to compensate for specific language-based abilities, intersex differences in these abilities may be more to do with the way in which this neural plasticity is able to compensate, rather than differences in specific language areas.

This notion, whilst more difficult to validate, is in line with some of the early theories of sexual dimorphism in language, specifically those of Sherman (1978). The bent twig hypothesis, as it was known, is based on the principal of small environmental cues, during critical periods of development, equating to large discrepancies in both ability and anatomy. This is in line with current theories of neural plasticity and has been seen in a number of other areas of neural development (Johnson and Meade, 1987). The theory continues to state that, with many studies showing that females talk at an earlier age than males, this early advantage causes a verbal reliance and results in larger post-developmental differences (Sherman, 1978). Though this theory does well to explain performance differences in terms of already-understood concepts of neural plasticity, it fails to address the cause for the initial, albeit marginal, precocity.

Developmental studies in this area, a seemingly previously neglected area, have looked at early sex differences in cerebral volume as well as specific hormone levels at ages between 4 and 18, the widely accepted critical period for a majority of cognitive development (Giedd et al., 1997). A number of sex differences were noted, not only in volume comparison of various areas, but in the growth characteristics (gradient and onset) of various cortical areas. A difference of interest was that of the basal ganglia caudate nucleus, which was found to be relatively larger in females. The caudate nucleus has been previously linked to language, specifically language control, a key area of importance in the first stages of speech (Friederici, 2006). With this initial language advantage, as well as differing rates of neural growth, sex differences could certainly be explained.

The notion of sex differences in areas of cognition being present from an early age is a common one, and though it is measurable on both an anatomical and performance level, an alternative theory looks at these performance differences as the result of this notion rather than as the cause of it (Halpern, 2013). Stereotypes are undeniably play a large confounding role in a number of gender, race and even social based studies (Flanagan and Ortiz, 2001). If we consider the method in which language ability, the basis of this dimorphism, is assessed, a self-preconceived idea of performance has the propensity to largely effect the findings of the test. As well as this, a number of tests are particularly susceptible to confirmatory bias due to their reliance on subjective criteria assessed by the examiner (Flanagan and Ortiz, 2001). It is possible that the gender-stereotypes precede gender performance differences and moreover, cause them. An additional issue with cognitive ability testing is that it is questionable as to whether ability is being directly assessed or merely achievement (Baron-Cohen, 2009). Though similar, the distinction between these two concepts is of paramount importance. The former indicates a predisposed or underlying capacity whilst the latter has the propensity to be entirely based on conditioning or learning-opportunities. Though these notions largely undermine the existence of neuroanatomical intersex differences, it could be seen to support the idea of neural plasticity playing a large role, whereby individuals perceived or expected performance would impact their actual performance, the knock on effect of which could cause long term differences in neural capacity for associated tasks.

With a number of theories seeking to explain these difference through various approaches with various degrees of success, it does raise the question of how these theories fit together. Despite each studies failings, they have all succeeded in indicating statistically significant variation between males and females language ability as well as highlight anatomical cortical differences. Whilst the end goal of these studies has consistently been to explain already-well-recorded differences in male-female cognitive performance in terms of measurable neural characteristics, the question should still be asked as to whether this is particularly pertinent. Scientific curiosity aside, the implications of these studies, lie in the consequences of these differences when examined under natural settings (Martell, Lane and Emrich, 1996). That being said, the concepts responsible for these differences do still hold some value as the implications of these sex differences do slightly vary with respect to the basis of the differences.

With the current evidence presented on sexual dimorphism with relation to language, it has been shown to a reasonable degree of certainty that these differences do exist, and whilst the neurological substrates responsible have not been formally identified, to a high degree of certainty, they have certainly been generally implicated. A number of studies have, therefore, focussed research into the implications of the existence of these differences (Baron-Cohen, 2005; Brown and Corcoran, 1996; Harrison and Tunbridge, 2007). One of the obvious areas it raises questions in is gender equality. Do the ability differences discussed provide grounds for some areas of inequality, or, conversely, are inequalities further promoting these differences? Brown and Corcoran (1996) used longitudinal studies to investigate differences in school content between males and females. It was found that, tending to the previously found areas of strength, males were exposed to more special tasks while females towards more language. The study continued to track their performance in these areas with the ultimate goal of translating these differences into wage differences in their career placements (Brown and Corcoran, 1996). Whilst evidence was found for an increase in male-female performance gaps, these did not equate to the wage gap present. As this inequality exists external to any measure of performance, it suggests that it is not as a result of sexual dimorphism but rather an issue unto itself. Studies highlighting the difference in role placement in jobs, however, would be more relevant. Education is one of the ideal areas to look at for this as, all things being equal, particular areas of performance should be reflected in the areas of teaching placement. In schooling reviews, it has been found that in general there are 75% more female teachers than males and yet males make up over 75% of maths and science teachers (U.S. Department of Education, 2014; Davies and Meighan, 1975). This is in line with respective areas of performance. Whether this is something that needs to be changed is largely subjective. If a particular gender is able to better fulfil a particular role then is it incorrect to capitalise on this. An understanding of generalised areas of strength between sexes would greatly facilitate this.

Because of the integral role language plays in human functioning, advantages in this area can in fact translate to seemingly unrelated areas. In this case, links have been made to the prevalence and effect of autism (Baron-Cohen, 2005). Whilst preliminary findings do not suggest treatment applications, greater importance has been placed on furthering the understanding of this disorder (Baron-Cohen, 2009). With females being generally stronger in abstract, language based areas and males in systemisation or maths based areas, autism, which exhibits the same characteristics as an extreme version of the male brain, can be investigated in terms of these neuroanatomical differences. From this, models of input-processing-output can be developed to aid understanding of how to best facilitate, and work to the strengths of both autistic individuals, and to a milder degree, different sexes.

Mertell et al. have developed computer simulation models of sex differences in input-processing-output characteristics. By using this as a tool to assess the impact of differences, pressure can be placed on correcting gender inequalities caused by these differences. By placing emphasis on the effects of differences rather than the causes, a greater understanding of the role they play, can be developed. Whilst being able to model the effects does not act as a perfect substitute for understanding the mechanisms at play, it largely facilitates the same benefits and capacity for further research.

The prevalence of language impairment in kindergarten children is estimated at 7.5% (n=2,084) of which boys make up a majority (Tomblin et al., 1997). With the previously demonstrated snowball effect language difference is able to adopt, assisting in correcting these impairments at an early stage would be greatly beneficial (Lenroot et al., 2007). The use of sexual dimorphism language models, if adjusted for and applied to, a younger demographic, could provide a unique insight into not only the effects of the impairment, but development of tailored treatment plans.

Whilst sexual dimorphism in language has been demonstrated in test results, as well as in various cortical areas, the full process behind this concept remains elusive. A resonating point made by a number of studies however, is that in areas of neural research, the rational or basis for the exploration has moved beyond exploration for exploration sake (Sherman, 1978). Implications of studies are becoming more important that the notion of furthering our understanding. On this premise, computerised models, able to simulate the discrepancies present in this area of sexual dimorphism, satisfy the criteria without the requirement of understanding neural processes in their totality. Though the validity of these models would need assessment, they have the propensity to offer humanity an external reference for understanding neurological differences, almost completely exempt from confounding factors (Flanagan and Ortiz, 2001; Martell, Lane and Emrich, 1996). With neural research proving to never be compartmentalised, as would be ideal, it is difficult to associate discrete areas of sexual dimorphism with anatomical differences. Overall the research base for sexual dimorphism has moved away from sensationalising gross anatomical differences, as they appear to offer little in the way of meaningful implications. With areas of research pushing forward in using sex differences as a tool to adjust current approaches for treatment, teaching and understanding based on gender (Vamvakopoulos and Chrousos, 1993).

References

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