What is the significance of the male preponderance of left handedness ? − Rakyat Biologi

It has long been known that there are more left handed boys and men than there are left handed girls and women. I have found close to a dozen studies having carefully investigated this issue, so that we can now estimate the gender-specific prevalence rates over tens of thousands of cases. The male/female ratio of left handedness in these studies varied from 1.3:1 to 2:1 (the average was 1.5:1) with every single study reporting higher prevalence in males, and the majority obtaining a statistically significant effect. The higher prevalence of right handedness in the female human is observed as early as it can be measured. Prior to this, in infancy, motoric lateralization has to be studied either by observing spontaneous movements, or by testing reflexes on each side of the body. One investigation found that there was a sex difference in asymmetry of three foot reflexes in newborns: females were right biased, but the majority of males were left biased.

Handedness is highly hereditary. This influence is the primary one. Indeed heritability estimates derived from concordance rates of homozygotic (from one ovum) and dizygotic (from two ova) twins are very high. Also, it appears that the hereditary determination of handedness can supercede the hormonal determination: there have been reports of cases of left and right handers with androgen insensitivity syndrome. In this syndrome, every cell of the body is insensitive to androgens, so that genetic males actually look very much like females. If testosterone were an important determinant of left handedness, as has been argued by Geschwind and his colleagues, then all these cases should be right handed.

A subtle prenatal hormonal modulation of handedness certainly exists. Prenatally masculinized women, whether by synthetic estrogen (diethylstilbestrol, i.e., DES) or by adrenal hyperplasia are more prone to left handedness (see chapter 7 for additional details). Likewise, Nass and colleagues (1987) found that women masculinized by congenital hyperplasia were more often left handed than their normal sisters.

However, another factor may be involved. To understand the neuropsychology of handedness, it is imperative to grasp that motor control is contralateral in mammals. The left hemisphere controls the muscles of the right side of the body, and the right hemisphere controls those on the left. The male sex is more at risk for prenatal diffuse brain damage (cerebral palsy, epilepsy, mental deficiency, hydrocephalus) and focal brain damage as well (cerebrovascular accident, localized dysplasia) (see chapter 6). Diffuse brain damage is associated with left handedness, independently of gender. For example, there are also slightly more left handers among stutterers, dyslexics, mental retardates, and autists. Incidentally, these are all male-prevalent conditions. It is supposed that early disruptions of brain development place the individual at risk for deviation of a delicate bifurcation leading to right or left handedness. Focal lesions have a more dramatic effect. When the prenatal or perinatal lesion is in the right hemisphere, the bearer of the lesion is even more likely to become a right hander than otherwise. However, when the lesion is "strategically emplaced" in the left hemisphere, motor control functions may shift to the opposite hemisphere, leading to left handedness. In either case, the male sex is more at risk.

In short, there are in fact three causes of left handedness: heredity, prenatal testosterone, and brain damage -and of course, these three factors interact.

There is no statistically significant difference between intelligence on IQ tests between left handers and right handers. The next logical question would be whether left and right handers differ as to cognitive profiles (verbal versus visuospatial). Unfortunately, IQ tests are not very adequate for answering this question: IQ test items were constructed or selected, at outset, so as to reduce, as much as possible, sex differences, and this had the effect of also reducing differences between left and right handers. It is more relevant, I think, to look at research which selected tests designed to measure hemispherically specialized functions -which is not the case of IQ tests. One set of studies carried out by Harshman and colleagues found that left handed men perform worse that right handed men on visuospatial (right hemisphere) tasks and yet better on certain verbal (left hemisphere tasks). However, these results could only be extracted when “reasoning ability” had been partialed out (i.e., statistically controlled). While these studies are important and useful, it is most pertinent, I think, to review investigations exploiting the two experimental neuropsychology techniques I outline in chapter 3: dichotic listening and tachistoscopy. A neuropsychologist called Jerry Levy has posited that women’s brain organization in charge of cognition resembles that of male left handers. Indeed, dichotic listening asymmetries and half-field effects of left handers generally resemble those of women. Levy proposed that cognitive functions are more bilaterally represented in women and that they are more hemispherically lateralized in men: verbal functions being located more in the left hemisphere and visuospatial functions being located more in the right hemisphere. She also argued that the visuospatial function is generally more efficient when it eschews verbal strategies. In other words, she stated that the visuospatial function is better processed by a specialized network located in one hemisphere and shielded from verbal interference (men), than by a non-specialized network located in both hemispheres (women). In short, the neuropsychological significance of left handedness is very perplexing: though men are more often left handed, left handers have cognitive profiles like women.

Sandra Witelson has proposed an intriguing neurodevelopmental model which seeks to explain sex differences not only in handedness but also in hemispheric asymmetry. She developed the hypothesis that naturally occurring loss of axons of the corpus callosum (either symmetric or asymmetric, with or without neuron death) may be one mechanism underlying the embryological development of hand preference and hemispheric anatomical and functional asymmetries in males. Specifically, she suggests that there is a subtle deficit in natural neuronal mortality, in healthy pruning of the male brain, during gestation. Supporting evidence for this hypothesis is noted from a report of increased prevalence of left-handedness in children born prematurely at the gestational age prior to the likely onset of axon loss.

Animal research into motoric asymmetry is globally concordant with the human handedness literature. Indeed female rats are generally more motorically lateralized than the males. Though there is no overall right or left paw preference in rats, each animal can be shown to prefer one or the other paw -when required to reach into a container for food. Females show stronger preference for their preferred paw. In other words, they are more consistently lateralized. Postural asymmetry is easy to measure in rats: the tail is either to the right or to the left. Here also, females are more lateralized. Rotational behavior can also be measured in terms of motoric asymmetry: rats turn more to the left or to the right. Females are more consistently asymmetric. These sex differences (which we suppose operate in the brain) are mediated by gonadal hormones. Indeed, gonadectomy attenuates the asymmetry in females but has no effect in males. Estradiol injections and naturally occurring estrous both increase the turning asymmetry observed in females.