Left-handedness is intriguing to many people because it represents a departure from the norm. Most people, around 90% worldwide, are right-handed. So what causes left-handedness and are the brains of left-handers different than right-handers? Research into left-handedness provides some fascinating insights into how the brain works.
Quick Answers
– Left-handers use the same parts of the brain as right-handers. There are no specific “left-handed” brain areas.
– However, the two brain hemispheres are often more symmetrically organized in lefties.
– Lefties show greater interconnectivity between the two sides of the brain. This allows lefties to access both hemispheres for tasks.
– Two main theories explain left-handedness: 1) natural variation due to genes, 2) birth stress affecting development.
– Lefties excel in some cognitive areas but also have a slightly higher risk of some mental health disorders.
– Around 10% of the population is left-handed. Left-handedness runs in families.
– The brains of lefties are functionally identical to righties but subtly structurally different.
– Culture and social pressures strongly influence handedness. Many lefties were forced to switch to right as children.
Left and Right Brain Hemispheres
The human brain is divided into two halves or hemispheres – the left and right. The left brain controls the right side of the body, while the right brain controls the left side. For most right-handers, the left hemisphere is dominant. It controls language abilities and logical processing. When right-handers perform a task, they predominantly use the left side of their brains.
For left-handers, the differences between the two brain hemispheres are less pronounced. The left and right hemisphere interact more evenly. Lefties utilize both sides of their brain more symmetrically than righties.
Research using MRIs shows the corpus callosum, the bridge between the two hemispheres, tends to be larger in left-handers. This allows for greater communication between the two sides of the brain. Some key differences have been found between left and right-handers in the structure and function of the corpus callosum.
Corpus Callosum Differences
| Left-Handers | Right-Handers |
|---|---|
| More symmetrical hemispheres | Left hemisphere dominance |
| Larger corpus callosum | Smaller corpus callosum |
| Better inter-hemispheric communication | Less inter-hemispheric communication |
This more balanced brain organization allows left-handers to access both hemispheres more evenly during tasks. For example, one study found lefties utilized both hemispheres of their brain when performing a simple motor task with their right hand. Right-handed people predominantly used their left hemisphere only.
So the brains of left-handers are not radically different, but rather subtly reorganized in key areas compared to righties. Next we’ll explore some of the leading theories that explain these left-right brain differences.
Theories on Left-Handedness Causes
There are two main theories that seek to explain the underlying causes of left-handedness:
1. Genetic and Natural Variation
Handedness has a strong hereditary component. If one or both parents are left-handed, their children are more likely to also be lefties. The chance is:
– 2 left-handed parents – 36% chance of left-handed child.
– 1 left-handed parent – 19% chance.
– 2 right-handed parents – 9% chance.
This suggests hand preference is influenced by genetics. However, no single “left-handed gene” has been identified that explains handedness. Instead, scientists propose left-handedness results from the complex interplay of multiple genes. The genetic basis gives rise to natural variation and a spectrum of hand preferences within the population.
2. Prenatal Hormone Exposure
According to this theory, exposure to different levels of testosterone before birth impacts handedness. Higher prenatal testosterone exposure increases the chances of being left-handed.
Events causing elevated maternal testosterone could explain higher rates of left-handedness:
– Birth stress. Difficult or lengthy labor may raise testosterone.
– Prior miscarriages. This also increases testosterone.
– Twin pregnancies. Twins show higher left-handedness rates.
Both genetic variation and prenatal hormones likely play a role in establishing handedness before birth. But other factors can also contribute after we are born. Let’s explore how handedness develops.
Development of Handedness
Hand preference starts to emerge very early in infancy. Before birth, arm and hand movements in the womb are generally symmetrical. Handedness patterns start establishing during the first 3 months of life.
70-80% of infants already favor their right hands during bilateral activities by 6-14 months of age. However, consistency in one dominant hand increases substantially around 3-4 years old for most kids.
While genetics clearly impact hand preference, cultural and societal pressures also play a role. In many cultures, left-handedness has been discouraged or even harshly forbidden. Some key historical trends include:
Negative Cultural Associations
– The right hand was seen as “proper” since ancient times. The Latin word for left is “sinister.”
– Left-handedness has been associated with evil, witchcraft, bad luck and clumsiness.
– Stigma and fear of lefties persists in some traditional cultures, for example in parts of the Middle East and Asia.
Forced Switching to Right Hand
– In the Western world, left-handed children were forced to switch to using their right hand for certain tasks or writing during the 19th and 20th centuries.
– School teachers strongly pressured students to conform and use their right hands.
– Children were sometimes tied down or had their left hands hit with rulers in efforts to change their handedness.
This pressure to switch away from their natural preference certainly pushed some left-handers to become right-handed. The overall rate of left-handedness was likely suppressed as a result. Let’s look at how common it is today.
Left-Handedness Rates
With decreased cultural stigma and pressure over the 20th century, the rates of reported left-handedness have risen. Today around 10-13% of people identify as left-handed. Rates vary between countries but remain minority everywhere:
| Country | Left-handedness Rate |
|---|---|
| USA | 11% |
| Canada | 13% |
| Australia | 11% |
| New Zealand | 11% |
| UK | 11% |
Rates are similar throughout cultures where left-handedness is now freely accepted. But in parts of Asia, the Middle East and Africa, left-handedness remains more strongly discouraged and the rates are under 5% in some populations.
Handedness is a spectrum and some people consider themselves ambidextrous. Full ambidexterity, where both hands show equal proficiency, is very rare at only around 1% of people. Partially ambidextrous individuals make up around 5-10% of the general population.
Now let’s explore how left-handedness influences cognition and mental health.
Cognitive Differences
Does being left-handed affect intelligence or cognitive abilities? There are no overall intelligence differences between left and right-handed people. However, some small cognitive variations have been noted:
Spatial Skills
Lefties show advantages in:
– 3D drawing – visualizing objects in relation to space.
– Map reading navigation skills.
– Representing imaginary viewpoints by mentally rotating objects.
Language Processing
The corpus callosum connectivity benefits lefties here too:
– Accessing meanings across both brain hemispheres during semantic tasks.
– Retrieving words from vocabulary stored in both hemispheres.
Multi-tasking
Left-handers seem able to handle doing multiple tasks at once better than righties:
– Study found lefties completed two tracking tasks concurrently with less interference.
– Lefties were less impacted when doing verbal and spatial tasks simultaneously.
However, advantages are small and inconsistent across all left-handers. And being left-handed may confer some disadvantages too.
Mental Health Differences
A number of studies have linked non-right-handedness with increased risk of certain mental health conditions:
– Depression – Lefties have a 73% higher risk according to a large-scale study.
– Bipolar disorder – 48% higher risk.
– ADHD – over 3 times higher risk.
– Schizophrenia – 40% higher incidence in non-right-handers.
– Autism – close to double the rate of autism in lefties compared to righties.
The reasons are unclear but likely involve the differences in brain structure and function we’ve discussed. Higher stress levels resulting from adapting to a right-handed world may also contribute.
In summary, while lefties are more at risk for some conditions, the large majority function productively with no mental health issues.
Advantages of Left-Handedness in Sports
Left-handers seem to excel in at least one area – interactive sports. Studies show lefties are overrepresented among successful athletes in many professional sports:
| Sport | % Left-Handed |
|---|---|
| Table tennis | 40% |
| Fencing | 33% |
| Tennis | 30% |
| Baseball | 25% |
| Cricket | 25% |
In one-on-one sports, studies suggest left-handers have an advantage in:
– The strategic and tactical elements – their moves are less predictable.
– Quickly processing visual information from both sides of the field.
– Excellent hand-eye coordination and ball skills with their dominant hand.
So while left-handedness is considered disadvantageous in many ways, when it comes to competitive and fast-paced sports lefties seem to have an edge.
Converting Left-Handers to Right-Handedness
Given the difficulties lefties have adapting to a right-handed world, some parents consider trying to convert their left-handed children to be right-handed. However, experts strongly recommend against forced conversion for several reasons:
Risk of Stuttering
Forcing left-to-right switching doubles the risk of stuttering. Stuttering occurs most often between ages 2-6 during speech development.
Stress and Emotional Distress
Trying to override hand preference causes tremendous stress and emotional upset in kids. They feel deeply frustrated and confused.
No Guarantee of Success
Even intensive training efforts throughout childhood often fail to successfully establish consistent right-handedness.
Natural Tendency Persists
A child’s underlying natural hand preference remains left so they may revert back upon entering school or as adults.
Experts emphasize it is futile and highly detrimental to force a left-handed child to change their handedness. Doing what comes naturally for each child is key.
Famous and Successful Left-Handers
While left-handers have faced discrimination throughout history, many famous figures across different domains were left-handed. Here are just some of the most influential and successful left-handed people:
US Presidents
– Barack Obama
– Bill Clinton
– George H.W. Bush
Artists
– Leonardo da Vinci
– Michelangelo
– Raphael
Musicians
– Paul McCartney
– Jimi Hendrix
– Kurt Cobain
Scientists
– Albert Einstein
– Isaac Newton
– Charles Darwin
This list clearly shows being left-handed is no obstacle to reaching the pinnacle of achievement in many fields.
Conclusion
Left-handers’ brains show intriguing differences in symmetry and inter-hemispheric communication compared to right-handers. This results in subtle cognitive variations between left and right-handers.
Lefties are often forced by circumstance to live in a right-handed world. But with decreased cultural stigma, left-handers are free to excel and thrive in all occupations. Research continues to uncover more about the fundamental basis and implications of left-handedness.
What is clear is that handedness represents a fascinating aspect of human diversity shaped by a mix of biological, social and cultural factors. Whether right or left-handed, appreciating this diversity should lead us to accept and celebrate the full spectrum of human beings.