A key question for personality psychologists is: “Are we what we are because of nature or because of nurture?” When the question is phrased in this manner, neither answer can be correct, for the answers are not mutually exclusive. The premise of the question itself is wrong. We are what we are because of how nature and nurture interact; it is not a question of either/or. There clearly are both inborn and social-cultural influences on the individual. Genetics and environment—nature and nurture—regulate and guide each person.
In Chapter 1 we examine some of the innate determinants of personality and behavior. We observe that, as products of a long evolutionary history, hu- man beings are predisposed to certain patterns of behavior. In addition, spe- cific genetic blueprints provide the foundation for personality and behavior.
But it is evident that individuals’ personalities are greatly influenced by the contexts that surround them and by the experiences they have had. Individu- als brought up in different cultures or social classes behave differently. If one wants to predict what an individual is doing or thinking at a given time, some of the best predictors are the point in history when the individual was born and where he or she is living. In Chapter 2 we examine some of these social and cultural influences as well as other kinds of environmental influences.
Culture not only influences individuals; it also influences psychological the- ories themselves. There is increasing reason to believe that the “grand theories” of Western psychology reflect certain biases about the nature of people that characterize Western culture.
Chapters 1 and 2 do not attempt to give complete accounts of the genetics of personality or the social determinants of action; full courses are devoted to these topics. Rather, we introduce the context of the person: an individual with unique predispositions who is nevertheless modifiable and shaped by surroundings.
Innate and Environmental Determinants of Personality 1PART
CHAPTER 1 Genetic Determinants of Personality
CHAPTER 2 Contextual Aspects of Personality and Behavior
Genetic Determinants of Personality
Charles Darwin introduced the idea that the human species is the product of a long period of evolution in The Origin of Species and Descent of Man. His arguments had an enormous influence on the field of personality. First, his theory of evolution assumed scientific determinism—that is, the theory assumed that the most complex aspects of behavior in all species are sub- ject to scientific and rational analysis and are not due to accident or divine intervention. This principle was accepted by psychologists in their study of both nonhuman and human behavior. Second, Darwin focused attention on the function or adaptive value of biological structures and behavior. Psychol- ogists have been guided by this viewpoint as they search for the usefulness of a particular pattern of action. Still another implication of Darwin’s work for the study of human personality is the importance of species differences and of individual differences within species. These issues are addressed in this chapter.
Darwin proposed a simple yet powerful theory to explain the process of evolution that linked the development of species with the concept of inheritance—the transmission of characteristics from one generation to an- other. He stated: “Any variation, however slight and from whatever cause proceeding, if it be in any degree of profitability to an individual . . . will tend to the preservation of that individual, and will generally be inherited by its offspring. The offspring, also, will thus have a better chance of surviv- ing” (1869, p. 61).
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CHAPTER1 Chapter Outline All Humans Are Alike All Humans Are Unique
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Nearly every theory of personality tries to describe why people are the way they are. Personality theories are going to make the claim that all humans are alike and the propositions apply to everyone. Simultaneously, personality theories are going to try to explain why each human is different. In our exploration of the influence of genetics on personality, we will see this fully on display. We will start with the ways in which we understand how people are alike, and then we will explore the ways in which people are unique.
In this chapter we examine some of the genetic determinants of personality structure and behavior. Although neurotransmitters and hormones, brain mechanisms, and other biological factors influence personality, we focus here on the topic of genetics because of the vast research in this area and because it well illustrates the role of biological givens in behavior.
It is helpful to frame a genetic approach to personality within the context of evolu- tionary theory. Among the questions we discuss in this chapter are: What limitations do inherited characteristics place on learning? Do humans have instincts, or innate urges and unlearned patterns of behavior? Are there universal facial expressions or behaviors that are exhibited across cultures? And can it be demonstrated scientifically that person- ality characteristics and behavioral problems are in part genetically influenced? We will present evidence that the answers are yes and that a complete theory of personality must consider innate factors.
All Humans Are Alike The human genome is made up of 23 chromosomes, which are then made up of a molecule called deoxyribonucleic acid (DNA). DNA has four bases: adenine, thymine, guanine, and cytosine. These are often denoted just by their first letter: A, T, G, and C. These four letters provide the instructions for mak- ing amino acids that then make up the various proteins and en- zymes that make up living organisms. The process by which this happens is beyond the scope of this book, but it is important to understand some of the basics.
DNA is copied via another molecule called ribonucleic acid (RNA). In the process of copying, mistakes can be made. Fur- thermore, mutations can also occur due to exposure to radia- tion and other things in the environment. Most of the time, the mutations are harmless; some of the time, the mutations are harmful. Importantly, sometimes the mutation leads to positive outcomes and increases the chances of the organism or its off- spring surviving.
Through the instructions on how to build the amino acids and how to arrange the amino acids into proteins and enzymes, the DNA provides the information needed to create brain struc- tures, neurotransmitters, and hormones. These then become the building blocks of behavior. All humans have a genome that codes for these things, but there will be subtle differences among each person, as everyone (except monozygotic twins) has a slightly different genome. However, unrelated humans still share somewhere between 99% to 99.5% of their DNA. The general pattern is what makes us human and will therefore make us similar in many ways.
It is important to recognize that most of what we will be examining will not be the result of a single gene, but rather
Our DNA is made up of four molecules: adenine, thymine, guanine, and cytosine.
genome The organized set of DNA that every living thing has.
Chapter 1 Genetic Determinants of Personality 5
the result of several genes acting together. The genes may be all pointing in a particular direction, or a few genes may point in one direction while the others point in the oppo- site direction. Consider the genes that are involved in height. Some genes may promote being tall, whereas other genes may promote being short. Some people will get many genes that point toward being tall; some people will get many genes that point toward being short. Most people will get a combination and end up somewhere in the middle. Traits that are made up of several genes are likely to be revealed as something that is normally distributed.
The Question of Instincts
Instincts are regarded as genetic givens. An instinct or, more appropriately, instinctive behavior can refer to either (1) an unlearned, fixed pattern of activity found in all members of a species, such as nest building in birds and web spinning by spiders; or (2) a specific motivational tendency that is in- ferred from overt behavior. An instinct in this second sense is a potential toward action and is considered an unlearned “want” or “urge” built into the structure of the organism. Although the ends, or goals, of instincts (urges) are presumed to be fixed, the means of expression can be quite diverse.
Instinctive Patterns of Behavior
Recall that instinct, in addition to meaning a genotypic urge, also refers to an unlearned pattern of behavior that is characteristic of all members of a species of the same sex at the same level of development. Birds’ responses to mating calls and squirrels’ hoarding of food are examples of such unlearned patterns of responses. These behaviors are caused primarily by genetically transmitted physiological states and function they are akin to the built-in behaviors exhibited by plants, such as movement toward light. These instinctive behaviors frequently are not single responses, but are action sequences that follow a pre- determined, predictable course. Furthermore, the observed behaviors often are crucial for survival and therefore are interpreted in an evolutionary sense. For example, it is known that the stickleback fish will attack any other fish of its color or even any object of this particular color. This behavior results in species-spacing, or spreading species members apart so that there is sufficient food available for survival. Distinctive birdsong often also has a species-spacing function.
An interesting set of studies that combines instinct and learning was conducted with Rhesus monkeys (Cook and Mineka, 1989; 1990). One group of monkeys was trained to fear either snakes or flowers. A different group of monkeys watched film of the first group while the first group was trained to fear that stimuli. The monkeys that observed the other monkeys learning to fear snakes acquired a fear of snakes, whereas those that watched the flower condition did not learn to fear flowers. This would seem to indi- cate that they have an instinctual predisposition to learn to fear things that may threaten survival.
It often is difficult to prove with certainty that a response is innate or unlearned rather than learned and experience-based. In addition, the higher the organism is in the nonhuman-human scale the more likely that the behavior is influenced by learning. Among human beings one might expect to find few unlearned patterns of behavior, other
instinct An innate, biologically based mode of response to certain stimuli.
species-spacing Spreading members of the same species across environment to enhance survival.
Traits that are due to multiple genes are likely to be distributed in a bell-shaped curve. People who get all the genes pointing toward being tall are likely to be tall; however, most people have a combination and are toward the middle of the distribution. Could we expect that people who have all their genes pointing toward shyness to be at one tail of the distribution of extraversion? Source: Shahreen/Shutterstock.
6 Part 1 Innate and Environmental Determinants of Personality
Childlike faces elicit positive responses. This is true in humans as well as pets, like puppies and kittens.
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Chapter 1 Genetic Determinants of Personality 7
than reflexive responses such as the eye blink or knee jerk. However, it is quite possi- ble that there are innate reactions—fears, aversions, preferences for particular stimulus situations—that have significance for human personality. Fear of the grotesque or a child’s responsiveness to the mother’s breast and the warmth of her body are possible examples of such reaction tendencies.
Infant Features and Maternal Care Whether behaviors such as maternal care among humans have genetic determinants can- not be demonstrated with certainty, although it is reasonable to suppose that they do. Such an inborn tendency would have great survival value for the species, in that the young would be protected from danger and starvation. It has been speculated that certain features of the young, particularly facial characteristics, operate as social signals and innately elicit complex emotions and behaviors (Berry and McArthur, 1986). A variety of facial features distinguish infants from adults. Infants have large heads in relation to their bodies, their faces are hairless and smooth—skinned, and they have a relatively large forehead and small chin so that the vertical placement of features is low in the infant’s face. In addition, infants’ eyes are large, their nose is small.
Berry and McArthur (1986) have shown that these infant facial features may elicit helping responses and positive emotions that, in turn, promote survival. Consider, for example, your reactions to the faces of infants, as opposed to their adult counterparts. It is evident that infant features call forth approach reactions and warmth. It has been contended that these features also influence responses toward adults. In one startling finding, it was reported that baby-faced criminal defendants were judged less harshly than mature-faced defendants, even when they had committed the same criminal offense (Berry and McArthur, 1986).
Evolutionary psychology has sought to establish that even such social behaviors as con- formity, altruism, and cheating have a biological or genetic basis using principles laid out by E. O. Wilson (1975, 1977). Wilson called his model Sociobiology, which helped lead to a field in psychology called Evolutionary Psychology. Evolutionary psychologists have noted that Darwinian principles account well for aggressive behavior. Aggressive individ- uals are likely to receive food and also may engage in more mating behavior. Hence, it is argued, those most aggressive will survive and pass on this trait to their offspring. How- ever, it is difficult for Darwinian principles to account for altruism. Those with altruistic tendencies decrease their own “fitness” relative to others, and thus altruism would tend to extinguish over time.
Like Darwin, evolutionary psychologists also accept the view that survival-promoting behavior is passed on from one generation to another. But they contend that survival of the genes, rather than the individual organism, is the prime evolutionary tendency. This belief enables them to account for apparently altruistic actions. For example, a bird may risk its own life to warn the rest of the flock of impending danger. In so doing, the individual bird might not survive, but the survival of others with like genes is aided. Why do human beings help others who apparently are not related to them? Evolutionary psychologists argue that an agreement of “reciprocal altruism” has evolved because mutual help-giving augments the genetic pools of all participating parties.
These ideas have been invoked to explain a vast array of puzzling facts. For example, why should female ants devote their lives to helping the queen to breed, instead of breed- ing themselves? Wilson (2000) has pointed out that female ants actually share more genes in common with their sisters than they would with their own offspring. Thus, in order to perpetuate genes identical to their own, it is in their self-interest to assist the queen in producing more daughters.
8 Part 1 Innate and Environmental Determinants of Personality
Evolutionary psychology concepts have also led to reinterpretation of various facets of human sexual behavior. The male, evolutionary psychologists may argue, has one prime goal: to transmit as many of his own genes as possible to the next generation. On the other hand, the female must invest a great deal of time in each birth and can have only a very limited number of offspring. Thus, males are in general more promiscuous than females because their promiscuity has a genetic payoff. Furthermore, because females must in- vest more of themselves in each pregnancy, they are important resources that males must “purchase.” As a result, in many cultures older males (who have more resources) marry younger women (who have many years of childbearing and child caring left), and women prefer men who are “good providers” (Buss, 2003, 2005; Fales et al., 2016). However, we should also examine this phenomena from a cultural vantage point. Historically, it has been difficult for women to accumulate wealth on their own, so the only way for them to get it was through a husband. Thus, a behavior of looking for a husband with access to wealth and resources can be explained through an evolutionary approach or a cultural approach.
It has been suggested that men might use two different reproductive strategies, some- times referred to as “dads” and “cads” (Draper and Harpending, 1982). The first strategy involves finding a reproductive partner and investing time and resources in helping to raise their children. The second strategy is the promiscuous strategy mentioned in the pre- vious paragraph: find as many reproductive partners as possible and not invest strongly in raising the children.
Likewise, women likely use two strategies. In one strategy, a woman looks for a reproductive partner who will invest in the offspring; thus, she is looking for a man who is using the dad strategy. In the other strategy, the woman is looking for a re- productive partner who will provide high-quality genes and will lead to “sexy sons” (Gangstead and Simpson, 1990; Weatherhead and Robertson, 1979). Those sexy sons could then conceivably use the cads reproductive strategy. Interestingly, women who
Chapter 1 Genetic Determinants of Personality 9
are ovulating are more likely to misperceive a cad as a dad (Durante, Griskevicius, Simpson, Cantú, and Li, 2012). There is evidence that there are consistent changes in reproductive partner choices during the ovulatory cycle such that women prefer the sexier cads during fertile parts of their ovulatory cycle (Gildersleeve, Hasselton, and Fales, 2014). This pattern has been named ovulatory shift (Gangestead, Thornhill, and Garver-Apgar, 2005).
Evolutionary psychologists are not reluctant to extend their ideas to explain seemingly unrelated phenomena. For example, they anticipate that child abuse will be most prevalent toward stepchildren because not only do these offspring not carry the genes of the abuser (most typically the stepfather), but they even decrease the “survival fitness” of the step- father (Daly and Wilson, 1996). Interestingly, data from Sweden (Temrin, Buchmayer, and Enquist, 2000) does not support this hypothesis.
Evolutionary psychologists point out that males have only one great disadvantage in breeding: they cannot be certain that the offspring are their own. Thus, sexual jealousy is aroused and courtship rituals have emerged to monopolize the female’s time. During this extended courtship the male also can determine if the female is already pregnant. Evolutionary psychologists suggest that the maternal grandparents (the parents of the mother) will be more attached to the offspring than the paternal grandparents (the parents of the father) because of uncertainty regarding the “true” father. It is generally accepted that the mother’s mother has prerogatives, such as the first visit to the baby!
Needless to say, these ideas have been controversial (Archer, 1988). The controversy involves primarily the extension of evolutionary psychology principles to complex human social behaviors. It is obvious that cultural factors greatly influence human actions. For example, how can evolutionary psychology account for the fact that today many men un- dergo voluntary sterilization and the birthrate in many countries is falling so rapidly that there is zero population growth? In spite of difficulties in explaining such facts, advocat- ing this extreme biological view has proved exceedingly provocative and has spurred the interpretation and reinterpretation of a variety of phenomena.
Facial Expressions and Emotions
Darwin argued that human facial expressions are inherited and are modified very little by cultural experience. For example, he pointed out that the baring of teeth by humans in situations of anger and contempt is similar to the display of teeth among carnivores prior to a hostile attack or during an aggressive defense.
A number of studies have attempted to verify Darwin’s insight. These studies bear on the biological basis of emotional experience as well as on the universality of emotional displays. Two basic paradigms have been followed in this research area: (1) examining the facial expressions of children born deaf and blind; and (2) analyzing the agreement concerning identification of emotions in different cultures.
Investigations reveal that blind and deaf children do display quite normal facial expres- sions (Eibl-Eibesfeldt, 1970, Freedman, 1964). Hence, these observations tend to support the universalistic, Darwinian explanation of facial expression. Of course, culture plays some role in the control of facial expression and in determining which situations are ap- propriate for eliciting and displaying particular emotions and facial expressions.
The cross-cultural research has been guided by identification of the so-called basic human emotions. Ekman and Friesen (1975) isolated six primary emotions—happiness, sadness, fear, anger, disgust, and surprise—and also identified the facial muscles associ- ated with these emotional states. Photographs showing the facial expressions associated with these emotions were then shown to people in five different countries—the United States, Brazil, Chile, Argentina, and Japan—and the subjects were asked to identify the emotion portrayed. The percentage of agreement with the labels supplied by Ekman and Friesen was quite high across all countries used.
ovulatory shift A woman’s preference for attractive healthy partners who may not invest in offspring during ovulation.
10 Part 1 Innate and Environmental Determinants of Personality
In addition, natives of New Guinea, who had very little exposure to Western culture and were not subject to the possible confounding of media exposure, responded simi- larly to people in the other five countries. (However, it must be noted that individuals are more adept at this labeling task when selecting the appropriate emotion from a given list, rather than supplying their own interpretation without the aid of a preselected list.) In sum, it appears that facial expressions of some emotions are to a large extent a genet- ically based characteristic of the species (Ekman, 1993, 2016). Although these appear to be the six facially displayed emotions across cultures, it does not mean they are the only emotions. There are likely to be emotions that are unique to different cultures (Niiya, Ellsworth, and Yamaguchi, 2006) and other emotions that are more effective if not fa- cially displayed.
The Need to Belong and Survival
Humans are a social species and survive and reproduce most successfully when they are part of a group. In the past, exclusion was likely to lead to death. Exclusion from a group is usually described using words associated with pain (MacDonald and Leary, 2005). Functional magnetic resonance imaging (fMRI) research has found that the same part of the brain is active when a person is in physical pain or experiences social exclusion (Eisenberger, Lieberman, and Williams, 2003).
Fear of exclusion also helps to explain one of the major fears that many people expe- rience: public speaking. If a person speaks in such a way that he or she loses status or belonging, there will be negative survival or reproductive consequences. Because of these perceived high stakes, people who believe that public speaking could have this conse- quence are likely to experience anxiety (Baumeister and Tice, 1990).
Leary and his colleagues (Leary and Baumeister, 2000; Leary, Tambor, Terdal, and Downs, 1995) have suggested that self-esteem acts as a way to monitor the extent to which a person belongs and is important to social groups. In this model, self-esteem mea- sures social belonging in the same way that a thermometer measures temperature; thus, they term this the sociometer theory of self-esteem. People who have a strong sense of
sociometer A theory that suggests the function of self- esteem is to measure the extent to which people fit in their social environment.
A baboon in full threat gesture, with teeth bared. Darwin pointed out that a human snarl has a similar facial pattern and suggested that emotional displays are genetically programmed.
Chapter 1 Genetic Determinants of Personality 11
The facial expressions of emotion that were identified across cultures are happiness, sadness, anger, fear, disgust, and surprise.
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12 Part 1 Innate and Environmental Determinants of Personality
belonging will have high self-esteem, whereas people who do not feel as if they belong will have low self-esteem. Self-esteem is, therefore, an evolved mechanism by which hu- mans are consistently checking belongingness.
Interestingly, there appear to be genetic differences in the ways in which people re- spond to social exclusion (Gallardo-Pujol, Andrés-Pueyo, and Maydeu-Olivares, 2013). They found that people with one gene respond strongly to being excluded, whereas those with the other version of the gene do not.
All Humans Are Unique Despite cross-cultural similarities and the universality of some behaviors, it can equally be said that each human is genetically unique. Given the billions of possible inherited gene patterns, each individual is provided with an idiosyncratic genetic configuration. The resulting variability in genetic structure is manifested in individual differences in characteristics and behavior. Two basic procedures have been followed in studying the genetic determinants of individual differences. One method is experimental, involving the selective breeding of nonhumans. The other is nonexperimental in the sense that there is no intervention in the lives of its human subjects. The nonexperimental procedures include both twin and adoption designs. Finally, there are new approaches looking at loci on the genome that are associated with behavior.
The Experimental Study of Heritability
For many years selective breeding has been unsystematically performed on domestic animals to enhance certain temperamental or behavioral characteristics, such as ferocity, docility, hunting ability, and so on. In more systematic laboratory settings, animals ex- hibiting a high or low degree of a certain behavior, such as emotionality, are selectively bred for generations and the emotionality of subsequent generations is then compared. The experimenter thus performs on a compressed time scale what natural selection is pre- sumed to do during the course of evolution. In natural selection, high or low extremes of a characteristic or directional selection, may be favored. For example, natural selection in opossums evidently favored inertness in the face of danger; visual aggressive displays or strength and speed are favored in other species. In nature, as in the laboratory, there may also be balancing selection, which favors intermediate values of the characteristic in question.
The logic of breeding experiments is straightforward and nicely illustrates what is meant by the concept of heritability—the degree to which genetics plays a role in the development of a particular aspect of behavior in a particular population. Assume, for example, that we observe a group of animals learning a maze and that they attain scores of 1–10, based on the number of trials before success. That is, one animal learns the maze in a single trial, another takes two trials, a third requires three trials, and so on. Now, for example, we take the “brightest” animals, which require only one trial to learn, and we inbreed them. Of their offspring, we again inbreed the fastest learners. Assume that this procedure is continued for thirty generations. Now, if the thirtieth generation of animals still gives us scores of 1–10 in maze learning, we know that the ability to learn this maze has no heritability. Rather, the differences among the animals are due entirely to environ- mental factors. Conversely, assume that for this thirtieth generation, all the animals learn the maze in one trial. In that case, we would assume that maze learning was due entirely to genetic factors; heritability is the determining factor. In actuality in this situation, the variability among the animals, i.e., their differences in how many trials it took them to learn, will most likely decrease over generations, but will not disappear entirely. Thus, there will be both genetic and environmental determinants of learning, with the decrease in variability indicating the degree to which this behavior is influenced by genetic factors.
selective breeding Choosing organisms with positive characteristics to reproduce.
directional selection An extreme version of a trait or behavior may be preferred in some instances of natural selection.
balancing selection An intermediate version of a trait of behavior may be preferred in some instances of natural selection.
heritability The degree to which genetics plays a role in the development of a particular aspect of a trait or behavior.
Chapter 1 Genetic Determinants of Personality 13
A representative study of selective breeding for a particular ability is illustrated in an investigation by Thompson (1954). Thompson measured the speed with which rats learned a maze for a food reward. Then he bred the low-error (maze-bright) rats with other maze-bright rats, and the high-error (maze-dull) rats with other maze-dull rats. The offspring of these matings were then tested on the maze. This procedure continued for six generations; by the sixth generation, the error scores of the two breeding pop- ulations were dramatically different. It is interesting to note that rats superior in maze learning do not necessarily master other problems as rapidly. Maze learning appears to be a specific ability.
Although the research by Thompson indicates that maze learning is influenced by he- redity, this should not be taken to mean that environmental factors have no effect on such learning. Cooper and Zubek (1958) took the strains of rats bred by Thompson and reared them in either enriched or impoverished conditions. The enriched environment contained ramps, tunnels, and many movable objects, whereas the restricted environment included only a food box and a water tin. When the maze learning of the rats was assessed, the experimenters found that when the groups were reared in either extreme environment, their performance did not differ greatly. The enriched environment primarily raised the performance of the dull rats, while the restricted environment chiefly lowered the perfor- mance of the bright rats. Only in the normal environment were there differences due to an innate ability factor. Hence, there is nothing absolute about heritability; an organism’s individual sensitivity to disparate environmental conditions may either mask or exacer- bate inborn tendencies. What is inherited, then, is the way in which the organism responds to the environment, and heritability is in turn a function of the environment in which the behavior is studied.
Very similar conclusions regarding heritability have been reached in the study of emo- tionality. Broadhurst (1961) inbred high- and low-emotionality rats, selected on the basis of their defecation in an open field test. In this procedure, rats are placed in an unfamil- iar environment, and emotional responses are inferred from the well-documented fact that fear causes defecation and urination. After six generations of inbreeding, there were dramatic differences between the populations, with the data resembling those reported by Thompson (1954) in terms of the magnitude of group differences and the increasing inequality in emotionality over the generations.
While it seems reasonable that learning ability and even emotionality may have a ge- netic basis, it is perhaps less sensible to believe that preferences or incentive values have a genetic component, as opposed to being entirely learned. However, breeding experiments have demonstrated that alcohol preference also may be inherited. It has been documented that particular strains of mice choose an alcohol solution over a water solution (Rodgers, 1966), while other strains avoid alcohol and prefer water. Furthermore, when strains with high and low alcohol preference are interbred, the offspring display an intermediate desire for alcohol as opposed to water (Rodgers and McClearn, 1962). Later in this chapter we will suggest that alcoholism among humans might also be determined in part by inborn tendencies.
The Study of Heritability Through Human Research
In contrast to studies of nonhumans, research using human populations is non- experimental in the sense that there is no intervention or manipulation of variables because of obvious ethical constraints. Rather, individuals who vary in their degree of genetic relationship, such as foster parents and their children, natural parents and their children, siblings, dizygotic twins, and monozygotic twins, are compared and contrasted. Genetic similarity is a function of degree of kinship, so for genetically influenced characteristics, similarity in behavior and character should increase with biological relatedness.
14 Part 1 Innate and Environmental Determinants of Personality
Many dog breeds were bred to be predisposed to particular skills. Border collies were bred for their herding instincts. Golden retrievers were bred to retrieve birds from the water; they even have webbed feet, which make it easier for them to swim.
Correlations One of the most important statistics that is used in personality psychology is the correlation coefficient. The correlation coefficient allows the similarity be- tween two variables to be quantified. The correlation coefficient has two components: sign and magnitude. The sign, positive or negative, tells us the direction of the relationship. If it is a positive correlation, as one variable goes up, the other variable also tends to go up; as one variable goes down, the other tends to go down. Height and weight are typically positively cor- related: tall people typically weigh more than short people. For a negative correlation, as one variable goes up, the other variable tends to go down. We might ex- pect classroom absences to be negatively correlated with grades; as absences increase, grades tend to go down. The magnitude can range from 0 to 1.0. When two variables are closely related, the magnitude will be near 1.0; when they are unrelated, they will be near 0. Considering both sign and magnitude, correlation coefficients can range from +1.0 to –1.0.
To create a simple example, let’s imagine that researchers have people take each of the following mea- sures: a measure of shyness, a measure of extraversion, and a measure of emotionality. They might want to know whether a relationship exists between the measure of shyness and the measure of extraversion. Using the
correlation coefficient, they can answer that question. They might find that the correlation between the mea- sure of shyness and extraversion is –0.78. This would indicate that there is a strong relationship between the two measures, and as shyness goes up, extraver- sion tends to goes down. They could then investigate whether extraversion and emotionality are related, and they might find the correlation to be +0.08. This would indicate that there is a very weak, or no, relationship be- tween the two variables.
By squaring the correlation coefficient, we can quantify how much of the variability is predicted by one variable to the other. Remember, we are squaring dec- imals, .90 squared is .81, .50 is .25, and .30 is .09. If we find a correlation of .30, then .09 of the variance is predicted—less than 10%.
Chapters 7 and 8 discuss these ideas further, but it is necessary to understand the basics to understand the research on twins. Most of the time in personality research, the same person provides both variables. However, in investigating heritability of personality, different people would be measured on the same scale. So both twins might take an IQ test, or a parent and child might both take a measure of shyness, or siblings would be measured on extraversion.
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Chapter 1 Genetic Determinants of Personality 15
The basic shortcoming of human genetic research is that the environments are not controlled. As a result, it is often impossi- ble to infer with certainty that observed behavioral similarity is a function of biological relatedness and is due to inborn charac- teristics. In nonhuman research using inbreeding techniques, the environments are controlled and identical for each group.
Consider, for example, a hypothetical research study demon- strating that emotional mothers have highly emotional chil- dren, whereas non-emotional mothers have children with low emotionality. These data might be interpreted as demonstrating that emotionality is heritable. However, it is quite possible that these children had different home environments; perhaps highly emotional mothers create unstable environments which produce highly emotional children, whereas non-emotional mothers es- tablish stable environments which give rise to children low in emotionality. In this investigation, the learning and the genetic contributions to emotionality cannot be separated. To help in- terpret the ambiguous results of such family studies, twin and adoption studies are often employed to disentangle genetic and environmental contributions.
The central research separating genetic from environmental contributions to human behavior involves a comparison be- tween twins from different eggs (fraternal, or dizygotic twins) with those from the same egg (identical, or monozygotic twins). The latter type share completely identical gene pools; dizygotic twins, on average, share only about 50% of their genes. Hence, monozygotic (MZ) twins should be more alike in behavior than di- zygotic (DZ) twins, if the behavior under study has a genetic component. This hypoth- esis assumes that the environments of DZ twins are as identical as the environments of MZ twins. However, such an assumption, although reasonable, is not always warranted. Identical twins may have more similar social learning histories; it has been found that they are more likely to dress alike, have common friends, and spend more time together than fraternal twins (Smith, 1965). However, this factor may be of limited importance, as parents of twins often incorrectly classify their children as MZ or DZ (Scan and Carter-Saltzman, 1979). See Table 1.1.
dizygotic twins Twins that develop from two fertilized eggs.
monozygotic twins Twins who develop from a single fertilized egg.
Identical (monozygotic, MZ) twins. Evidence indicates that many aspects of their temperament and behavior also will be similar.
Relationship (A) Shared genes (C) Common environment
MZ together 100% 100%
MZ apart 100% 0%
DZ together 50% on average 100%
DZ apart 50% on average 0%
Siblings together 50% on average <100%
Siblings apart 50% on average 0%
Half-siblings together 25% on average <100%
Half-siblings apart 25% on average 0%
Parent-child 50% <100%
grandparent-grandchild 25% <100%
Adopted siblings 0% <100%
16 Part 1 Innate and Environmental Determinants of Personality
The genetic contibution that a person has at birth.
Common Environment. C
All the common experiences as a result of being raised in the
E Nonshared Environment.
The parts of life not shared. Unique experiences. Measurement error.
The observed or measured
By knowing the genetic relatedness, it is possible to begin making estimates about the contributions of the genotype to the phenotype. Different degrees of family relatedness lead to different amounts of shared genes. Again, remember we are examining only those genes that vary.
In principle, it is possible to study the effects of shared genes by comparing identical twins reared in similar versus different environments. Although such a sample is seldom available, some investigations have examined the similarities of siblings adopted in in- fancy and reared in foster homes, compared with unrelated individuals reared in these same homes. Any differences in the similarities within these two groups, given that all else is equal, are logically attributable to the genetic similarity of the sibs. Conversely, any differences in the behavior of the identical twins shows environmental and experiential influences on behavior.
The phenotype is going to be expressed as a function of the genotype and the environ- ment. If we consider monozygotic