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Linkshaender Beratung (a.k.a. Lefthander Consulting) warns that trying to convert someone from being left-handed to right-handed (or vice versa) leads to a number of direct and indirect harms.

The possible primary consequences of converting handedness are as follows:

  • memory disorders (especially in the recall of learned material)
  • disturbances in concentration (being easily tired)
  • legasthenic problems or dyslexia (i.e. problems in reading and writing)
  • spatial disorientation (uncertainty concerning the left and right)
  • disorders in fine motor skills that manifest themselves in handwriting
  • disturbances in speech (ranging from stammering through stuttering)

[Emphasis added]

They claim to have empirical evidence to support these claims:

In the Consulting Center for Left-handers and Converted Left-handers, an attempt was made to precisely quantify this increased amount of average energy required. In contrast to the many earlier scientific studies, a wide range of data from 4,510 subjects was available for study.

There are too many claims for one question, so to select an arbitrary one:

Is it true that a left-hander trying to convert to using their right hand leads to memory disorders?

I found a similar question (Is there any harm in forcing/re-educating a left-handed child to use their right hand?), but it has only one answer and conflicts with some things I've read and supports others.

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    Whether something is 'bad' is a value judgement. Please find a specific claim, and ask if it is true. – Oddthinking Sep 17 '16 at 0:18
  • I felt I did that... if the bad shown by studies is there, trained ambidexterity is bad; otherwise it is good, as there are obvious benefits such as those I listed. I do not think it is a value judgment to say that something which causes memory, coordination, and socialization problems is bad. Is doing meth daily for 10 years bad? I guess the title and some of the phrasings are opinion-based. I can change them to "bad for the brain," I guess. – Lupe Sep 17 '16 at 0:57
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    Where did you get that trained ambidexterity causes memory problems? Find that claim - sounds like an interesting one to investigate! "Is meth bad?" <- Poor question. "Does meth cause decreased performance on verbal memory tests, as claimed [here]?" <- Good question. – Oddthinking Sep 17 '16 at 2:02
  • Well, okay. I see that at the very least my question was way too broad; this site isn't really meant to support what it asked. I found that claim here: linkshaender-beratung.de/english/Problem.htm If I later edited my question to ask whether the listed consequences have truth, would that be okay? – Lupe Sep 17 '16 at 15:35
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    I just realized I ignored your point that trained left-right and ambidexterity aren't the same. But the speculation behind the supposed studies for both says that it's "confusing the brain" / ..., applicable for both. Harm in trained right handedness of one's own choice may be a better post than ambidexterity. – Lupe Sep 18 '16 at 3:22
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Current evidence:

  1. Non-dominant hand training activates the dormant motor control areas in the brain.

The path from brain to dominant hand is well-traveled, she explains. But when a person starts using the non-dominant hand more than usual, the brain has to start activating areas that have remained comparatively dormant. Like an atrophied muscle, the motor control areas connected to the off hand are smaller and less developed than those associated with the dominant one. Despite our genetic predispositions, however, many people do change handedness. Mostly, they are forced to switch as a result of injury, Porac says. Source: Probing Question: Can you change the handedness you were born with?

  1. With reference to Alex Goddard, graduate student, Neuroscience, Harvard Medical School, there is no significant harm when a left-handed (dominant) person is forced to write with their right (non-dominant) hand.

As far as I can tell, making a left-handed person write with their right (non-dominant) hand does not cause any significant harm. The little research I could find suggested that a spectrum of handedness exists; people fall along a right handed -- left handed continuum. Furthermore, the more dominant the left hand (as measured by reaction times, strength, etc), the less likely the person will be able to switch to writing with their right. Thus, if you're writing with your right hand, the implication is that your left hand is slightly less dominant than a person who couldn't make the switch. Source: Re: What is the effect of changing the child from left handed to right handed

  1. Referring to Michael Corballis, professor of cognitive neuroscience and psychology at the University of Auckland in New Zealand, humans are dextral by nature and tampering (trying to force or train) with this is postulated to lead to psychological problems.

The two hemispheres of the brain are not interchangeable. The left hemisphere, for example, is typically responsible for language processing, whereas the right hemisphere often handles nonverbal activities. These asymmetries probably evolved to allow the two sides of the brain to specialize. To attempt to undo or tamper with this efficient setup may invite psychological problems. Source: Can Training to Become Ambidextrous Improve Brain Function?

  1. Converted left handed writers show features of lefthandedness during right-hand writing.

Adult converted left-handers show persistent features of lefthandedness during right-hand writing. Extending previous studies, which emphasized functional asymmetries at the executive level of the motor system (e.g., the SM1), our results provide evidence for a neural substrate of human handedness in premotor and parietal motor association areas. Source: Long-Term Consequences of Switching Handedness: A Positron Emission Tomography Study on Handwriting in “Converted” Left-Handers

  1. Switched individuals share both left-hander and right-hander features regarding their motor control architecture and hand preference profiles.

In both left- and right-handed subjects, the preferred hand is controlled mainly by the hemisphere contralateral to that hand, whereas the nonpreferred hand is controlled by both hemispheres. The switched individuals share features of both left-handers and right-handers regarding their motor control architecture and hand preference profiles. Increasing levels of complexity of motor activity results in an increase in the volume of consistently activated areas and the involvement of the ipsilateral in addition to contralateral activations, especially in thedominant hemisphere. Source: Switching handedness: fMRI study of hand motor control in right-handers, left-handers and converted left-handers

  1. In adult "converted" left-handers who were forced as children to become dextral writers, changes occur not only in primary sensory-motor cortex but also in the deep structures of the brain.

Our results show that a specific environmental challenge during childhood can shape the macroscopic structure of the human basal ganglia. The smaller than normal putaminal volume differs markedly from previously reported enlargement of cortical gray matter associated with skill acquisition. This indicates a differential response of the basal ganglia to early environmental challenges, possibly related to processes of pruning during motor development. Source: Nurture versus nature: long-term impact of forced right-handedness on structure of pericentral cortex and basal ganglia.

TL;DR:

  1. Although it is not proven by research at this point that converting handedness causes memory disorders, it is notable that there are differences between the structures of the brain adult "converted" left-handers and consistent right- and left-handers for a specific task of handwriting. (Correlation does not imply causation)

  2. Advise from both medical professionals and logical reasoning is that it is best not to force children to change their "natural" handedness.

Researchers still don’t understand why around 10 per cent of the population turn out to be left-handed. Most children have a preference for using one hand or the other by the age of about 18 months, and are definitely right or left-handed by about the age of three. If your child is naturally left-handed, don’t try to force them into using their right hand. Source: Left-handedness

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