By LOUISE CARR, Associate Editor and Featured Columnist
With around 10 percent of the global population born left-handed it's a wonder we don't know more about the condition, but left handedness is still shrouded in mystery.
Theories and associations between health and handedness --- the technical name for the hand you choose to use most in your life --- abound, but we are far from being clear about the consequences of left handedness. Is being born left handed a threat to your health? Do left handed people have a greater risk of certain diseases and do they suffer more accidents? Or do left handed people live longer, and live smarter?
Six of the last 12 US Presidents, including Obama and Bush, have been lefties and their handedness hasn't held them back of course. Jim Carrey, Tom Cruise (and Nicole Kidman), Angelina Jolie (and Brad Pitt), Keanu Reeves, Julia Roberts, Oprah Winfrey, David Bowie and Sting are all left handed, to name a few. How does being left handed affect your health, and why?
Historical Prejudice Against Lefties
Ever since left handed people showed up in the population (which is probably as long as humans have been in existence) left handedness has been stigmatized in some way.
The words "left" and "left hand" have negative meanings in most global languages. Left means "sinister" in Latin and "dishonest" in Mandarin. In 1903 Italian criminologist Cesare Lombroso linked left-handedness with being a criminal and afflicted by mental illness and his views have been surprisingly prominent in some form ever since.
Up until recently parents and teachers were so afraid of their children's left handedness they would force them to favor their right hands. If we are to believe all of the theories that have been attached to left handedness - the cause of schizophrenia, greater risk of autism or ADHD, earlier death etc - we could assume that being born left-handed is one of the greatest risks to our health we could ever experience.
Why are Some People Left Handed?
Left handedness is linked to genetics but cannot be explained completely by our genes - identical twins don't always favor the same hand. Experts also link handedness to stress in the womb and other environmental factors that affect us before birth.
A 2003 study by Aarhus University Hospital, Aarhus, Denmark showed that women suffering stressful life events during pregnancy were more likely to give birth to left handed or mixed handed children.
And a 2008 study from Uppsala University, Sweden showed that mothers who had depressive symptoms or were under stress when pregnant were more likely to have left handed children.
Does being born left handed affect your health? In what way could left handedness deal you an ace, or the wrong cards? We looked through the scientific research into left handedness to see how favoring the left hand over the right makes a difference to your health and well-being.
1. Left Handedness Puts You at Greater Risk of ADHD
Looking at the way we favor one hand over the other sheds light onto how our brains are wired and may provide an insight into brain development conditions such as attention deficit hyperactivity disorder, or ADHD.
According to a 2008 study from Mid Sweden University in Ístersund left-handed children had a greater risk of language difficulties and ADHD symptoms. However, a 2012 study by Shiraz University of Medical Sciences, Iran showed a lack of association between inattention and hyperactivity symptoms in ADHD and handedness - being left handed made no difference to reported hyperactivity or inattentiveness.
It could be that mixed handedness - favoring one hand for certain tasks like writing and the other for other things - is more likely to be linked with ADHD symptoms than left handedness. A 2010 study by researchers at Imperial College London and Uppsala University, Sweden came to this conclusion.
2. Is Being Left Handed Linked with Schizophrenia?
This theory is drawn from the fact that while left handed people make up 10 percent of the population, 20 percent of people with schizophrenia are left handed. But does the theory stand up to scientific analysis?
A 2010 study from the National Institute of Mental Health, National Institutes of Health said no - left handedness or non-right handedness was not seen as a greater risk for schizophrenia in the 1,445 patients studied.
More evidence to refute the link comes from a 1995 study by Finch University of Health Sciences/Chicago Medical School which concluded that handedness was not an important consideration in the study of schizophrenia or psychosis. (Read more about foods that fight schizophrenia.)
3. Do Lefties Live Longer or Die Younger?
It's a popular theory that left handed people have shorter life spans than their right handed counterparts, mainly down to population studies showing how the percentage of left handed people falls when the population ages. For example, around 10 percent of a population of young adults will report left handedness, while only around 1 percent of 80-year-olds will favor their left hand. What happens to all the left-handers? Why do 99% of lefties die before age 80?
One conclusion is that lefties die younger due to accidents caused by using tools or following the norms of a right-handed world. Another theory states that the stress during fetal development which led to left handedness also causes people to suffer certain health conditions - this theory is put forward in 1991 research by the University of British Columbia, Vancouver, Canada.
However, it could be that left handed people live just as long as right handed people but they describe themselves as right handed as they get older due to stigma or because they were forced to switch hand dominance when younger.
Researchers at the University of Bergen, Norway in 1993 leaned towards this hypothesis although couldn't rule out the idea that lefties died younger. Handedness was not significantly linked to life expectancy, according to a 2011 study into baseball players by the School of Community Health Sciences, University of Nottingham, UK.
But according to a 2007 study by University Medical Center Utrecht, Utrecht, The Netherlands, left handedness is associated with higher mortality in women. Do lefties live shorter lives than righties? At present no one is really sure but the evidence seems to suggest that, for unknown reasons, lefties die sooner.