Left-Handed People Die Young
TYPES OF HANDEDNESS
Redirected handedness = learned handedness
Natural handedness = genetic or inherited
Pathological handedness = caused by brain injury or other pathology
HISTORY
left-handed people have been treated
or forced to convert
sinister comes from the latin for left
INCIDENCE
8-10% of people are left-handed
data and research show variations of that number across demographics and time periods
No evidence has ever been found showing that all left-handers have anything in common.
negative hand paintings in Paleolithic caves
oldest from western Europe, 10,000 to 35,000 years ago
507 hand paintings: 79/343 (almost 1/4) are unambiguously of the left hand
Research in France in 2002 gave this same task to college students, found same distribution
same students 9% leftie when throwing a ball, 8% left handed when writing
the ones who held the tube in right hand to image the left were normally righties
of those who held it in left hand only 1/3 were normally lefties
400,000 ya: Homo neanderthalensis held meat in teeth, pulled meat with left and cut with right
sometimes left mark on teeth showing direction of cut. Mostly right handed.?
8% not at all consistent worldwide, 10% reported by many studies
In "formal" countries (handedness enforced in schools)-->few lefties
3.5% China, 2.5% Mexico, 0.7% Japan
In "informal" countries: up to a high of 12.8% in Canada
MARKED DIFFERENCES IN LIFE EXPECTANCY
males lefties more often than females
fewer lefties in older age groups
~15% of 10-year-old children are left-handed, then percentage declines
age 90: virtually no left-handers left
women live longer than men; add handedness, the difference is startling
famous article published in 1991 in Psychological Bulletin
right-handed women have a life expectancy of around 77
left-handed men only live to about 62
left-handed women and right-handed men have nearly identical life expectance of 72
overall right-handers live 9 years longer than left-handers!
SPECULATION ON MECHANISM
Popular suggestion: lefties switch, but stats don't show how this matters
Death certificates don't include handedness: no data
accidents caused by left-handed living in a right-handed world?
in all 5 categories of life-threatening accident types lefties are 1.2-1.8x more likely to suffer fatal accidents than righties (sports, work, home, tools, and driving)
STRESS DURING PREGNANCY AND BIRTH CORRELATE WITH LEFT HANDEDNESS
1988 survey found that in 30 of 33 publications: infants who had undergone birth stress were significantly more likely to be left-handed
Lower Apgar scores - a measure of a baby's overall condition at birth - have been clearly associated with left-handedness
1987 study found that 1/3+ of 4yo born prematurely were left-handed
54% of children born with extremely low birth weights were left-handed
Left-handers are twice as likely as right-handers to have had a stressful birth.
long-term neurological damage?
Hypoxia: mothers who smoke during pregnancy, which causes hypoxia to the fetus, are more likely to produce left-handed offspring
OTHER HEALTH ASSOCIATIONS
certain immune deficiencies more prevalent
elevated testosterone levels in the fetus? T seems to slow the development of neurons in the left hemisphere of the brain, causing the right hemisphere to develop better, resulting in dominance of the left side of the body.
WHY LEFTIES DIE OUT
combination of factors put them at higher risk of impaired longevity:
accidents, neuropathology, immune deficiencies, other causes
Heritability: left-handed children are more likely to have left-handed parents.
No genetic markers confirmed, lots of studies showing possible markers
handedness among identical twins, fraternal twins, general population: all equal
Identical twins: are discordant (one leftie one rightie) just as often as fraternal and all other sibs
one leading theory is that is has to do with physical positioning within the womb
Twin fetuses usually face each other
limbs adopt mirror-image positions
Handedness often predicted by which hand is most often held closest to the fetus' mouth
this may explain why identical twins are so frequently discordant
SPECIAL LEFTIE APTITUDES
weak evidence except the most commonly believed ones
no good evidence that left-handers are especially creative or less analytical (R brained)
College educated left-handers seem to earn more money
more likely to be homosexual or to have gender identity problems
better at geometry and spatial analysis
better able to think outside the box better
nearly all such conclusions were based on a small number of small studies countered by others
the author remains skeptical
SPORTS AND COMBAT
athletes and soldiers trained to compete against righties
lefties have advantage
top athletes have a larger distribution of left-handedness, esp in sports like baseball
no dt talent, rather dt participants being less skilled at countering them
SOURCE
http://skeptoid.com/episodes/4310 AKA Skeptoid #310, May 15, 2012
Redirected handedness = learned handedness
Natural handedness = genetic or inherited
Pathological handedness = caused by brain injury or other pathology
HISTORY
left-handed people have been treated
or forced to convert
sinister comes from the latin for left
INCIDENCE
8-10% of people are left-handed
data and research show variations of that number across demographics and time periods
No evidence has ever been found showing that all left-handers have anything in common.
negative hand paintings in Paleolithic caves
oldest from western Europe, 10,000 to 35,000 years ago
507 hand paintings: 79/343 (almost 1/4) are unambiguously of the left hand
Research in France in 2002 gave this same task to college students, found same distribution
same students 9% leftie when throwing a ball, 8% left handed when writing
the ones who held the tube in right hand to image the left were normally righties
of those who held it in left hand only 1/3 were normally lefties
400,000 ya: Homo neanderthalensis held meat in teeth, pulled meat with left and cut with right
sometimes left mark on teeth showing direction of cut. Mostly right handed.?
8% not at all consistent worldwide, 10% reported by many studies
In "formal" countries (handedness enforced in schools)-->few lefties
3.5% China, 2.5% Mexico, 0.7% Japan
In "informal" countries: up to a high of 12.8% in Canada
MARKED DIFFERENCES IN LIFE EXPECTANCY
males lefties more often than females
fewer lefties in older age groups
~15% of 10-year-old children are left-handed, then percentage declines
age 90: virtually no left-handers left
women live longer than men; add handedness, the difference is startling
famous article published in 1991 in Psychological Bulletin
right-handed women have a life expectancy of around 77
left-handed men only live to about 62
left-handed women and right-handed men have nearly identical life expectance of 72
overall right-handers live 9 years longer than left-handers!
SPECULATION ON MECHANISM
Popular suggestion: lefties switch, but stats don't show how this matters
Death certificates don't include handedness: no data
accidents caused by left-handed living in a right-handed world?
in all 5 categories of life-threatening accident types lefties are 1.2-1.8x more likely to suffer fatal accidents than righties (sports, work, home, tools, and driving)
STRESS DURING PREGNANCY AND BIRTH CORRELATE WITH LEFT HANDEDNESS
1988 survey found that in 30 of 33 publications: infants who had undergone birth stress were significantly more likely to be left-handed
Lower Apgar scores - a measure of a baby's overall condition at birth - have been clearly associated with left-handedness
1987 study found that 1/3+ of 4yo born prematurely were left-handed
54% of children born with extremely low birth weights were left-handed
Left-handers are twice as likely as right-handers to have had a stressful birth.
long-term neurological damage?
Hypoxia: mothers who smoke during pregnancy, which causes hypoxia to the fetus, are more likely to produce left-handed offspring
OTHER HEALTH ASSOCIATIONS
certain immune deficiencies more prevalent
elevated testosterone levels in the fetus? T seems to slow the development of neurons in the left hemisphere of the brain, causing the right hemisphere to develop better, resulting in dominance of the left side of the body.
WHY LEFTIES DIE OUT
combination of factors put them at higher risk of impaired longevity:
accidents, neuropathology, immune deficiencies, other causes
Heritability: left-handed children are more likely to have left-handed parents.
No genetic markers confirmed, lots of studies showing possible markers
handedness among identical twins, fraternal twins, general population: all equal
Identical twins: are discordant (one leftie one rightie) just as often as fraternal and all other sibs
one leading theory is that is has to do with physical positioning within the womb
Twin fetuses usually face each other
limbs adopt mirror-image positions
Handedness often predicted by which hand is most often held closest to the fetus' mouth
this may explain why identical twins are so frequently discordant
SPECIAL LEFTIE APTITUDES
weak evidence except the most commonly believed ones
no good evidence that left-handers are especially creative or less analytical (R brained)
College educated left-handers seem to earn more money
more likely to be homosexual or to have gender identity problems
better at geometry and spatial analysis
better able to think outside the box better
nearly all such conclusions were based on a small number of small studies countered by others
the author remains skeptical
SPORTS AND COMBAT
athletes and soldiers trained to compete against righties
lefties have advantage
top athletes have a larger distribution of left-handedness, esp in sports like baseball
no dt talent, rather dt participants being less skilled at countering them
SOURCE
http://skeptoid.com/episodes/4310 AKA Skeptoid #310, May 15, 2012