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The Heritability Index

Twin research is important to scientists studying genes. There are two types of twins: fraternal (dizygotic) and identical (monozygotic) twins. The term "fraternal" may falsely imply that dizygotic twins must be males, when they could be females, or a male/female pair. So the technical term dizygotic (coming from two different eggs or zygotes) is preferred. Dizygotic twins come from two different sperm and two different eggs and share 50% of their genetic material, like any brother or sister. Identical twins, by contrast, come from one zygote (fertilized egg) that splits and grows into two individuals. Identical twins, alone among humans, share 100% of their genetic material.

What are influences that can cause identical twins can develop differently?

This does not mean identical twins develop in identical ways, however. For one thing, they may be mirror images of each other. This can be determined by examining the thumb prints. If they swirl in opposite directions, then the identical twins are the mirror image type. That can affect behavior because, to a limited degree, the two hemispheres of the brain are specialized for different tasks. One identical twin may be especially good with language, for example, while the other is better with math.

Epigenetics (environmental effects on expression of the DNA) may also lead twins to develop differently. And, of course, twins may choose to emphasize differences. Some twins like to dress alike and act alike, others emphasize their differentness. Sometimes twins disagree about which of those options is preferable. (That happened with the identical twins and famous advice columnists Ann Landers and Dear Abby, according to one of their daughters.) Different choices can lead to different lifestyles, looks, health, and mannerisms.

However, other things being equal, identical twins are strikingly more likely to make similar life choices or experience similar life events. For all their differences and occasional acrimony, Ann Landers and Dear Abby both chose the same profession, and both were highly successful with it.

The distinction between dizygotic and monozygotic twins is used to calculate a measure of "genetic influence" called a heritability index. The heritability index is the amount of statistical variability (in test scores or other measurements) accounted for by genes. . In other words, the heritability index tells you how well you could predict a measurement obtained from one identical twin, given a measurement of the same varable obtained from the other twin.

What does the heritability index tell us?

The heritability index is what people are usually referring to when they make statements such as, "the genetic component of homosexuality is somewhere between 30% and 70%." The component referred to here is a statistical concept. It is like saying, "the expected error in your prediction of sexual orientation is reduced by 30-70%, if you know the sexual orientation of the identical twin."

How can something like religiosity produce a high heritability index, when it is plainly learned?

Heritability indexes can be calculated for any psychological trait that can be measured. For example, Waller, Kojetin, Bouchard, Lykken and Tellegen (1990) showed genes contributed about 50% to measures of religiosity (the tendency to be religious). They explored this variable because Plomin, an expert on behavior genetics, suggested that religiosity and political beliefs probably would not be related to genes. Taking this as a challenge, Waller and colleagues compared the religious and political views of identical and dyzygotic twins. They concluded that religiosity actually had a high heritability index.

Does this mean you inherit a tendency to be religious or not religious? Not specifically. Religiosity simply correlates with temperament, personality, or some other factor that is influenced by genes. That could be any biological factor, or any cultural factor that correlates with a biological factor.

To make a similar point about the limitations of the heritability index, Plomin, Corley, DeFries, and Fulker (1990) calculated a heritability index for television-watching. As they suspected, it had a high heritability index. They wrote:

What point did Plomin and colleagues make with the example of television viewing?

Of course, there are no genes for television viewing just as there are no genes for performance on IQ tests or for height....We do not inherit genes that code for vocabulary words or for height, and we cannot inherit genes that code for television viewing. Genes only code for sequences of amino acids......Finding genetic influence on individual differences in children's television viewing means that some unspecified genetic differences among children indirectly affect the extent to which children watch television.

The moral of the story is that high heritability indexes do not demonstrate that a behavior is "coded in the genes." A high heritability index merely indicates that, for the variable being measured, identical twins are much more similar than fraternal twins.


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