Saturday, March 20, 2010

Dr. Spock

Nature, Nurture, and Handedness

by Robert Needlman, M.D., F.A.A.P.
reviewed by Robert Needlman, M.D., F.A.A.P.

We're used to thinking of human characteristics as either inherited or learned--in other words, the old "nature vs. nurture" controversy. Unfortunately, for many of the characteristics that matter most--such as temperament and personality, and physical, artistic, and intellectual abilities--the best explanation is a complex mixture of both.

In the case of handedness, the nature-nurture controversy isn't just academic. If handedness is learned, then it can be unlearned. That line of thinking would support teaching all children to use their right hands, even though the left might feel more comfortable. But if handedness is inherited, then trying to change a child's preference might be seen as going against nature.

Neither a wholly genetic nor a learning-based explanation fits for handedness. Here are some of the facts, and what they mean.

Handedness runs in families. If both parents are right-handed, 92 to 96 percent of their children will be right-handed, too. If both parents are left-handed, 45 to 50 percent of their children will be left-handed (which also means that roughly half will still be right-handed). When one parent is a lefty and the other a righty, about 80 percent of their children will be right-handed. All of the above fits nicely with a genetic explanation.

Of course, the fact that handedness runs in families could be explained by learning. Since most people in society are right-handed, right-handed parents would have an easy time teaching their children to be right-handed. Left-handed parents would be working against the tide, and so a greater percentage of children would turn out right-handed, despite their parents' efforts. But what the learning explanation can't explain is why any children of right-handed parents would turn out to be left-handed! Also, very young infants show left- or right-sided preferences (see How Handedness Develops in Infants) that are related to their eventual handedness. It's hard to imagine that parents are teaching their children these things.

Among adopted children, the patterns of handedness are related to their biological parents, not their adoptive ones--more strong support for a genetic cause.

On the other hand, the evidence from studies of twins goes against genetics. Identical twins, who develop from a single egg, share the same genes. Fraternal twins, who develop from two different eggs, have about half of their genes in common. If genes were directly responsible for handedness, you'd expect identical twins to always have the same handedness--both being either right-handed or left-handed. You would think that fraternal twins, however, would only have the same likelihood of handedness as any other siblings.

In fact, the number of pairs where one twin is left-handed and the other right is about 20 to 30 percent regardless of whether they're identical or not. To further complicate the matter, there is controversy about a possible connection between left-handedness and twinning (which I won't get into!), but that doesn't change the fact that straightforward genetics can't account for how identical twins can be so different when it comes to handedness.
Making sense of the puzzle
There are lots of theories about how genes and learning or other influences might combine to produce these patterns of handedness. The one that makes the most sense to me is that most people carry a gene that increases the chance that they will be right-handed. Those children who have a full dose of the gene (that is, they inherited it from both parents), are very likely to be right-handed, but still have some small chance of ending up left-handed. Those who don't have the gene at all are about equally likely to develop as left-handed or right-handed. Those who have a half dose of the gene (say, from their mother but not their father) have better than average odds of becoming right-handed, although a small chance still remains that they'll develop into left-handers.

This theory can explain how two left-handed parents can have a right-handed child, and how two right-handed parents can have a left-handed child, and also why twins with identical genes could develop different handedness. We're not used to thinking about things happening just by chance, but that is probably how a lot of development comes about.

What it means for parents
Learning probably plays only a small role in handedness. It may be possible to teach some left-handed children to act as right-handers (or at least to write with their right hands), but the process is bound to be difficult and may not offer any real benefits.

Credit to Chin Yee Jah, T5

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