Are environmental ones. This is normal: twin

Are you destined to inherit your mother’s penchant for collecting old postcards or your father’s constant worry that something bad is about to happen? Find out with this lesson, which delves into the world of genetic and environmental behaviors and personality traits.

Twin Studies

Compared to your parents, how frequently do you tend to worry, and to experience negative emotions? What evidence is there for personality being at least partially determined by genetics? Consider this example: in the early 1950s, a German-British psychologist named Hans Jurgen Eysenck was the lead author on a published study about whether neuroticism is inherited.

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Neuroticism is a term that psychologists use to label a general tendency to experience negative emotions. Eysenck’s test was a twin study, which used 25 pairs of fraternal twins, and 25 pairs of identical twins. Identical twins have the same genes, whereas fraternal twins have only about half the same genes.

So, here’s the value of a twin study like this: if identical twins are raised in the same households, and fraternal twins are raised in the same households, then characteristics that are determined largely by environmental factors should be about equally pronounced in both kinds of twins. By contrast, assuming again that both identical twins and fraternal twins are each raised in the same households, characteristics that are more influenced by genetics should be more similar between identical twins than between fraternal ones. In other words, the goal of identical vs. fraternal twin studies is to understand the significance of genetics for a particular characteristic. Identical twins share twice as much genetic code as fraternal twins, so by comparing the degree of similarity between the two different kinds of twins, researchers can better understand the role that genetics play in determining that characteristic.

Eysenck’s study found that neuroticism seemed to be overwhelmingly determined by genetics, with about 80% stemming from biological factors, and only 20% from environmental ones. This is normal: twin studies like this usually find more similarity between identical than between fraternal twins.

Statistics in Behavioral Genetics

Twin studies such as Eyesenck’s belong within a field known as behavioral genetics, which studies the heritability of traits in animals and humans. Behavioral genetics is not exclusively a sub-field of psychology; it’s more of an interdisciplinary field that impinges on disciplines including psychology, biology and statistics. And studies in behavioral genetics do rely heavily on statistics. For example, twin studies often must establish concordance rates, which is the proportion of twin pairs in which both individuals exhibit the trait being considered. Now, with some traits, such as red hair, you either have it or you don’t.

With others, though, such as height, you can exhibit the trait to varying degrees. With these traits that can differ in degree, another type of statistic is needed. It’s called a correlation coefficient. You don’t need to worry about how to calculate it at the moment; just know that a correlation coefficient measures the extent to which the trait in one twin compares or contrasts with the same trait in the other twin.

Family Studies

Aside from twin studies, another type of research method that behavioral geneticists use to study humans is a family study. Probably you can guess from the name what this type of study is. A family study begins by isolating a single trait, or even a single illness, in a single person.

This first person is known as the proband. Then that person’s parents, siblings and children are studied with reference to the same trait. These relatives are known as first-degree relatives. Sometimes, family studies will even consider more distant relatives, such as aunts and grandparents, and so forth. These more distant relatives are known as second-degree relatives. Obviously, family studies aim to identify whether a trait runs in a family. But can you think of a complication with them? Families tend not only to be similar in terms of genetic makeup; they also tend to live in similar, if not the same, environments.

So even though family studies can determine whether a given trait runs in a given family, they cannot easily determine whether it runs in the family due to environment or due to genetics. Twin studies, such as Eyesenck’s, are better for distinguishing that.

Adoption Studies

Another variation of a family study is an adoption study. Adoption studies can happen in two different ways.

First, biologically related people who have been raised in different environments can be studied. For example, what if identical twins were adopted by different families, and raised in different households? Since they have the same DNA, studies like these are well-positioned for drawing conclusions about effects those different environments have exerted on those twins. One well-known example of a study like this is known as the Minnesota Twin Study.

It studied a pair of identical twins who were raised separately, and who did not meet until they were 40. The results were astonishing. One researcher concluded, ‘(On) multiple measures of personality and temperament, occupational and leisure-time interests and social attitudes, (identical) twins reared apart are about as similar as (identical) twins reared together.’ Second, adopted children can be compared to their adopted parents, to their biological parents, or both.

The Texas Adoption Project is one well-known adoption study.

Lesson Summary

To summarize, behavioral genetics is a field that attempts to ascertain whether, or the extent to which, traits, including personality traits, are inherited. To help determine this, researchers study families in family studies, twin studies and adoption studies. These studies are heavily reliant on statistical measurements, including concordance rates, which indicate the proportions of pairs in given studies in which both individuals exhibit the trait in question, and correlation coefficients. A correlation coefficient measures the extent to which the prevalence of a trait in one person matches or diverges from the prevalence of the same trait in another.

Lesson Objectives

After watching this lesson, you should be able to:

  • Explain the goal of behavioral genetics
  • Describe twin studies, family studies and adoption studies
  • Define the terms concordance rate and correlation coefficient and understand how they are used in behavioral genetics
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