Chapter 6
Genetics and Personality

Chapter Outline

 

The Human Genome

• Genome refers to the complete set of genes that an organism possesses
• Human genome contains 30,000–80,000 genes on 23 pairs of chromosomes
• Human Genome Project is designed to sequence the entire human genome—i.e., identify the particular sequence of DNA molecules in human species
• But identifying sequence of DNA molecules does not mean identifying the function of each molecule
• Most genes in a human genome are the same for all humans
• Small number of genes are different for different individuals, including genes that indirectly code for physical traits and for personality traits

 

Controversy About Genes and Personality

• Behavioral geneticists attempt to determine the degree to which individual differences in personality (for example) are caused by genetic and environmental differences
• Highly controversial
• Ideological concerns
• Concerns about renewed interest in eugenics
• Modern behavioral geneticists who study personality are typically very careful about addressing implications of work and are sensitive to ideological concerns
• Knowledge is better than ignorance
• In addition, finding that a personality trait has a genetic component does not mean the environment is powerless to modify the trait

 

Goals of Behavioral Genetics

• Determine the percentage of individual differences in a trait that can be attributed to genetic differences and the percentage that can be attributed to environmental differences
• Determine the ways in which genes and environment interact and correlate with each other to produce individual differences
• Determine precisely where in the “environment” environmental effects exist—e.g., parental socialization, different teachers to which children are exposed

 

What Is Heritability?

Overview

• Proportion of observed variance in group of individuals that can be explained or accounted for by genetic variance, OR
• Proportion of phenotypic variance that is attributable to genetic variance
• Environmentality = proportion of observed variance in group of individuals attributable to environmental variance

Misconceptions About Heritability

• Heritability CANNOT be applied to single a individual
• Heritability is NOT constant or immutable
• Heritability is NOT a precise statistic

Nature-Nurture Debate Clarified

• No such debate at the individual level
• Influence of genes and of environment is only relevant for the discussion of group-level variation

Behavioral Genetics Methods

            Selective Breeding—Studies of Humans’ Best Friend

• Can only occur if a desired trait is heritable
• Selective breeding studies of dogs
• Cannot be ethically conducted with humans

 

Family Studies

• Correlates the degree of genetic overlap among family members with the degree of similarity in personality trait
• If a trait is highly heritable, family members with greater genetic relatedness should be more similar to one another on the trait than family members who are less closely genetically related
• Problem: Members of a family who share the same genes also usually share the same environment—confounds genetic with environmental influences
• Thus, family studies are never definitive

Twin Studies

• Estimates heritability by gauging whether identical (monozygotic or MZ) twins, who share 100 percent of genes, are more similar than fraternal (dizygotic or DZ) twins, who share only 50 percent of genes
• If MZ twins are more similar than DZ twins, this provides evidence of heritability
• Calculating heritability—many formulas, simple one: Two times difference between correlation (“r”) for MZ twins and DZ twins, or 2 (rmz – rdz)
• Two assumptions of the twins method
• Equal environments assumption
• Representativeness assumption

Adoption Studies

• Positive correlations on traits between adopted children and adoptive parents provide evidence of environmental influence
• Positive correlations between adopted children and genetic parents provide evidence of genetic influence
• Adoption studies are powerful because they get around the equal environments assumption—genetic and environmental causes are unconfounded
• Assumption that adopted children and their adoptive and genetic parents are representative of the general population—questionable
• Problem of selective placement of adopted children
• Design that combines strengths of twin and adoption studies = twins reared apart

Major Findings from Behavioral Genetic Research

            Personality Traits

• Summaries of behavioral genetic data yield heritability estimates for major personality traits (extraversion, agreeableness, conscientiousness, neuroticism, openness to experience) of about 20–45 percent
• Sexual orientation
• Controversial and developing area
• Current evidence suggests that genes provide modest and indirect influence (via childhood gender nonconformity) on adult sexual orientation

Attitudes and Preferences

• Wide variance in heritability of attitudes
• Some attitudes (e.g., traditionalism) show high heritability (about .60), whereas others show low or no heritability (e.g., beliefs in God, attitudes toward racial integration)
• Not clear why only some attitudes appear to be heritable

Drinking and Smoking

• Behavioral manifestations of personality traits such as sensation seeking, extraversion, neuroticism
• Drinking alcohol and smoking cigarettes are stable over time
• Both show evidence of heritability

           

Shared Versus Nonshared Environmental Influences: A Riddle

• Same studies that suggest moderate heritability also provide good evidence of the importance of environmental influences
• Personality characteristics show heritabilities in 30–50 percent range; hence, showing substantial degree of environmentality—50–70 percent
• Two key types of environmental influences
• Shared: In family environment, features of the environment shared by siblings (e.g., number of books in home)
• Nonshared: In family environment, features of the environment that differ across siblings (e.g., different friends, different teachers)
• For most personality traits, the environment has major influence, but this influence is primarily in the form of nonshared and not shared variables
• For most personality traits, the shared environment has little impact
• We do not know which nonshared experiences have a key impact on personality

 

Genes and the Environment

            Genotype-Environment Interaction

• Differential response of individuals with different genotypes to the same environments
• For example, task performance of introverts versus extraverts in loud versus noisy conditions
• Individual differences interact with environment to affect performance

Genotype-Environment Correlation

• Differential exposure of individuals with different genotypes to different environments
• Three types of genotype-environment correlations
• Passive: Parents provide both genes and environment to children, yet children do nothing to obtain that environment
• Child’s verbal ability and the number of books in home
• Reactive: Parents (or others) respond to children differently depending on the child’s genotype
• Baby’s liking for cuddling and the mother’s cuddling behavior
• Active: Person with particular genotype seeks out a particular environment
• High sensation seekers expose themselves to risky environments
• Genotype-environment correlations can be positive or negative

Molecular Genetics

• Techniques designed to identify specific genes associated with personality traits
• D4DR gene located on the short arm of chromosome 11, codes for dopamine receptor
• Most frequently examined association between D4DR gene and a personality trait involves “novelty seeking”
• Individuals with the “long repeat” version of D4DR gene are higher on novelty seeking than individuals with the “short repeat” version of gene
• But several failures to replicate association and, when replicated, association is weak
  

Behavior Genetics, Science, Politics, and Values

• Findings that some personality traits are heritable seemed to violate prevailing environmentalist view that personality is determined by socialization practices, such as parenting style
• People also worried about political and ideological misuse of behavioral genetics findings
• Much controversy surrounding individual differences in intelligence
• In past decade, attitudes shifted somewhat so that behavioral genetics are fairly mainstream (recent exception is sexual orientation studies)
• Because scientific research can be misused for political and ideological goals, scientists bear special responsibility, but
• Science can be separate from values
• Knowledge is better than operating in ignorance

 

SUMMARY AND EVALUATION

• Most compelling evidence for heritability and environmentality of personality comes from findings generated across methods that do not share the same problems and limitations
• Personality variables such as extraversion and neuroticism have moderate heritability, as do drinking, smoking attitudes, and sexual orientation
• These studies suggest that these same variables have moderate to strong environmentality
• Much of the environmental influence is due to nonshared variables—experiences unique to siblings
• Genotype-environment interaction and correlations, as well as the new field of molecular behavior genetic analysis, are promising areas for future work

           
KEY WORDS

            Genome                                              Equal Environments Assumption
Genetic junk                                       Adoption Studies
Eugenics                                             Selective Placement
Percentage of Variance                      Gender Identity Disorder (GID)
Heritability                                         Shared Environmental Influences
Phenotypic Variance                          Nonshared Environmental Influences
Genotypic Variance                           Genotype-Environment Interaction
Environmentality                               Genotype-Environment Correlation
Nature-Nurture Debate                      Passive Genotype-Environment Correlation
Selective Breeding                            Reactive Genotype-Environment Correlation
Family Studies                                   Active Genotype-Environment Correlation
Twin Studies                                     Molecular Genetics
Monozygotic (MZ) Twins                 D4DR Gene
Dizygotic (DZ) Twins                                   Environmentalistic View

 

Chapter Overview

This chapter provides students with an introduction to research and theory at the interface of genetics and personality. The authors begin with a discussion of the human genome, then move to a consideration of the controversy surrounding genetic influences on personality. The authors note that much of this controversy is generated by misinformed political and ideological concerns about the application of the findings from behavioral genetics. The authors review the scholarly goals of modern behavioral geneticists, then provide a definition for key terms such as heritability, environmentality, phenotypic variance, and genotypic variance. Next the authors detail several key misconceptions about heritability, including that it can be applied to a single individual, that it is constant and unchangeable, and that it is precise. The authors then review the four key research designs used by behavioral geneticists, including the selective breeding design, family designs, twin designs, and adoption designs. Each design has advantages and disadvantages, and these are reviewed in turn. Next the authors review the major findings generated from behavior genetics research, including findings in the areas of traditional personality traits, sexual orientation, attitudes and preferences, drinking, and smoking. The authors then discuss and differentiate shared and nonshared environmental influences. Next the authors review work on genotype-environment interaction and correlations and then address recent work in the new area of molecular behavior genetics. The authors close with a discussion of the inter-relationships among science, politics, and values.

 

Learning Objectives

 

• Discuss historical and current controversy surrounding the genetic analysis of personality.

• Discuss why behavioral genetics research is controversial, with reference to issues surrounding ideology conflicts and fears about eugenics.

 

• Identify and describe the actual scholarly goal of modern behavioral geneticists.

• Define and discuss the concept of heritability, with reference to phenotypic variance, genotypic variance, and environmentality.

 

• Identify and discuss three key misconceptions about heritability.

• Discuss the nature-nurture debate, from the perspective of modern behavioral genetics.

• Identify and discuss the four key research deigns used by behavioral geneticists, including a discussion of the advantages, disadvantages, and assumption of each design.

 

• Identify and discuss the major findings from behavioral genetics research, including findings in the areas of personality, sexual orientation, attitudes and preferences, drinking, and smoking.

• Discuss and differentiate shared environmental influences from nonshared environmental influences.

 

• Discuss the impact of shared versus nonshared environmental influences on personality.

• Define and provide examples of genotype-environment interactions.

 

• Define and provide examples for each of the three types of genotype-environment correlation, including passive, reactive, and active correlations.

• Discuss the emerging field of molecular behavior genetics, including its goals, methods, and recent findings.

 

• Discuss why science, politics, and values sometimes seem to be in conflict with respect to behavioral genetic findings.

• Provide a clear rationale for why behavioral genetics research should continue, and why knowledge is better than operating in ignorance.

 

Lecture Topics and Lecture Suggestions

 

• Heritability of Happiness (Lykken & Tellegen, 1996). Students are likely to find a presentation of research documenting the heritability of happiness intensely interesting and controversial. A brief overview of the study can be presented, as provided below. The instructor should plan to spend at least several minutes of class time discussing the results. What do students think of these results? Are they upsetting? Inspiring? What do they mean? Be sure to monitor student comments for a clear and correct understanding of modern behavior genetics. For example, one certainly wrong conclusion based on this research is that happiness is set at birth and there is nothing one can do to change it—that is, happy people will always be happy and unhappy people will always be unhappy. This is not a proper conclusion of this research (and also remind students about the misunderstanding of behavioral genetics presented by Larsen and Buss). Instead, this research indicates that, in the population studied, about 80 percent of the individual differences in happiness “set-point” are attributable to genetic differences. This means, of course, that about 20 percent of these individual differences are attributable to environmental differences. Ask students to consider what some of these environmental differences might be.

• Lykken & Tellegen (1996) measured happiness, or subjective well-being, on a birth-record-based sample of 2,310 middle-aged twins
• Measured happiness using the Well-Being (WB) scale of the Multidimensional Personality Questionnaire
• Also measured SES, educational attainment, family income, marital status, and religious commitment
• None of these variables could account for more than about 3 percent of the variance in well-being
• From 44–52 percent of the variance in well-being, however, is associated with genetic variation
• Re-tested smaller samples of twins after intervals of 4.5 years and 10 years
• Authors estimate that heritability of stable component of subjective well-being approaches 80 percent

 

Reference:

Lykken, D., & Tellegen, A. (1996). Happiness is a stochastic phenomenon. Psychological Science, 7, 186–189.

• Genetic Influence on Risk of Divorce (McGue & Lykken, 1992). Another topic that students are likely to find both interesting and controversial is that of the heritability of divorce. A brief overview of the McGue and Lykken (1992) study can be presented, as provided below. As with the presentation of the heritability of happiness, the instructor should plan to spend at least several minutes of class time discussing the results. What do students think of these results? Are they upsetting? Inspiring? What do they mean? As with the discussion of the heritability of happiness, be sure to monitor student comments for a clear and correct understanding of modern behavior genetics. For example, one certainly wrong conclusion based on this research is that the likelihood that one will divorce is set at birth and there is nothing one can do to change it—that is, your are born either doomed to divorce or destined not to divorce, following marriage. This is not a proper conclusion of this research (and also remind students about the misunderstanding of behavioral genetics presented by Larsen and Buss). Instead, this research indicates that, in the population studied, about 50 percent of the individual differences in risk of divorce are attributable to genetic differences. This means, of course, that about 50 percent of these individual differences are attributable to environmental differences. Ask students to consider what some of these environmental differences might be.

 

• McGue and Lykken (1992) explored the separate influence of genetic and environmental factors on risk of divorce
• They examined divorce status of 1,516 same-sex twin pairs (722 monozygotic [MZ] and 794 dizygotic [DZ]), their parents, and their spouses’ parents
• Concordance for divorce was significantly higher in MZ than DZ twins
• The robustness and magnitude of the MZ-DZ difference in divorce concordance indicates a strong influence of genetic factors in the etiology of divorce
• Moreover, family background of both spouses contributed independently to couples’ divorce risk
• Latter result suggests that, in many cases, divorce may be largely the result of characteristics the two spouses bring to the union rather than to interaction effects
• Results also suggest that adjustment difficulties seen with some children of divorced parents may be due to an interaction between genetic and environmental factors rather than environmental influences alone

Reference:

            McGue. M., & Lykken, D. T. (1992). Genetic influence on risk of divorce. Psychological Science, 3, 368–373.

Classroom Activities and Demonstrations

 

• Larsen and Buss present a simple formula for calculating heritability. One simply takes the difference between the correlation on some trait for monozygotic (MZ) or identical twins and the correlation on that same trait for dizygotic (DZ) or fraternal twins. Distribute Activity Handout 6-1 (“Calculating Heritability from Twin Studies”). Review this formula for students (which appears on the Activity Handout) and give them about 10 minutes to calculate heritability for the three examples. Ask students to volunteer their answers. Work out the examples on the board or on an overhead transparency, so that students who calculated an incorrect heritability can understand what they did wrong. The point of this exercise is not to quiz students on their math skills. Instead, the point is to demystify the calculation of heritability, so that students see that there is nothing magical about these calculations. Once you have established the correct answer for each example, discuss with students what each answer means—namely, the degree to which individual differences in the trait can be attributed to genetic differences or environmental differences. Ask students to tell you why this formula gives you an estimate of heritability. Instructors may also wish to use this activity and the discussion as a springboard for addressing what heritability means and what it does not mean.

• Larsen and Buss note that there are two key assumptions of the twin method used by behavioral geneticists. If either assumption is not met, then the results from the twin study might be called into question. These assumptions are the equal environments assumption, and the assumption that twins are representative of the general population. Distribute Activity Handout 6-2 (“Assumptions of Twin Studies”). Give students five minutes to describe these assumptions in their own words. Then ask for volunteers to share their responses. Use this activity as a springboard for discussing the twin design of behavioral genetics. Challenge students to identify why these are assumptions of the twin method, and why it is problematic if these assumptions are not met. Finally, close your discussion by noting that recent research suggests that neither of these assumptions is violated in most twin studies of personality.

 

• Behavioral geneticists distinguish between two types of environmental influences: Shared environmental influences and nonshared environmental influences. Distribute Activity Handout 6-3 (“Shared and Nonshared Environmental Variables”). Give students about five minutes to complete the handout. Ask students to volunteer their responses and carefully correct incorrect responses. Use this activity as a springboard for discussing the difference between shared and nonshared environmental influences on personality. Ask students to speculate why it is the case that most of the environmental influences on personality have been identified as nonshared, rather than shared environmental influences. Finally, ask students to address why this latter set of findings is controversial among social scientists, particularly among those who argue that parenting and various socialization processes are critical determinants of personality.

 

Questions for In-Class Discussion

• Larsen and Buss note that, although some observed differences between people are attributable to genetic differences, this does not mean that the environment plays no role in modifying the extent of such differences. Larsen and Buss provide the example of height. Height is about 90 percent heritable. Over the past century, however, the average height of people living in the United States has increased by about two inches, probably due to increases in the nutritional value of food eaten by U.S. citizens. This nutritional increase is an environmental variable. Ask student to provide other examples of how a trait with high heritability can be modified by the environment. Start by asking about other physical traits that are highly heritable that might also have been impacted by the increased nutritional value of food eaten by U.S. citizens over the past century.

 

• Larsen and Buss present three common misconceptions about the concept of heritability. One misconception is that heritability can be applied to a single individual. A second misconception is that heritability is constant and immutable or unchangeable. A third misconception is that heritability is a precise statistic. Ask students to discuss why these are misconceptions and have them correct these misconceptions using one or a couple examples. Ask them to consider why people, even trained psychologists, continue to believe these gross misconceptions. Instructors may wish to use this final part of the discussion as a springboard for discussing ideological and political agendas that thwart a clear understanding of behavior genetics.

• Larsen and Buss present four basic research designs used by behavioral geneticists who study personality (selective breeding studies, family studies, twin studies, and adoption studies). Ask students to identify and describe each research design. Ask them to discuss the advantages and disadvantages of each design. Finally, ask them to consider one particular personality trait, say agreeableness, and how the heritability and environmentality of that trait might be documented using each of these designs. Suggest to students that they discuss the selective breeding design with reference not to humans (for which this design is unethical), but instead with reference to dogs, following the presentation by Larsen and Buss. A clear understanding of these four research designs is central to understanding and appreciating behavioral genetic studies of personality. Students will gain a clearer and more comprehensive understanding of this material if they discuss it as a class, with the instructor serving as moderator, in addition to hearing it in lecture form and reading about it in the textbook.

 

Critical Thinking Essays

 

• Larsen and Buss present several possible reasons why behavioral genetic research in personality generates controversy. In your own words, why is behavioral genetic research so controversial? Discuss the possibility that this controversy lies primarily in several key misconceptions about behavioral genetics.

• Larsen and Buss present four key research designs used in the behavioral genetic study of personality. Briefly review the key features of these designs, including discussions of the advantages and disadvantages of each design. Why would it be particularly powerful for a set of results (such as heritability and environmentality estimates) to be documented using more than one design? Refer to a specific personality trait in formulating your answer to this question.

 

• Larsen and Buss review recent research documenting that some attitudes, such as traditionalism, have moderate heritability (30–60 percent), whereas other attitudes, such as beliefs in God, involvement in religious affairs, and attitudes toward racial integration are much less heritable, with heritabilities approaching 0 percent. What might account for the variance in the heritability of different attitudes? If individual differences in some attitudes are accounted for almost entirely by environmental differences, what might some of these environmental differences be? Include in your response a discussion of the difference between shared environmental influences and nonshared environmental influences.

 

Research Papers

• Larsen and Buss discuss four key research designs used by behavioral geneticists who study personality. These are selective breeding studies, family studies, twin studies, and adoption studies. Conduct a search of the psychological research literature and locate four research articles published within the last five years, each of which uses only one of the four types of designs. For each article, first summarize what the researchers investigated, how they investigated it, and what they found. Then suggest how the researchers might have used each of the remaining three research designs. Finally, address whether you think the results might have turned out differently if they had used different research designs and why.

 

• Larsen and Buss distinguish between shared environmental influences and nonshared environmental influences. Larsen and Buss note that, for most personality variables, the shared environment has either little or no discernible impact. Instead, environmental influences are usually of the nonshared type. First, briefly discuss, describe, and distinguish between shared environmental influences and nonshared environmental influences. Next, and in your own words, speculate as to why nonshared environmental influences so frequently account for more of environmental impact than do shared environmental influences. Finally, conduct a search of the psychological research literature and identify three articles that address empirically shared and nonshared environmental influences on personality (Try searching by using the key words “personality and nonshared environment”). For each article, summarize what the researchers investigated, how they investigated it, and what they found. Highlight the distinction between shared and nonshared environmental influences.

• Larsen and Buss discuss molecular behavior genetics, the most recent development in the science of behavioral genetics. Larsen and Buss note that these techniques are designed to identify, for example, the specific genes associated with, for example, personality traits. Conduct a search of the psychological literature and identify three articles that report research at the interface of molecular genetics and personality. For each article, summarize what the researchers investigated, how they investigated it, and what they found. In your own words, what are some of the advantages and disadvantages of studying personality using molecular behavioral genetics?

 

Recent Research Articles and Other Scholarly Readings

 

 Bailey, J. M., Kirk, K. M., Zhu, G., et al. (2000). Do individual differences in sociosexuality represent genetic or environmentally contingent strategies? Evidence from the Australian twin registry. Journal of Personality and Social Psychology, 78, 537–545.

Borkenau, P., Riemann, R., Angleitner, A., et al. (2001). Genetic and environmental influences on observed personality: Evidence from the German Observational Study of Adult Twins. Journal of Personality and Social Psychology, 80, 655–668.

Bucholz, K. K., Hesselbrock, V. M., Heath, A. C., et al. (2000). A latent class analysis of antisocial personality disorder symptom data from a multi-centre family study of alcoholism. Addiction, 95, 553–567.

Comings, D. E., Johnson, J. P., Gonzalez, N. S., et al. (2000). Association between adrenergic alpha-sub(2A ) receptor gene (ADRA2A) and measures of irritability, hostility, impulsivity and memory in normal subjects. Psychiatric Genetics, 10, 39–42.

Coolidge, F. L., Thede, L. L., Young, S. E. (2000). Heritability and the comorbidity of attention deficit hyperactivity disorder with behavioral disorders and executive function deficits: A preliminary investigation. Developmental Neuropsychology, 17, 273–287.

Geijer, T., Frisch, A., Persson, M.-L., et al. (2000). Search for associations between suicide attempt and serotonergic polymorphisms. Psychiatric Genetics, 10, 19–26.

Goldsmith, H. H., & Lemery, K. S. (2000). Linking temperamental fearfulness and anxiety symptoms: A behavior-genetic perspective. Biological Psychiatry, 48, 1199–1209.

Jocklin, V., McGue, M., & Lykken, D. T. (1996). Personality and divorce: A genetic analysis. Journal of Personality and Social Psychology, 71, 288–299

Johansson, B., Grant, J. D., Plomin, R., et al. (2001). Health locus of control in late life: A study of genetic and environmental influences in twins aged 80 years and older. Health Psychology, 20, 33–40.

Jorm, A. F., Henderson, A. S., Jacomb, P. A., et al. (2000). Association of a functional polymorphism of the monoamine oxidase: A gene promoter with personality and psychiatric symptoms. Psychiatric Genetics, 10, 87–90.

Lake, R. I. E., Eaves, L. J., Maes, H. H. M., et al. (2000). Further evidence against the environmental transmission of individual differences in neuroticism from a collaborative study of 45,850 twins and relatives on two continents. Behavior Genetics, 30, 223–233.

Lykken, D., & Tellegen, A. (1996). Happiness is a stochastic phenomenon. Psychological Science, 7, 186–189.

McCrae, R. R., Costa, P. T., Jr., Ostendorf, F., et al. (2000). Nature over nurture: Temperament, personality, and life span development. Journal of Personality and Social Psychology, 78, 173–186.

McGue. M., & Lykken, D. T. (1992). Genetic influence on risk of divorce. Psychological Science, 3, 368–373.

McGuffin, P., Riley, B., & Plomin, R. (2001). Toward behavioral genomics. Science, 291, 1232–1249.

O’Connor, T. G., Caspi, A., DeFries, J. C., et al. (2000). Are associations between parental divorce and children’s adjustment genetically mediated? An adoption study. Developmental Psychology, 36, 429–437.

Plomin, R. (2001). Genetics and behaviour. Psychologist, 14, 134–139.

Plomin, R., & Craig, I. (2001). Genetics, environment and cognitive abilities: Review and work in progress towards a genome scan for quantitative trait locus associations using DNA pooling. British Journal of Psychiatry, 178, s41–s48.

Price, T. S., Eley, T. C., Dale, P. S., et al. (2000). Genetic and environmental covariation between verbal and nonverbal cognitive development in infancy. Child Development, 71, 948–959.

Sander, T., Ostapowicz, A., Samochowiec, J., et al. (2000). Genetic variation of the glutamate transporter EAAT2 gene and vulnerability to alcohol dependence. Psychiatric Genetics, 10, 103–107.

Turkheimer, E., & Waldron, M. (2000). Nonshared environment: A theoretical, methodological, and quantitative review. Psychological Bulletin, 126, 78–108.

Weiss, A., King, J. E., & Figueredo, A. J. (2000). The heritability of personality factors in chimpanzees (Pan troglodytes). Behavior Genetics, 30, 213–221.

Wills, T. A., Sandy, J. M., & Yaeger, A. (2000). Temperament and adolescent substance use: An epigenetic approach to risk and protection. Journal of Personality, 68, 1127–1151.

Activity Handout 6-1:

Calculating Heritability from Twin Studies

Instructions: Larsen and Buss present a simple formula for calculating heritability. One simply takes two times the difference between the correlation (“r”) on some trait for monozygotic (MZ) or identical twins and the correlation on that same trait for dizygotic (DZ) or fraternal twins. Here is the formula:

Heritability = 2 (rmz – rdz)

Calculate the heritability for the following three examples:

• rmz = .90, rdz = .10

 

• rmz = .60, rdz = .40

 

• rmz = .30, rdz = .30

 

Activity Handout 6-2:

Assumptions of Twin Studies

Instructions: Larsen and Buss note that there are two key assumptions of the twin method used by behavioral geneticists. If either assumption is not met, then the results from the twin study might be called into question. These assumptions are the equal environments assumption, and the assumption that twins are representative of the general population. In your own words, define each of these assumptions—that is, what does each assumption mean?

 

• Equal Environments Assumption:

 

• Assumption of Representativeness:

 

Activity Handout 6-3:

Shared and Nonshared Environmental Variables

Instructions: Under the label “Shared Environmental Variables” below, write down five shared variables that you had in common with your siblings while you were growing up (if you are an only child, write down five things that might be shared environmental variables if you had siblings). Next, and under the label “Nonshared Environmental Variables,” write down five environmental variables that you did not share with any of your siblings while you were growing up (again, imagine you had siblings if you are an only child).

 

Shared Environmental Variables

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Nonshared Environmental Variables

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