Do Genes Infl uence Our Behavior and Mental Processes?
It’s obvious that children inherit many of their physical characteristics from their par- ents. Light or dark skin, blue or brown eyes, tall or short stature—these are all traits we routinely expect to be passed from parents to children. Inheritance is also impor- tant to psychology because many aspects of our behavior are infl uenced by our genes (Bouchard, 2004). Humans do not inherit specifi c patterns of behavior; rather, inheri- tance seems to infl uence broad dimensions of our behavior, such as general intelli- gence (Plomin, 1989; 1999). Although it was long suspected that positive and negative characteristics of our personalities also might be infl uenced by genetic factors, little solid evidence was available until the latter part of the 20th century. Evidence from many studies strongly suggests that heredity infl uences normal and abnormal aspects of broad dimensions of our personalities, including sociability, aggressiveness, alco- hol and drug use, kindness, depression, and anxiousness (Bouchard, 2004). As we will discuss later in detail, heredity is never the sole cause of our behavior, however.
Heredity always operates in conjunction with the effects of the environment.
Genetic Studies of Nonhuman Animal Behavior
Gregor Mendel was an Austrian monk who helped found the science of genetics in the 1860s. When he wanted to study genetic infl uences on the physical characteristics of pea plants, he was able to selectively breed plants with a particular characteristic, such as smooth skin, to see what the next generation would be like. Selective breeding our diversity, we fi rst need to understand why we are more similar to some people than to others. In particular, why do some characteristics “run in fami- lies.” During the 1970s, Felipe Alou and his younger brothers, Matty and Jesús, were all outstanding National League baseball players, and Filipe’s son, Moisés Alou, is still a star outfi elder. Ken Griffey senior and junior are another notable father-son pair of major leaguers. Negative traits also tend to run in families.
Sixty years ago, Harvard University psychologists Eleanor and Sheldon Glueck compared boys who had committed juvenile crimes to public school boys. Two- thirds of the fathers of the delinquents also had been convicted of a crime, compared to only one-third of the school boys. Not everyone with an athletic or a criminal parent will share those characteristics, of course, but many studies show that people in families do resemble one another in many ways more than would be expected by chance. Why? If you are good in math like your mother, did you inherit her talent or did she create an environment that helped you learn math? The evidence is now clear that family resemblance is due to both inheritance (nature) and experience (nurture). This chapter describes the inter- play of nature and nurture in determining family resemblance, but also in cre- ating human diversity. To understand the role of “nature,” we will study the structure and functions of genes. To understand the role of nurture, we will study the range of human diversity associated with being female or male, being a member of different race and ethnic groups, and living in different cultures.
Key Terms
chromosomes 94 culture 99 dizygotic twins 93 dominant gene 96 Down syndrome 97 ethnic group 99 ethnic identity 99
evolutionary psychology 111 fertilization 94
gender identity 104 gene expression 103 genes 94
monozygotic twins 93 nucleotides 94 polymorphic gene 94 recessive gene 96 social-role theory 114 zygote 94
also has been used successfully with nonhuman animals, showing, for example, that aggressiveness and the ability to learn to fi nd food in mazes in mice and emotionality in monkeys are infl uenced by genes (Petitto & others, 1999; Suomi, 1988).
For example, in a classic study, Patricia Ebert and Janet Hyde (1976) captured wild house mice and then began a program of selective breeding. In the fi rst gen- eration, females were tested for aggressiveness (when placed in a cage with another strain of mice) and divided into two lines based on their aggressiveness and bred with randomly selected unrelated males. In each generation, the 10 most aggressive female offspring in the high-aggression line and the 10 least aggressive offspring in the low- aggression line were bred with randomly selected males. The researchers predicted that each successive generation in the high-aggression line would be more likely to receive genes related to aggression from their mothers and would be more aggressive than previous generations. As shown in fi gure 4.1 , the successive offspring of the selectively bred high-aggression line became increasingly more aggressive, and the offspring of the low-aggression line because less aggressive over successive genera- tions. These fi ndings suggest that genes infl uence aggression.
Is it possible, though, that the fi ndings of Ebert and Hyde (1976) were actually caused by something other than genes? Perhaps the female mice in the aggressive line provided different rearing environments (or even different environments in the womb before birth) that caused each offspring generation to be more aggressive. A number of clever experiments have shown that such alternative explanations are unlikely, how- ever (Gariepy & others, 2001). For example, an ingenious group of Dutch psycholo- gists transplanted the embryos of aggressive females into the wombs of nonaggressive females, and vice versa, and allowed them to rear the “adopted” mice pups (Sluyter &
others, 1996). If the increases in aggression across generations were being caused by the way in which aggressive females reared their young, the offspring should resemble their “adoptive” mothers more than their biological mothers. The results of the Dutch study were the same as in Ebert and Hyde’s (1976) study, which provides strong evi- dence that genes infl uence aggression in female mice.
Genetic Studies of Human Behavior
For obvious ethical reasons, selective breeding experiments cannot be carried out with humans. Therefore, it’s much harder to study the roles of nature and nurture in human behavior. Instead, researchers interested in genetic infl uences on humans must use research designs that allow them to separate genetic and environmental infl uences.
These designs are based on unusual situations that are not arranged by the experi- menter. Because such studies do not allow the same degree of experimental control as Figure 4.1
The fi ndings of a study by Ebert and Hyde (1976) show that when the most aggressive female house mice in each generation were selected for breeding, each successive generation became increasingly more aggressive.
Conversely, when the least aggressive female house mice in each generation raised under the same conditions were separately selected for breeding, each successive generation became less aggressive. These fi ndings suggest that genes infl uence aggression.
Aggression scores
Selectively bred generations Parent
High Aggression Line Low Aggression Line 7
6.5 6 5.5 5 4.5 4 3.5 3
1 2 3 4
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formal experiments, any conclusion drawn from them must be viewed cautiously until confi rmed by other kinds of studies. The two most common types of “natural experi- ments” involve the study of twins and the study of adopted children (Bouchard, 2004;
Rutter, 2006).
Studies of Twins. There are two kinds of twins that are formed in very different ways. In the case of identical, or monozygotic twins , a single fertilized egg begins to grow in the normal way through cell division in the mother’s womb. Ordinarily, the growing cluster of cells develops over the course of about 9 months until the baby is born. Monozygotic twins result when the growing cluster of cells breaks apart into two separate clusters early in pregnancy. If conditions are right, each cluster grows into a baby and its twin. These twins are identical not only in appearance but also in genetic structure, because they came from the same fertilized egg.
Dizygotic twins , in contrast, are formed when the female produces two separate eggs that are fertilized by two different sperm cells from the father. These two fertil- ized eggs grow into twin babies who are born at about the same time but who are not genetically identical. Dizygotic twins are no more alike genetically than are siblings born at different times, because they come from two separate eggs and two separate sperm cells. Like other siblings, dizygotic twins share 50% of their genes on average.
Monozygotic and dizygotic twins provide psychologists with an informative “nat- ural experiment,” because both types of twins grow up in essentially the same home environment. They have the same parents, live in the same neighborhood, have the same sisters and brothers, and are raised during exactly the same time period in his- tory. But the two kinds of twins differ in their degree of genetic similarity. If a charac- teristic of behavior is infl uenced to some degree by heredity, monozygotic twin pairs (who share 100% of their genes) will be more similar to each other than will dizygotic twin pairs (who share 50% of their genes, on average, like typical siblings born at dif- ferent times).
The many experiments conducted using twins have revealed the infl uence of heredity on behavior. For example, studies of twins have suggested that intelligence, or IQ, is partly determined by heredity (Bouchard, 2009; Plomin, 1999). Figure 4.2 summarizes the fi ndings of a number of studies indicating the degree of similarity in the intelligence test scores among various types of twins and siblings (Bouchard, 2004; Bouchard & McGue, 1981). Monozygotic twins who have identical genetic structures have almost identical IQ scores. Dizygotic twins and ordinary siblings share only half of their genes and have considerably less similar IQ scores than monozy- gotic twins. Similarly, there is evidence from other twin studies that aerobic fi tness,
monozygotic twins
(mon´´oˉ-zIˉ-got ´ ik) Twins formed from a single ovum; they are identical in appearance because they have the same genetic structure.
dizygotic twins (dIˉ´´zIˉ-got ´ ik) Twins formed from the fertilization of two ova by two sperm.
Identical, or monozygotic, twins are formed when a single fertilized egg breaks apart into two clusters of cells, each growing into a separate person.
Figure 4.2
The degree of similarity among monozygotic twins, dizygotic twins, and other siblings on measures of intelligence.
Monozygotic twins
Dizygotic twins (same-sex pairs only)
Siblings
Unrelated children
Correlation coefficient
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0.00
0.60
0.47
0.86
muscular strength, and athletic ability are partly infl uenced by our genes (Silventoinen
& others, 2008), and many other human behavioral and psychological characteristics are infl uenced partly by our genes (Rutter, 2006).
Studies of Adopted Children. Studies of adopted children also have shown that genes infl uence human behavior (Angoff, 1988; Plomin, 1994). Take the case of IQ again. Studies of adopted children have revealed that the IQs of adopted children are more similar to the IQs of their biological parents (with whom they share half of their genes) than to the IQs of the adoptive parents who raised them since infancy but are genetically unrelated to them (Plomin, 1994). Because the children spent no time liv- ing with their biological parents, the most likely explanation for the similarity in IQs is that genes play a role in infl uencing intelligence. Although we must be very cautious in interpreting naturally occurring experiments, the fact that many twin studies and many adoption studies consistently suggest that genes play a role in infl uencing intel- ligence is reassuring.
Molecular Genetic Mechanisms of Inheritance
In the past, it was believed that inherited characteristics were transmitted through the blood—hence old sayings such as: “He has his family’s bad blood.” We now know that inheritance operates through genetic material, called genes, found in the nuclei of all human cells. The existence of genes was guessed more than a century ago by Gregor Mendel. It has been only during the last half of the 20th century, however, that genes have actually been seen, with the aid of electron microscopes.
Genes, Chromosomes, and DNA. All cells of the body contain microscopic structures called chromosomes (see fi gure 4.3 a). Chromosomes are long strands of deoxyribonucleic acid, or DNA for short. As shown in fi gure 4.3 a, DNA usually takes the form of a curved ladder that doubles back on itself, known as a double helix. The outside rails of the ladder are composed of a type of sugar. Chemical compounds called nucleotides are located on the twin rails of the double helix. There are four dif- ferent nucleotides in DNA: adenine (A), thymine (T), guanine (G), and cystine (C).
The many possible sequences of A, T, G, and C carry the genetic code, much like the combinations of letters on this page convey verbal information to you. Segments of DNA on the chromosomes that contain the information needed to infl uence some aspect of the body are called genes . Thus, genes are the basic biological units of inher- itance. Each of the chromosomes of a normal human cell contains thousands of genes.
Human chromosomes are arranged in 23 pairs (see photo in fi gure 4.3 ). When cells divide in the normal process of tissue growth and repair, they create exact copies of themselves. But, when sex cells (sperm or ova) are formed, the chromosome pairs split, so that the resulting sex cell has only 23 unpaired chromosomes. When a sperm unites successfully with an ovum in the process of fertilization , the new cell that is created is called a zygote . The zygote has a full complement of 23 pairs of chromo- somes, with one member of each pair of chromosomes from the mother (ovum) and half from the father (sperm). If conditions are right, the zygote implants in the lining of the mother’s uterus, and the embryo develops.
Polymorphic Genes. At various times in the long evolutionary history of human beings, small changes, called mutations, occurred in the DNA of sex cells. Most muta- tions damage the organism so much that it cannot live, but some mutations to genes help the organism survive and reproduce and, therefore, are passed on to future gen- erations. Sometimes, more than one version of a gene is created by separate mutations and passed on. Genes with more than one version are called polymorphic genes .
chromosomes (kroˉ ´ moˉ-somz) Strands of DNA (deoxyribonucleic acid) in cells.
nucleotides (noo ´ kli-uh-tIˉ ds) The four chemical compounds (adenine, thymine, guanine, and cystine) located on the double helix of DNA.
genes (jeˉnz) Segments of
chromosomes made up of sequences of base pairs of adenine, thymine, guanine, and cystine, which are the basic biological units of inheritance, because they contain all the coded genetic information needed to infl uence some aspect of a structure or function of the body.
fertilization (fer ´ tˉ -li-zaˉ ´ shun)I The uniting of sperm and ovum, which produces a zygote.
A human egg cell.
Human sperm, the tiny cells with the long tails.
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Figure 4.3 (a) The nucleus of each human cell contains 46 chromosomes united in pairs, 23 from the sperm and 23 from the ovum.
In this photograph, the 23rd pair is labeled X and Y. (b) DNA in cell bodies typically takes the form of the double helix, which resembles a curved ladder that doubles back on itself. The outer “rails” of the double helix are made of a type of sugar that provides structure to DNA. The rails are connected at intervals by bonded base pairs of adenine (A), thymine (T), guanine (G), and cystine (C). Different sequences of the base pairs of A, T, G, and C carry the genetic code.
Chromosome Nucleus
Cell
G Base pairs
A A
A
C A G
G
G
G G C
C
C
C C G
T
T
T T
Sugar ribbon backbone
(a)
(b)
zygote (zˉ´goI ˉ t) The stable cell resulting from fertilization; in humans, it has 46 chromosomes—
23 from the sperm and 23 from the ovum.
polymorphic gene
(pah´eˉmoˉ mor´fi k) Gene that has more than one different version.
There is only one version of about 99% of all human genes. The most interesting genes are the polymorphic genes, because different versions of some polymorphic genes are partly responsible for differences among people in their behavior and mental processes.
For example, think of each of the polymorphic genes infl uencing eye color that you receive from each of your parents as being labeled A and B. Three genes that infl uence eye color have been discovered, but to simplify our example, let’s focus on the gene that infl uences whether you have blue or brown eyes, which is located on chromosome 15. The version of polymorphic genes that you receive from each parent is a matter of chance. That is why siblings can have substantially different genes. You might inherit two copies of one version of this gene, that we will call 15A, from both your mother and your father. In contrast, your sister might inherit two copies of 15B from your parents. Therefore, you and your sister would have no versions of this gene in common. A second brother might inherit one copy of 15A from your mother and one copy of 15B from your father. Because siblings receive versions of polymorphic genes randomly from each parent, brothers and sisters have an average of 50% of their entire set of polymorphic genes in common.
Dominant and Recessive Genes. What if the two versions of a polymorphic gene that we inherit from each of our parents confl ict? What if the version of a gene infl uenc- ing eye color from the father codes for blue eyes and the version of the same gene from the mother codes for brown eyes? The answer depends on which is the dominant gene . In the case of eye color, the version of the gene coding for brown eyes is typically domi- nant over the version for blue eyes. The version of gene coding for blue eyes is said to be recessive. A dominant gene normally reveals its trait when it is present. A recessive gene is revealed only when the same recessive gene has been inherited from both par- ents and there is no dominant gene giving instructions to the contrary. Dark hair, curly hair, farsightedness, and dimples are other common examples of dominant traits. In contrast, blue eyes, light hair, normal vision, and freckles are recessive traits. To have these traits, persons must inherit the same version of the gene from both parents.
Polygenic Traits. To this point, we have discussed only simple aspects of genetic inheritance. Although traits such as eye color and physical height are controlled by only three or four genes, other traits are infl uenced by many more genes. Traits controlled by large numbers of genes are termed polygenic traits. It is likely that almost all impor- tant behavioral traits, such as intelligence traits and personality traits, are polygenic.
The basic principles described here are the same for both simple traits and polygenic traits, but it will take us much longer to discover all of the many polymorphic genes involved in polygenic traits and how these genes work together (Jabbi & others, 2007).
X and Y Chromosomes and Sex. The diversity of human behavior and mental processes also is infl uenced through another genetic mechanism. The biological sex of each person is determined by the chromosomes that are referred to as “X” and “Y”
chromosomes because of their shapes (see photo of chromosomes in fi gure 4.3 ). Males have one X chromosome (and one Y chromosome), whereas females have two X chro- mosomes. The chromosomes determine many profound physical differences between females and males, such as the presence of different glands (ovaries versus testes), the ability to bear and nurse children, and characteristic sex differences in height and muscle strength. Recently, geneticists have determined that sex differences in our physical characteristics are determined not only by the number of X chromosomes but also by sex differences in the expression of many genes on the other 22 chromo- somes (Ellegren & Parsch, 2007). That is, the same version of the same gene can be expressed differently in females and males.
dominant gene Version of a polymorphic gene that produces a trait in an individual even when paired with a recessive gene.
recessive gene Version of a polymorphic gene that produces a trait in an individual only when the same recessive gene has been inherited from both parents.
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