Handedness

from Wikipedia, the free encyclopedia
Dominant (leading the tool) and non-dominant (supporting) hand

In humans and animals, handedness is the preferred use of a certain hand, especially for demanding and fine motor activities. Depending on which hand is the dominant hand, one speaks of left-handed or right-handed .

The degree of handedness can vary greatly. In addition, the handedness can change in the course of life. B. through upbringing or unconscious copying and with adults through conscious learning. People who are or were actually left-handed and who have learned to write with the right are also called retrained left-handed people .

People with no or only weak dominance of one hand are often referred to as two -handed. This ability to use one's own hands equally is known as two-handedness (also called ambidexterity ).

Frequency of left and right handed people

A clear overrepresentation of right-handedness can be found in all well-researchable human cultures. The proportion of left-handers in the European population is given by the German Society for Occupational Medicine and Environmental Medicine at 10 to 15%. The proportion of so-called two-handed people is a matter of definition: over 60% of right-handed people and around 99% of left-handed people also use the non-dominant hand to carry a broom or a toothbrush, but the more demanding the tasks, the more the proportion of functional two-handed people tends to zero .

Historical differences cannot be identified either. The analysis of archaeological finds (such as the examination of tools for signs of wear) suggests that even the Neanderthals , extinct relatives of modern humans, were predominantly right-handed. This cannot be clearly established for other “ancestors” of humans, since the finds (bones, tools, ...) are often insufficient to make statements about handedness or even the preferred hand in a population group. The oldest evidence of right-handedness is a 1.8 million year old upper jaw of a Homo habilis . For the present, a majority right-handedness can be determined in all civilizations.

Determination of handedness

The information on the proportion of left and right-handers in the population can vary greatly from study to study. In large part, this is because there are different definitions of handedness and different methods of determining them. The methods range from simple questions from the participants, to observing certain activities, to testing the accuracy and speed of the hands in various tasks.

The definitions range from the division into two classes (left and right-handed) or three classes (left, right and two-handed) to scales that measure any degree of handedness. In addition, definitions of handedness usually relate to specific activities because a person does not necessarily prefer the same hand in different activities. In the simplest case, this can be a single activity. Often right-handed or left-handed people simply mean people who write with their right or left hand . If several activities are to be taken into account, one often falls back on standardized lists that, in addition to writing, also include many other everyday activities such as Include throwing, brushing teeth, or drinking from a cup. More commonly used test methods include e.g. B. the Crovitz-Zener Inventory, the Annett Handedness Questionnaire, the Edinburgh Inventory and the Waterloo Handedness Questionnaire.

causes

It is not easy to answer why a person is left-handed or right-handed. Even in ancient times there were different views about the cause of handedness. So went z. B. Aristotle assumes that handedness is innate. Plato, on the other hand, assumed that at birth both hands are still completely the same and that handedness is a result of upbringing. He demanded that care should be taken in education that the dexterity of both hands is trained.

To this day, the question of whether the genes or the environment are responsible for handedness has not been conclusively clarified. The observation that left-handed parents are more likely to have left-handed children than right-handed parents can be explained by the influence of genes or the environment. Studies with twins, however, show that both genes and the environment influence handedness. On the one hand, identical twins have the same handedness more often than dizygoti, on the other hand, it is not uncommon for identical twins to have different handedness.

Genetic causes

The two best known models of the genetic cause of handedness are the "right shift" theory by M. Annett and the "dextral / chance" theory by McManus. Both models assume that only a single gene is responsible for handedness and both are based on the idea that there is not a gene variant for the left and a gene variant for the right, but only one for the right (R) and a neutral one (N ). Since humans have two sets of chromosomes ( diploidy ), there are four possible combinations. The combination R / R leads to right-handedness, R / N and N / R lead to right-handedness more often than to left-handedness and with N / N left and right-handedness are equally likely. This theory explains why there are more right-handed people than left-handed people, and why identical twins with the same genes can have different handednesses.

Whether handedness is actually determined by a single gene and which one it is remains unclear to this day. The PCDH11Y gene on the Y chromosome and the LRRTM1 gene on chromosome 2 were proposed as candidates .

Conditions during development

The Geschwind-Behan-Galaburda model assumes that an increased testosterone level during embryonic development inhibits the growth of the left hemisphere, which leads to a dominance of the right hemisphere and thus to left-handedness. This model can help explain why men are more likely to be left-handed than women.

Another theory explaining handedness with conditions during development was suggested by FH Previc. In the last trimester of pregnancy, most children lie with their right ear forward (from the mother's point of view). This is said to specialize the left hemisphere in speech because the right ear is stimulated with more sounds, and the left hemisphere specialize in motor skills because the child's sense of balance on the left side is stimulated more when the mother walks.

Education and culture

The influence of upbringing, and thus also culture, on handedness can be observed in the change over time in the proportion of left-handers, which increased noticeably during the second half of the 20th century. Whereas in the past all children were taught to write with the right hand, the opinion that prevailed in western countries was that schoolchildren should not be forced to write with the right hand. This effect can also be seen in the higher proportion of left-handers among children of Asian immigrants in the USA compared to children in Asia.

"Handedness" in animals

Typical asymmetries of the nervous system can already be observed in molluscs and crustaceans . In higher mammals there are many examples of handedness, which is mostly evident in the procurement of food. In walruses , for example , it has been observed that they predominantly use the right front fin to dig for mussels on the sea floor. Rats , cats, and monkeys all prefer one of their limbs to perform more demanding tasks, but right-handed and left-handed people are roughly equally represented. Great apes (like humans) do not yet show pronounced handedness for simple tasks; the more complex the task, the clearer the standardized distribution of roles between the hands becomes. A predominant right- or left-handedness at group level was only observed occasionally: for example, dwarf chimpanzees should prefer the left hand for carrying, and common chimpanzees for throwing the right hand.

Wild kangaroos and related marsupials show a preference for the left front paw in everyday life, such as scratching the nose, picking a leaf or bending down a branch. The degree of handedness in marsupials is surprising because, unlike placenta animals, marsupials do not have a cerebral bar connecting the halves of the brain. The left-handedness of the red giant kangaroo and the eastern gray giant kangaroo is particularly striking . The smaller red-necked wallaby seems to prefer the left front paw for fine manipulation, but prefer to use the right paw for applications that require strength. This expression does not appear in arboreal marsupials or marsupials, which usually move on all four legs.

In dolphins , when turning to food at the bottom, a "legal fluidity" greater than 90% was found.

Laterality of the brain and hand

Left and right hemisphere of the brain ( transverse section through a human brain - single image of a magnetic resonance tomography )

Findings from studies on the lateralization of the brain show that the left hemisphere is mainly responsible for motor control of the right side of the body and the right hemisphere for that of the left side of the body.

The “dominant” hemisphere of the brain is the one that is primarily responsible for processing language. In 95% of right-handers it is on the left and in 2% on the right. In 3% the language center is divided between both hemispheres. Even with the majority of left-handers, the language center is on the left, namely in 70%, in 15% it is distributed over both hemispheres, in a further 15% it is on the right. So there is a certain correlation between the “dominant” half of the brain and that half of the brain that controls the strong hand. Nevertheless, for the majority of left-handers, the language center is on the left side, which is responsible for the “weak” right hand. So there is no clear law in the connection between handedness and lateralization of the brain. Assuming a share of 10% left-handers, around 7% of the population have control of the “strong” hand and the language center in different halves of the brain - without any disadvantages.

Targeted magnetic impulses ( transcranial magnetic stimulation , TMS) over the motor cortex made it possible to temporarily “switch” the choice of hand for a simple activity.

advantages

In evolution , the hemispherical specialization of the brain presumably prevailed because the two brain hemispheres could not compete with each other. Advantages of one-handedness over two-handedness are seen in the greater precision of the manual work through specific training. The advantages of majority right-handedness are unclear. The "battlefield theory" is said to come from Thomas Carlyle , according to which soldiers carried weapons on the right and shields on the left , which enabled them to better protect their hearts. However, shields have only been in use since the Bronze Age , i.e. for about 5000 years, and even primitive humans who lived much earlier (about two million years ago) were predominantly right-handed.

Effects on the functionality of tools

Some tools are adapted to the right hand, such as the corkscrew with a right-hand thread. A flipbook is right held in the binding and tense there by bending the left thumb provides counter-support and dosed the leaves - a book is kept exactly the opposite, counter-paged through and you look at it, the front sides of the sheets. The cutting edges of a pair of scissors cut better when they are pressed together by the torques normally exerted by the thumb and fingers of the right hand. Heavy tailor's scissors, but also some household scissors with plastic handles (eyes), have a tunnel that is diagonally oriented to the transverse direction, especially for the thumb, so that the left thumb does not fit through when gripping. Left-handers need exactly the mirror-image construction.

Eyes, ears and feet

Ankle bone and swing leg

Other lateralizations besides handedness are eyesight, eariness and footing. These lateralizations are usually determined using questionnaires ("With which eye do you look through a telescope?"). When people prefer different sides of different organs, one speaks of "crossed lateralization". Of people who are “clearly lateralized” with regard to all four pairs of organs, a good 63% show a right congruence, a good 3% a left congruence and a good 33% an incongruence. A third of these people therefore show crossed lateralization.

In sport one speaks of right and left footedness, sometimes also of right and left leggedness, or one makes discipline-dependent distinctions. B. between play and support leg, in athletics between ankle and swing leg. The footing is more evenly pronounced than the handedness. About half of the people are right-footed, the other more left-footed.

literature

  • FR Wilson: The hand - a stroke of genius in evolution. Rowohlt Taschenbuch Verlag, Reinbek bei Hamburg 2002, ISBN 3-499-61338-7 .

Individual evidence

  1. ↑ German Society for Occupational Medicine and Environmental Medicine e. V .: Guideline handedness - meaning and investigation (status 11/2014).
  2. ↑ a b c d J. F. Stein and CJ Stoodley: Neuroscience. An Introduction . John Wiley & Sons, Chichester 2006, ISBN 1-86156-389-2 , p. 428.
  3. ↑ Brain research: even Neanderthals preferred the right hand. In: Spiegel Online. April 21, 2011, accessed October 22, 2016 .
  4. ↑ a b L. Cashmore, N. Uomini and A. Chapelain: The evolution of handedness in humans and great apes: review and current issues (PDF file; 889 kB) In: Journal of Anthropological Sciences . Vol. 86, 2008, pp. 7-35 (article on isita-org.com, as of July 1, 2010).
  5. ↑ David W. Frayer, Ronald J. Clarke , Ivana Fiore, Robert J. Blumenschine, Alejandro Pérez-Pérez, Laura M. Martinez, Ferran Estebaranz, Ralph Holloway, Luca Bondioli: OH-65: The earliest evidence for right-handedness in the fossil record . In: Journal of Human Evolution . tape 100 , November 2016, October 18, 2016, p. 65-72 , doi : 10.1016 / j.jhevol.2016.07.002 .
  6. ↑ HF Crovitz & K. Zener: A group test for assessing hand and eye dominance . In: American Journal of Psychology . tape 75 , 1962, pp. 271-276 .
  7. ↑ M. Annett: A classification of hand preference by association analysis . In: British Journal of Psychology . tape 61 , 1970, pp. 303-321 .
  8. ^ RC Oldfield: The assessment and analysis of handedness: The Edinburgh inventory . In: Neuropsychologia . tape 9 , no. 1 , 1971, p. 97-113 , doi : 10.1016 / 0028-3932 (71) 90067-4 .
  9. ↑ P. Bryden: Measuring handedness with questionnaires . In: Neuropsychologia . tape 15 , no. 4-5 , 1977, pp. 617-624 .
  10. ^ Aristotle, Magna Moralia , Book 1, 1194.b.32.
  11. ^ Plato, Nomoi , Book 7, 788a – 795d.
  12. ↑ a b I.C. McManus & MP Bryden (1992). The genetics of handedness, cerebral dominance and lateralization. In: I. Rapin & SJ Segalowitz (Eds.), Handbook of Neuropsychology, Volume 6, Section 10: Child neuropsychology (Part 1) (pp. 115-144). Amsterdam: Elsevier
  13. ↑ Walter McKeever, British Journal of Psychology (2000). A new family handedness sample with findings consistent with X-linked transmission.
  14. ↑ M. Annett, British Journal of Psychology, 63, 343-358 (1972). The distribution of manual asymmetry.
  15. ^ IC McManus, Psychological Medicine, Monograph Supplement no. 8, 1-40 (1985). Handedness, language dominance and aphasia: a genetic model.
  16. ^ The genetics of lateralization. In: The Economist. April 26, 2001, Retrieved July 9, 2016 .
  17. ↑ Tinka Wolf: Researchers discover left-handed gene. In: Welt Online. August 2, 2007
  18. ↑ FH Previc, Psychological Review 98, 299-334 (1991). A general theory concerning the prenatal origins of cerebral lateralization in humans.
  19. ^ A b V. Llaurens, M. Raymond, C. Faurie, Philosophical Transactions of the Royal Society B, Vol. 364, Issue 1519 (2009). Why are some people left-handed? An evolutionary perspective
  20. ↑ JF stone and CJ Stoodley: Neuroscience. An Introduction . John Wiley & Sons, Chichester 2006, ISBN 1-86156-389-2 , p. 432.
  21. ↑ Nette Levermann, Anders Galatius, Göran Ehlme, Søren Rysgaard, Erik W Born: Feeding behavior of free-ranging walruses with notes on apparent dextrality of flipper use . In: BMC Ecology . 3, No. 1, 2003, p. 9. doi : 10.1186 / 1472-6785-3-9 .
  22. ↑ JF stone and CJ Stoodley: Neuroscience. An Introduction . John Wiley & Sons, Chichester 2006, ISBN 1-86156-389-2 , pp. 428 and 433.
  23. ↑ Most kangaroos are 'left-handed' , BBC.com , June 18, 2015; accessed on December 31, 2015
  24. ↑ parallel Emergence of True handedness in the Evolution of Marsupials and Placentals in Current Biology , Volume 25, Issue 14, pages 1878-1884, July 20, 2015
  25. ↑ Most dolphins are Rechtsflosser orf.at, November 28, 2019, accessed November 28, 2019.
  26. ^ PA Young, PH Young and DL Tolbert: Basic Clinical Neuroscience . 2nd Edition. Lippincott Williams & Wilkins, Philadelphia 2008, ISBN 0-7817-5319-8 , page 364.
  27. ^ All percentages in JF Stein and CJ Stoodley: Neuroscience. An Introduction . John Wiley & Sons, Chichester 2006, ISBN 1-86156-389-2 , pp. 427-440.
  28. ^ Richard B. Ivry, Julie Duque, Timothy Verstynen, Jörn Diedrichsen, Flavio TP Oliveira: Transcranial magnetic stimulation of posterior parietal cortex affects decisions of hand choice . In: Proceedings of the National Academy of Sciences . tape 107 , no. 41 , October 12, 2010, ISSN  0027-8424 , p. 17751-17756 , doi : 10.1073 / pnas.1006223107 , PMID 20876098 .
  29. ↑ JF stone and CJ Stoodley: Neuroscience. An Introduction . John Wiley & Sons, Chichester 2006, ISBN 1-86156-389-2 , p. 435.
  30. ^ Paul Geraedts: Motor development and control. An introduction for physiotherapists, occupational therapists and trainers. Springer, Heidelberg 2019, p. 4.
  31. ↑ Anke Siefer, Walter H. Ehrenstein, Birgit E. Arnold-Schulz-Gahmen, Jürgen Sökeland, Alwin Luttmann: Population statistics and association analysis of sensory motor side preferences and their relevance for various professional fields of activity. In: Zentralblatt für Arbeitsmedizin, Arbeitsschutz und Ergonomie 53 (2003), pp. 346–353, here: p. 349.
  32. ↑ Eugen Morf: limb, ankle bone, and their mutual relationship to one another. In: Quarterly publication of the Natural Research Society in Zurich 96 (1951), pp. 223–239.