The Mystery of Language Lateralization, Solved!: A Brain-based Explanation of Language Lateralization (and Handedness)

Note: This theory is an extension of the theory of left-handedness. The related article should be reviewed prior to reading this article.

A Concise Explanation of Language Lateralization

Similar to why some of us become left-handed, the underlying cause of atypical (i.e., rightward or symmetrical) language lateralization seems to be a person’s learning rate in early childhood.

People with atypical language lateralization presumably had a slower learning rate in early childhood, at least in regard to learning language. Due to this slower rate, the typical right-to-left shift of the associated information happens to a lesser degree, and seemingly not at all at times in some regards.

This presumably results in more language content than usual being permanently housed in the right-brain, enough so as to lead to atypical language lateralization (see note 1 below). The slower a person comes to learn language then, presumably the more rightward the person’s language lateralization (see notes 2 and 3 below).

In short, relatively few of us come to have atypical language lateralization, as most of us learn enough language in early childhood to establish the left-brain as the predominant side for it.

Note 1: Similar occurs in regard to presumably genetic right-handed animals without a corpus callosum that become left-handed (e.g., kangaroos). In such cases, the right-brain presumably comes to house enough routinization-related content such that its corresponding hand, the left one, becomes the preferred one, thereby countering the animal’s genetic factor for right-handedness. See the handedness-related article for additional details.

Note 2: Similar occurs in regard to mixed-handedness. In such cases, the less a genetic left-hander develops the left-brain in early childhood, the increased strength of the person’s left-handedness. Thus, more left-brain development leads such a person to be more right-handed than left, whereas less development leads them to be more left-handed than right.

Note 3: Of note, though, sex differences presumably exist, as males learn language seemingly slower in early childhood, yet they come to have perhaps greater leftward language lateralization. Possible explanations include sex-related differences, such as males having relatively smaller language-related brain areas and greater leftward neuroanatomical asymmetry (as detailed in another article).

Support from Left-Handers

According to this theory, atypical language lateralization as well as increased strength of left-handedness are more likely to manifest in individuals with a slower learning rate during early childhood. Fittingly, an increased prevalence of atypical language lateralization has been shown to be related to increased strength of left-handedness (i.e., it occurs in consistent left-handers more than it occurs in inconsistent ones, the so-called mixed-handers).

Consistent left-handers, though, can still have typical language lateralization. Based on this article’s theory, such people did not develop the left-brain enough to counter their genetic factor for left-handedness, but they did develop it enough in regards to language. In essence, these people seemingly had different learning rates for each characteristic — fast enough for language aspects but slow enough for hand-use ones.

Conversely, mixed-handers with atypical language lateralization are the opposite: These individuals appear to have learned quick enough when it comes to hand use but slow enough in terms of language development.

It’s worth noting that right-handed individuals can also experience atypical language lateralization, presumably for the same reasons as others — a slower learning rate in early childhood regarding language (see note 1).

Of note, though, right-handers perhaps exhibit a relatively lower degree of atypical language lateralization compared to left-handers. Although quite speculative, perhaps a factor is a genetic preference for an associated side of the neocortex: Left-handers with a relatively greater degree of atypical language lateralization thus perhaps owe it in part due to genetically favoring its associated side of the neocortex, the right-brain (see note 2).

Note 1: Studies have revealed that fewer right-handed individuals (4–8%) exhibit atypical language lateralization compared to left-handed individuals (15–30%). This difference, though, seems in part due to the erroneous classification of right-handers that countered their genetic factor for left-handedness.

(In short, such people are mistakenly grouped with people that have a genetic factor for right-handedness. This inflates the number of right-handed individuals in the studies, resulting in a lower percentage of atypical language lateralization among them and a higher percentage among left-handers.)

Note 2: Similarly, right-handers perhaps genetically favor the left-brain, the side associated with both the right hand and typical language lateralization. Consequently, right-handers perhaps have a relatively higher degree of typical language lateralization compared to left-handers.

At least one study has shown this pattern, although it was not significant. However, perhaps it would be significant if not for the erroneous classification of genetic left-handers in the group of right-handers (similar to as described in Note 1). In particular, such people presumably lower the group’s degree of typical language lateralization.

Support from Developmental Disorders

Many individuals with developmental disorders often exhibit a slower learning rate during early childhood. Consequently, a higher proportion of them tend to develop atypical language lateralization. For instance, children with autism frequently display a greater degree of rightward language activation and reduced leftward activation compared to typically developing peers.

Another illustrative example is found in cases of agenesis of the corpus callosum. Individuals with complete agenesis of this structure are more likely to exhibit rightward language lateralization, while those with partial agenesis are inclined towards symmetrical language lateralization.

This aligns with the expectation that complete agenesis of the corpus callosum would result in a relatively slower initial learning rate compared to partial agenesis (due to the related right-to-left shift of information when learning, and the resulting reduction of it due to an absent vs. partial corpus callosum).

Support from Identical Twins Discordant for Language Lateralization

Identical twins discordant for language lateralization provide support too. In such cases, the discordant twins had different enough learning rates in early childhood to lead to such discordances. (As with identical twins discordant for handedness, the difference in development rate could be due to numerous factors, perhaps most notably one twin having been exposed to comparatively more hormones in utero.)

Support related to Skilled Movements (Praxis)

Recent findings indicate that individuals with atypical language lateralization also tend to exhibit atypical lateralization in skilled movements, such as gesturing (i.e., most of us use the right hand, but they use the left one instead). This fits with the thinking that sufficiently slow enough learning of language leads the right-brain (and its associated left hand) to handle at least some of the “routinization” information associated with language.

(Similarly, it’s worth noting that most left-handers wave and gesture with their right hand, which might seem unexpected. However, this is primarily because most left-handers have typical, leftward language lateralization, which leads them to use its associated right hand in skilled movements — even though it’s not their dominant hand.)

Support from Specific Brain Regions and White Matter Tracts

Further support for this theory emerges from investigations into white matter tracts and brain regions associated with language processing. In both of these cases, although an atypical rightward asymmetry of these brain features does not always occur in people with rightward language lateralization, a relationship nonetheless likely exists (see note 1). This fits with the thinking that such people have a slower learning rate in early childhood, and that it’s perhaps due to having an atypical brain.

Similarly, compared to people with typical language lateralization, those with atypical language lateralization have shown an opposite pattern of activation related to the default mode network (DMN). This opposite pattern is perhaps due to such people housing more of the “routinization” information associated with language in the right-brain instead of the left-brain.

Additionally, rather than be mirror images of people with typical language lateralization, those with atypical language lateralization activate different areas during some language tasks. They also tend to use more cortical tissue during some language tasks. This includes more activation of the contralateral hemisphere in such tasks than observed in people with typical language lateralization. This all fits with the thinking that atypical language lateralization leads to some atypical use of the right-brain (i.e., as the “routinization” side) but still uses the left-brain somewhat in its customary capacity.

Note 1: The differences are perhaps due in part to the likelihood that such features are not solely related to language, and if the brain area is a primary or higher-order one. In addition, different brain areas are involved in different aspects of language, such as production vs. comprehension. Studies must properly account for such distinctions if they are to be accurate and reveal associated patterns.

Partial Support for the Influence of Early Childhood Learning Rate

Compared to typical language lateralization, atypical language lateralization has been shown to be related to worse language performance in healthy children. This fits with such children having a slower learning rate in early childhood than children with typical language lateralization.

Similarly, left-handers have been shown to have reduced language ability at age five. Given that left-handers are more likely to exhibit atypical language lateralization, this finding aligns with the notion that they may have experienced a slower learning rate during their early childhood.

Note: For more information about left-handers, and a continuation of this encompassing theory of laterality, see the related article.

Closing Remarks

I invite you to explore my other articles that reinforce/support the “novelty-routinization” theory of lateralization.

In particular, one article provides a much-needed neurological-based deeper understanding and associated revision to the thinking that we’re left- or right-brained (Spoiler: We are!). Another article provides fresh insights and brain-based explanations for sex-related differences, while a third one led to a brain-based unifying model of personality types / societal roles!

Feel free to ask any questions or contribute to the discussion otherwise. Together, let’s continue unraveling the mysteries of the human brain and uncover the profound implications they have for our lives.