Reading Practice 12

[1] Research suggests that humans aren’t the only members of several species in the animal kingdom which exhibit handedness, meaning the preference for one hand over the other. Handedness is related to lateral asymmetry, which refers to physical and functional aspects between the sides of the brain. It appears that nine out of ten humans throughout history have preferred to use their right hand and that eight of ten were right-foot dominant. A genetic link, or some kind of inherited trait, has been attributed to hand preference, though there is no guarantee that a right or left-handed person’s child will have the same hand preference. On the other hand, left-handedness does tend to run in families. In fact, around 6 percent of children whose parents are both right-hand dominant will end up left-handed. Meanwhile, among children with two left-handed parents, up to 40 percent grow up left-handed, too. The occurrence of left-handed children with one left-handed and one right-handed parent typically ranges from about 15 to 20 percent. Amazingly enough, one in six pairs of identical twins will differ in their handed preference in spite of having the same genes.

 

[2] If hand preference goes beyond genetics alone, there must be other unknown variables at play. To learn more, researchers have closely studied the brain in regard to this subject. French anthropologist, Dr. Paul Broca, found that patients who had lost the ability to speak due to a stroke would also become paralyzed on the right side of their body. From this, Broca realized that because the left hemisphere of the brain controls the right half of the body and vice versa, the brain damage must have been in the brain’s left hemisphere. Psychologists have theorized that 95 percent of right-handed people’s language centers are in the left hemisphere of their brains, while 5 percent have language centers on the right side. Surprisingly enough, left-handed people do not show the exact reverse of this, and instead, a majority have their language center located in the left hemisphere, with some 30 percent in the right hemisphere.

 

[3] At the Australian National University in Canberra, Dr. Brinkman has suggested that human speech evolved alongside a preference for the right hand. From her research, Brinkman believes that one side of the brain became specialized for fine motor skills, which are necessary for speech and as the brain continued to evolve for speech, the right-hand preference emerged. According to Brinkman, the majority of left-handed people exhibit left-hemisphere dominance with some capacity in the right hemisphere as well. Dr. Brinkman has observed that a left-handed person with brain damage to the left hemisphere is often able to recover speech ability more effectively. She thinks this phenomenon can be explained by left-handed people’s tendency towards bilateral speech function.

 

[4] Dr. Brinkman’s research has expanded her research into primates as well. In her studies of macaque monkeys, she found that the year-old babies appeared to learn either hand preference from their mothers. In humans, on the other hand, specialization of the two hemispheres’ functions actually creates physical differences in the anatomy of the brain, with areas associated with speech production often being larger on the left side than on the right. Because monkeys and apes do not have the ability of speak, one would not expect to see such a variation in these animals. However, Brinkman believes that she has discovered a trend in monkeys’ brains that suggests the same asymmetry seen in human brains.

 

[5] Recent research on human embryos led to the discovery that a left-right asymmetry is evident before birth. However, there are many other variables that can affect brain development along the way. Initially, all brains begin as female, becoming male brains if the male fetus secretes hormones. The sides of the brain mature at different rates; the right hemisphere develops first, then the left. Moreover, a female brain develops slightly more quickly than a male brain. During a fetal brain’s development, the male brain is more likely to be affected, and the left hemisphere is also more likely to be affected. The brain may lose some lateralization, resulting in left-handedness as well as a tendency towards skills that are typically associated with the left brain hemisphere, such as logic, rationality, and abstraction. It is possible that this explanation captures why professionals such as mathematicians and architects tend to be left-handed more often, as well as a higher occurrence of left-handed males than females.

 

[6] Throughout our history, our language subliminally reinforces the notion that the right side is good, while the left is potentially dangerous. In fact, the Latin word “sinister” actually means “left.” It is no coincidence that left-handed children, forced to use their right hand, often develop a stammer as they are robbed of their freedom of speech.” However, as more research is undertaken on the causes of left-handedness, attitudes towards left-handed people are gradually changing for the better.