What Causes Left-handedness

An individual more skilled in using her left hand for day-to-day activities is known as a left-hander. Studies show that almost 10 to 13% adults in United States are lefties, and the number of males with left-handedness is higher than that of females. It is said that the condition is likely to develop in identical and fraternal twins, and those with neurological disorders like epilepsy and mental retardation. Left-handed people are said to have an edge in sports and other such activities.

Normally, babies use both their hands and it is difficult to differentiate a left-handed one. A common belief is that, hand preference develop in children by the age of three, but according to recent researches, the preference may arise during the fetal stage itself. 90% of babies prefer to suck their right thumb than the left one, and this tendency later transforms to hand preference in their life.

Even though, there is no specific cause for left-handedness, there are many theories supporting the same.

• If both the parents are left-handed, the chances of having a left-handed offspring is higher than that of right-handed parents. If the parents are right-handed, there is only a 9% chance of having left-handed children. It is 26% for left-handed parents and 19% for a left-handed and right-handed couple.
• Another contention is that too much exposure to the male hormone testosterone, during the fetal stage, may slow down the growth of left brain. In such cases, the right brain would be more developed than the left. As the right brain controls the left side of the body (and vice versa), the dominant right side would make the child a left-hander. This theory can support the fact that left-handers are more in males than in females, as the amount of testosterone is higher in the former.
• A recent development in this area is the discovery of the gene LRRTM1, which is said to be associated with the left-handedness of a person. The study also suggests that one particular form of this gene may increase the risk of psychotic mental disorders.
• Another speculation is about the role of ultrasound during pregnancy. It is said that ultrasound of uterus can affect the brain development of fetus, which causes childhood malignancies and left-handedness in the child.
• Some studies link this condition to difficult birth experiences and effects on the brain. The first point is supported by higher occurrences (20%) of left-handedness among twins, who undergo difficult births. The second point is backed by the fact that, 30% of mentally retarded kids are left-handed.
• Long term use of left hand by right-handers, due to some injury or impairment of the right hand, may turn them left-handed, even after the curing of the impairment. Here, long-term use means, for a period of eight months or more.
• Some kids may try to imitate their left-handed parents or baby-sitters and end up as lefties.

Hence, there is no specific reason for left-handedness. It can be said that hand preference can be attributed to partly genetic and partly environmental factors.