Why Most People Are Right-Handed, But Left-Eyed

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Whether you’re left-wing, right-wing, or ambidextrous, “handedness” is part of our identity. But many people don’t realize that we have other biases, and they’re not unique to humans. My colleagues and I have published a new study showing that aligning our biases with other people’s can have social benefits.

Across cultures, human populations have high levels of right-handedness (around 90%). We also have a strong population bias in how we recognize faces and their emotions.

A large portion of the population recognizes identities and emotions faster and more accurately when they are in the left visual field than when they are in the right visual field.

These types of biases develop in our brains in early childhood. The left and right hemispheres of the brain control motor actions on opposite sides of the body. If your left visual field is dominant, it means that the right hemisphere of your brain is dominant for recognizing faces and emotions.

Until recently, scientists thought that behavioral biases were unique to humans. But animal research over the past few decades has shown that behavioral biases exist across all branches of the vertebrate tree of life.

For example, chicks that peck with an eye bias to distinguish food are better at distinguishing grain from pebbles. Also, chicks with an eye bias to detect predators are less likely to be eaten than chicks without an eye bias. Studies show that animals with biases generally perform better on survival-related tasks in laboratory experiments, which likely translates into a better chance of survival in the wild.

But the chicks with the best advantage are those that keep one eye on the ground (to find food) and the other eye on the sky (to watch for threats). One advantage of the “divided brain” is that wild animals can both search for food and watch for predators – important multitasking.

Why then do animals have such behavioral prejudices?

Research suggests that hemisphere biases arise because it allows the two hemispheres of the brain to simultaneously control different behaviors. It also protects animals from confusion. If both hemispheres had equal control over critical functions, they could simultaneously direct the body to perform incompatible responses.

Thus, biases free up some of the resources or ‘neural capacity’ that allow animals to find food more efficiently and protect themselves from predators.

Animal studies suggest that it is the presence, not the direction (left or right) of our biases that matters for performance. But that doesn’t explain why so many people are right-handed for motor tasks and left-handed for face processing.

Everyone should have a 50-50 chance of being left- or right-leaning. Yet, in the animal kingdom, the majority of individuals within a species go in the same direction.

This suggests that tuning biases to others in your group can have a social advantage. For example, animals that tune in to the population during cooperative behavior (shoaling, flocking) reduce the chance of being picked off by a predator. The few that turn away from the herd or shoaling become obvious targets.

Close-up of a woman's face

Although people, regardless of their ethnic or geographical background, are highly lateralized, there is always a significant minority within the population, suggesting that this alternative bias has its own advantages.

The prevailing theory is that being different from the population gives animals an advantage during competitive interactions, by creating an element of surprise. It could explain why left-handedness is overrepresented in professional interactive sports such as cricket and baseball.

In the first study of its kind, scientists from the universities of Sussex, Oxford, Westminster, London (City, Birkbeck) and Kent put our human behavioural biases to the test. We examined relationships between strength of hand bias and performance, as well as the direction of bias and social skills. We chose behaviours that are consistent with animal research.

More than 1,600 people of all ages and ethnicities participated in this study.

You don’t always use your favorite hand: some people are mild, moderate, or strong. So we measured our participants’ handedness with a timed color-matching pinboard task. Not everyone knows whether they have a visual field bias, so we assessed this for participants using images of faces expressing different emotions (such as anger and surprise) presented on a screen.

People with a mild to moderate hand bias (left or right) placed more color-matched pegs correctly than people with a strong or weak bias. These results suggest that in humans, unlike wild animals, extremes may limit our performance flexibility.

The majority of participants had a standard bias (right-handedness for motor tasks, bias toward the left visual field for face processing). But not everyone.

To test the associations of social skills and bias direction, participants were categorized based on their hand and visual side biases into one of four groups: standard (right hand, left visual), press right (right hand, right visual), press left (left hand, left visual), and reverse (left hand, right visual). They also completed a survey assessing their social difficulties.

The default profile, found in 53% of participants, was not associated with a social advantage over the busy left or right groups. However, the reversed profile, which was relatively rare (12%), was associated with significantly lower social scores compared to the other groups. People in the reversed group were four times more likely to have a self-reported diagnosis of autism or attention deficit hyperactivity disorder (ADHD).

We cannot conclude from this study whether there is a causal relationship between the reverse profile and autism and ADHD. However, we are planning research to investigate whether bias profiles can serve as an early risk marker for autism and ADHD in childhood, which could pave the way for earlier screening, diagnosis and the development of new interventions.

This study is a reminder that we humans have an evolutionary history, much of which we share with other animals. We must study ourselves within the context of the broader animal kingdom if we are to truly understand our modern brains and behavior.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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This research was funded by the Wellcome Trust (ISSF/Birkbeck) and the Waterloo Foundation (REF: 917-3756).

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