Psychology Evolution and Genes Introduction, Principles & Types of Genetic Studies
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Introduction
Which has the greater effect on human behavior: nature or nurture? Hair color, height, and many other physical characteristics depend on genes, or nature, but the origin of behavior, intelligence, and personality is not so clear. Most scientists agree that both genes and the environment play a role in behavioral development, but disputes still rage over the degree of influence exerted by each.
A branch of psychology known as behavior genetics examines the genetic base of behavioral and personality differences among people. Behavior genetics is a controversial field, since misuse of psychological research into the genetic roots of behavior can have horrifying results. Several generations ago, psychologists and other scientists used arguments about the genetic influence on behavior and intelligence to support racist theories about the superiority of Anglo Americans. Moreover, these theories often became the foundation of public policies that discriminated against African Americans and Native Americans. Today, behavior geneticists carefully consider the potential political repercussions of their work.
Principles of Genetics
Behavior genetics is a branch of psychology that examines the genetic base of behavior and personality differences among people. An understanding of genetics begins with the following basic concepts:
- A vast number of cells make up the human body. Each cell has forty-six chromosomes, which come in twenty-three pairs. The only exceptions are sex cells.
- Sex cells are sperm in males and eggs in females. Each sex cell has only twenty-three chromosomes.
- Chromosomes are made up of thin strands of deoxyribonucleic acid (DNA). Each chromosome pair contains thousands of genes.
- Genes are segments of DNA that function as hereditary units. Genes are carried on chromosomes.
- DNA is made up of units called nucleotides. There are only four different nucleotides, labeled A, C, G, and T. Long strings of nucleotides make up genes.
- Genes get translated into proteins, which carry out various functions in our bodies. For instance, some proteins serve as the building blocks of cells. Other proteins function as enzymes or hormones.
Who Shares Genes?
No two people share the exact combination of genes unless they are identical twins. However, all family members share some genes with one another. The closer the biological relationship between individuals, the more genes they share. The chart below shows the percentage of genes any person shares with his or her close relatives:
Percentage of Shared Genes
| Identical twin | 100 percent |
| Parent | 50 percent |
| Brother or sister | 50 percent |
| Nonidentical twin | 50 percent |
| Grandparent | 25 percent |
Monogenic and Polygenic Traits
Some characteristics or traits are controlled by a single gene, which means they are monogenic. A single gene, for example, can be part of what brings about alcoholism or schizophrenia.
Most traits are controlled by the actions of several genes, which means they are polygenic. For example, a person’s intelligence is linked to the combination of several genes.
The environment also shapes traits, and later in this chapter we will discuss how genes interact with the environment to produce psychological traits.
Heritability
In a group of people, a particular psychological trait, such as intelligence, usually varies a lot. Differences in groups may be due to genes or the environment, and researchers use a statistic called heritability to see which has the largest influence. Heritability is a mathematical estimate that indicates how much of a trait’s variation can be attributed to genes. There are three important principles of heritability:
- Heritability estimates don’t reveal anything about how much genes influence a person’s traits. These figures tell us only to what extent trait differences between people can be attributed to genes.
- Heritability depends on the similarity of the environment for a group of people. In a group of people who share similar environments, heritability of a particular trait may be high. However, that same trait may have low heritability in a group of people who operate in different environments.
- Even if a trait is highly heritable, it can still be influenced by environmental factors.
Example: Imagine that ten people live in identical environments. Somehow, they experienced identical prenatal environments while in their mothers’ wombs, were raised in identical homes by parents who were identical in every way, and had all the same childhood and adulthood experiences. Suppose that these ten people turn out to be different with respect to one trait, such as the rate at which they can wiggle their ears. Since both genes and environment can influence traits, these differences would have to be genetic, since they could not be due to differences in environment. In such a case, heritability of the ear-wiggling trait would be close to 100 percent. Now suppose some of these ten people enter different ear-wiggle training camps. The camps vary in effectiveness, so the subjects in some camps increase their ear-wiggling rates, while other subjects remain the same. After the camp training, environment would account for some of the differences among the ten people in ear-wiggling ability. A smaller proportion of the differences would be due to genes alone. Therefore, heritability would be lower.
Types of Genetic Studies
Researchers do different kinds of studies to see whether and to what extent a characteristic might be genetically transmitted.
Family Studies
In family studies, researchers look at similarities among members of a family with respect to a particular trait. If the trait is genetically inherited, it should be similar in blood relatives. The closer the blood relationship, the more similar people should be.
Family studies alone don’t reveal whether a trait is genetically inherited. A family shares genes, but they also share similar environments. When researchers find trait similarity in a family study, their findings may suggest that the trait is genetically inherited, but the study can’t prove it.
Twin Studies
Compared to family studies, twin studies give researchers more solid evidence about whether a trait is inherited. In twin studies, researchers compare pairs of identical twins to fraternal, or nonidentical, twins. When doing these studies, researchers assume that identical twin pairs share the same environment, just as fraternal twin pairs do. However, identical twins share all of their genes with each other, while fraternal twins share only half of their genes. When a trait shows more similarity between identical twins than between fraternal twins, the greater similarity probably comes from shared genes, not shared environment.
One problem with this type of study is that identical twins may not in fact share an identical environment while fraternal twins do. People tend to treat identical twins in unusual ways. For example, people may treat identical twins as if they are similar in every respect, or they might focus intensely on differences between them.
Studies of Separated Twins
In order to avoid uncertain environmental factors, researchers sometimes study separated twins. Twins who are separated when they are very young and brought up in different families have different environmental influences but identical genes. Trait similarities between separated twins result mostly from genes.
However, separated twin studies can also be problematic. The environments of separated twins may not actually be that different from each other for the following reasons:
- The twins shared a similar prenatal environment before they were born.
- Adoption agencies may tend to place twins in similar households.
- Since they are similar in appearance and in genetically inherited abilities, the twins may evoke similar responses from people around them.
As in other types of studies, trait similarities in separated twins may be due to both similar genes and similar environments.
Experiences and Behavior
Experiences affect behavior partly because environmental stimulation forms and maintains neural connections. For example, psychological research shows that babies need consistent, loving contact with a caregiver in order to achieve optimal brain development. Neglected babies, lacking attention and physical contact, experience unpleasant emotions that are not simply transitory. Their experiences determine the development of their neural connections. Similarly, soldiers on active combat duty can suffer mental damage from the continuously stressful environment, even if they never experience physical injury.
Adoption Studies
In adoption studies, researchers compare adopted children to their biological parents and to their adoptive parents. Adopted children share more genes with their biological parents. The children’s living environments, however, more closely resemble the environments of their adoptive parents. When adoptive children resemble their biological parents more than their adoptive parents with respect to a certain trait, researchers can hypothesize that the trait has a genetic basis.
Interaction of Genes and Environment
In conducting all these types of studies, researchers have found that while genes influence psychological traits, they don’t act alone. Highly influential environmental factors also play a major role. These factors include: Next Page
