topics: |  neuro-science | brain | biology | laterality | left-handedness

Abounds with interesting asides.

As an example of left-right asymmetry, try this on yourself:

hand-clasping:
quickly clasp your hands together with the fingers and thumbs interlaced. Which thumb is on top; the right or the left? Now try doing it the other way round and you'll find you have to pause a little and think.

Well, isn't that quite a grabby hook? This book is full of these.

Excerpts

Hand clasping doesn't seem to be learned; but it starts early in life [pic of his daughter Franziska at 6 weeks].

In Britain about 60% people clasp with left-thumb on top, and the proportion is the same for L- and R-handed people

Arm-folding: which wrist is on top - L or R? Now try to do it the other way around. When I use this demonstration in lectures there is invariably a delayed burst of laughter as people rotate their arms in front of them, only to realise they have ended up exactly where they started, with the same wrist still on top.

Development of Handedness

Charles Darwin [who was RH] 

   Now eleven weeks old take hold of his sucking bottle with right hand,—when
   nursed either on right arm or left— He has no notion of clasping it with
   left hand, even when it is placed on body— this baby has had no sort of
   practice in using its arms. ...

   When one day under 12 weeks took hold of Catherines finger with his right
   hand & drew it into his mouth.

   Now exactly 12 weeks & on following day old clasped his bottle with left
   hand, just like he did formerly with right hand— Therefore his right hand
   is at least one week in advance of left.— I say at least for I am not
   quite sure, the first time he used his right hand, was observed.

From these observations Darwin decided that his son would be right-handed.
But he relates, 'This infant proved to e left-handed, the tendency being no
doubt inherited - his grandfather, mother, and a brother having been
left-handed.  149

Check if you are left-handed

Which tasks do you do with hand? 

	Write
	Draw
	Throw a ball
	Brush your teeth
	Hold scissors
	Hold a knife (no fork)
	Hold a spoon
	Hold a cup
	Use a TV remote
	Open a coke can w ring pull

Add up the R and L hand usage.  Typically, about 2/3ds don't do any of the
tasks with their LH -> strong RH-ers.  Drops to about 1% around 5/10, and
then rises to about 3% for LH-only. 

 
results of above questionnaire on 3000+ 
schoolchildren in N. London.  
[dark=Female, light=male.]
(image from upside-down-world)

Definition of Left-handed: 
Those who use LH for half or more tasks. [11.6% of males, 8.6% of Females]

 
Tapley and Bryden handed-ness measure.  
Take a printout.  Make a dot in as many circles as you can. 
[Almost no one does this task equally well with both hands]
 
Language and Left-brain: 
   
	     | L-brain   |  R-brain  |
 -----------------------------------
  RH-ers   |	~95%	 |  ~5%	     |
  LH-ers   |	 ~5%	 |  ~30%     |
 -----------------------------------

Also more LH-ers are male than female. 

Do lefties die young?

It is a popular myth, and is almost certainly wrong. 

The myth started in 1988, with a paper in Nature by Diane Halpern and Stan
Coren, based on an encyclopedia of baseball players, handedness, and age of
death.  A further paper by the same authors in 91 gave this graph: 

 
NJ J of Medicine article; from tracking a group of elderly.  
Suggests a difference of as much as 7 years on average
between RH people and LH/mixed -  5 yrs for F, 8 years 
for M.  [Halpern & Coren 90]. 

[Coren suggested several possibilities, including the “Right Hand World”
theory, which states that the obstacles lefties experience in their day to
day life in a right-hand-oriented world are what lead to accidents, stress,
illness, and eventually increased mortality in lefties.  
]

[McManus p. 292:]
Results are entirely the result of a statistical artefact.  If correct, it
would be the most substantial influence on human lifespan known to modern
healthcare, equivalent to smoking 120 cigarettes a day... 

There is nothing wrong in the data per se - at the time of death, the
left-handers were indeed younger. 

The problem arises because one is looking at death cohorts - studying a group
of people who die at the same time.  "Birth cohort" studies have no such
problem.  Many differences in society and development are different.
Handedness is in fact a marker of being born later in the 20th c.  [Those
born earliers would be less likely to be marked RH]. 

[but, on the contrary, from 
Ramadhani, MK, etal (2007). Innate handedness and disease-specific
mortality in women. Epidemiology, 18(2), 208-12. 
	The relative risk for breast cancer in left handed women compared with
	non-left handed women was 1.39 overall. 

	What is already known on this topic: 
	High levels of sex hormone in utero may induce
	left handedness and may change breast tissue as a
	source for breast cancer. 

summary from http://healthpsych.psy.vanderbilt.edu/2008/LeftHand.htm : 

Ramadhani and colleagues studied the association between innate handedness
and total mortality, and cause-specific mortality, using 12,178 middle-aged
Dutch women who were followed for about thirteen years.  The cause-specific
mortality categories included total cancer, breast cancer, colorectal cancer,
circulatory system, and cerebrovascular issues.  Also, such potential
covariates as socioeconomic status, age, and BMI were adjusted for.  Overall,
the study was very thorough, and included a large representation of people,
albeit all female.  And, importantly, the study found a statistically
significant increase in mortality in these women, and an increase in disease
frequency (Ramadhani et al., 2007).  
]

Left handers suffer more from disease

from above website

There are more studies that conclude increased disease and disability in
left-handed individuals.  One group of biologists claims to have isolated the
gene that causes left handedness, the gene LRRTM1.  And, this gene is
associated with schizophrenia (Francks et al., 2007).  Another study examined
270 school-aged children and showed that right handed students statistically
outperformed left handed students in terms of language ability (Natsopoulos,
Kiosseoglou, Xeromeritou Alevriadoc, 1998).  Finally, there has been shown to
be a link between left handedness and increased PTSD symptoms; one study
randomly sampled 596 individuals, diagnosed 51 of them as having PTSD
symptoms, and found there were significantly more left handers among this
subgroup (Choudhary, O’Carroll, 2007).
]

[McManus p. 295, on immune disorders]:  In 1994, Phil Bryden and I unearhed
89 different studeis, by 25 teams, and involving over 21K patients and 34K
controls.  The results were clear: LH-ers showed no systematic tendency to
suffer from immune disorders.  

The right hands of primates

Touch the pad (tip) of your thumb against the pad of the other four
fingers. This action is unique to humans, and is called opposition.
Whereas you can lay the hand flat on the table and see all five
fingernails, when held in opposition the fingernails are facing in opposite
ways.  This is because the thumb rotates (and so does the little finger).
218

 
   Tulaia is no longer considered a primate. 
   Lemur to Tarsius are prosimians. 
   Leontecebus to Ateles are New World monkeys. 
   Macaca to Colobus are old world monkeys; 
   Hylobates to Gorilla are apes.  p. 219 

McManus Interview: Asymmetry in the brain

	http://www.abc.net.au/quantum/scripts98/9807/feature2.htm

CHRIS McMANUS: . . . the brain is symmetric in most animals as far as we can
tell and that the two halves of the brain each controls the opposite of the
body and there's exact equivalents between the two sides and it's only in
humans that one half of the brain has suddenly started doing something which
is utterly different to the other half and that's basically language. So that
I'm a right hander and as I sit talking here it's the left half of my brain
which is producing the language and for most of the audience who are
listening who are right handed it's the left half of their brain which is
interpreting what I'm saying. . . . about ten percent of the population are
left handed suggests there must be some advantage in being left handed but at
the moment we don't know what that is.

CHRIS McMANUS: Well I think first of all we probably developed asymmetry
because it came along with a lot of other powerful things at the same
time. And if we look at the evolutionary record, if we go back about two and
a half million years ago in the African rift valley we're finding for the
first time evidence of humans with a large brain. They're walking up right,
they're making stone tools; they're right handed stone tools as far as we can
tell. There are subtle asymmetries in their brain which suggests that the
left half is different to the right half and all the evidence suggests that
they're probably communicating through language.  . . . my reading is that is
what happened was that most animals are fifty/fifty right handers left
handers. Two million years ago all humans became right handed - that's one
hundred percent of them and that had its advantages.

QUANTUM: Your genetic model. Can it explain the relative proportions of right
and left handers?

CHRIS McMANUS: Well we think basically there's a gene for right handedness
which accounts for the fact that most people are right handed. And so most of
us have a double dose of this right handed gene, and that makes us right
handed and that's straight forward. But the other gene is not a left
handedness gene in any simple sense. Instead is what we call a chance
gene. And people who have a double dose of this chance gene do not end up as
either right hand or left hand. They have a fifty fifty chance of being right
or left handed. It's as if a coin is being tossed and the brain will only go
this way or that way. And it could be either way. There's no control there at
all.

The other thing that we can explain quite nicely is the fact identical twins
are not identical very often for their handedness. And this is always a
mystery to people. Why is that i've got two identical twins. Ones right
handed, ones left handed? Well the answer is that we expect that from this
genetic model because if those identical twins have the same genes and that
gene happens to be the chance genes then one twin tosses a coin and goes one
way and the other twin can toss the coin in uterus and go the other way and
when we do the mathematical calculations we get almost exactly the
proportions we'd expect.

QUANTUM: What are the implications of all this for language?

CHRIS McMANUS: Well what seems to be happening in language is that if you
read the text books you'll find that most people are right handed and most
people have language in the left hemisphere. And that's true. And probably if
you've got this double dose of the right handedness gene then indeed that's
the pattern you have. And so the text book descriptions are applied to the
people with the double right handedness gene. But if you've got that chance
gene then its a fifty fifty chance whether you have right or left handedness,
and it also is a fifty fifty chance whether language goes into your right or
left hemisphere. But those two fifty fifty chances are independent. Two
separate coins are being tossed if you like. And so for those people with
that gene a quarter are right handed in language in the right hemisphere a
quarter are right handed language in the left hemisphere and so on. So we get
all possible combinations and so left handers then are more likely to have
language in the right hemisphere than are right handers.

QUANTUM: So it's not entirely mythical then that left handers are more likely
to have, say, reading difficulties?

CHRIS McMANUS: I think that's probably right. There, there's no doubt when
you look at the statistics that left handers are somewhat over represented
amongst people with some language ah problems. And dyslexia is the one that's
always quoted but stuttering is another one that probably applies. You also
find some effects on childhood autistic and other conditions where there
seems to be an excess of left handers. It's not a very strong effect.

If you really want some evidence that handedness is biological rather than
environmental, nice study by Peter Hepper in Belfast who is looking at real
time ultra sonic scanning of foetuses in utero. And he found that foetuses of
course sit in the foetal position, their hands up like this, and they do what
all babies do; they suck their thumbs and you can see them sucking their
thumbs on the ultra, the dynamic ultra sound records and 90 percent of babies
suck their right thumb. Ten percent of babies suck their left thumb. Now
that's clearly not learned. Its not environmental or anything like that.