Sistemas basados en CBR aplicados al sector salud

The Twin Method

Most often, heredity studies use the twin method to assess genetic influence. This method simply compares the degree of similarity on measures of personality obtained for genetically identical twins—those who are from the same egg or monozygotic (MZ)—as opposed to twins who are fraternal or dizygotic (DZ), that is, with each from a different fertilized egg (Plomin et al., 1997). Identical twins are as close to genetically identical as people get, but fraternal twins—like other siblings—are only 50% similar genetically. To the degree that genetic factors affect personality, it follows that identical twins must be more similar than fraternal twins with regard to that characteristic.

5.5 How do twin studies shed light on genetic factors in personality?

Results of Twin Studies

A pioneering study with nearly 800 pairs of adolescent twins and measuring dozens of personality traits reached a conclusion that has stood the test of time (Loehlin &

Nichols, 1976): Identical twin pairs are much more alike than fraternal twin pairs.

The resemblance within identical twin pairs tends to be strongest for general ability.

The resemblance is somewhat lower for personality inventory scales and lowest for interests, goals, and self-concepts (see Table 5.1). For personality, twin correlations are about .50 for identical twins and .25 for fraternal twins. This same study also found that nearly all personality traits measured by self-report questionnaire show moderate genetic influence.

5.6 What do twin studies find regarding the degree of resemblance in personality as compared with other

characteristics?

The Big Five

Genetic research on personality has focused on five broad dimensions of personality— the Big Five (Goldberg, 1990), discussed in Chapters 3 and 4. Extraversion and Neuroticism

Studies comparing identical twins and fraternal twins help specify the role of genes in personality.

(Source: (left) Photodisc/Getty Images, Inc; (right) Todd Warnock/Taxi/Getty Images)

Twin Studies 䉳 101 TABLE 5.1 Resemblance of Identical and Fraternal Twin Pairs: Typical

Correlations within Pairs

Trait Area Identical Twins Fraternal Twins

General ability .86 .62

Special abilities .74 .52

Personality scales .50 .28

Ideals, goals, interests .37 .20

Source: Adapted from Heredity, Environment, and Personality: A Study of 850 Sets of Twins, by John C.

Loehlin and Robert C. Nichols, Copyright ” 1976. By permission of the University of Texas Press.

have been studied the most. Extraversion includes sociability, impulsiveness, and liveliness. Neuroticism (emotional instability) includes moodiness, anxiousness, and irritability. Table 5.2 summarizes results for Extraversion and Neuroticism (Loehlin, 1992). Results from five twin studies in five different countries, using a total of 24,000 pairs of twins, consistently indicate moderate genetic influence. Correlations are about .50 for identical twins and about .20 for fraternal twins.

The role of heritability in extraversion and neuroticism has been studied extensively, but much less genetic research has been done for the other three Big Five traits, namely, Agreeableness (likeability, friendliness), Conscientiousness (conformity, will to achieve), and Culture (openness to experience). These qualities have been investigated with diverse measures (rather than one standardized test) that also make it more difficult to compare results across different studies. Nevertheless, results of twin and adoption studies with measures related to these three traits also suggest genetic influence for agreeableness, conscientiousness, and culture at least to a moderate degree (Loehlin, 1992).

TABLE 5.2 Resemblance of 24,000 Pairs of Reared-Together Twins in Five Countries and Identical Twins Reared Apart

Correlations within Twin Pairs

Identical Twins Fraternal Twins Identical Twins Reared Together Reared Together Reared Apart

Extraversion .51 .18 .38

Neuroticism .46 .20 .38

Source: Adapted from Loehlin, J. C. Genes and environment in personality development.

” 1992, reprinted by permission of Sage Publications, Inc.

Temperaments

The term temperaments refers to traits that are visible in early childhood (Buss &

Plomin, 1984), and seem especially relevant to the individual’s emotional life (Allport, 1961; Clark & Watson, 1999). Dispositions usually considered temperaments include the general level of emotionality, sociability, and activity. These temperaments are usually assessed through parental reports about their children on temperament rating scales (Rothbart, Derryberry, & Posner, 1994). In adults, they typically are assessed by self-report measures (e.g., Buss & Plomin, 1984), with items like those shown in Table 5.3.

TABLE 5.3 Self-Report Items to Assess Temperament in Adults

EMOTIONALITY (easily aroused physiologically to experience negative emotions) Many things annoy me.

I get emotionally upset easily.

SOCIABILITY (seeks social interaction) I prefer working with others rather than alone.

I like to be with other people.

ACTIVITY (overall energy level, tempo/speed, intensity or vigor) My life is fast-paced.

I usually seem to be in a hurry.

Note: Participants rate on five-point scales the degree to which items like these apply to them, from ‘‘not typical’’ of me to ‘‘very typical’’ of me. Based on Buss, A. H., & R. Plomin.

(1984). Temperament: Early developing personality traits (Table 7.3). Hillsdale, NJ: Erlbaum.

Copyright, 1984, reprinted by permission.

Emotionality, also called emotional reactivity, is often defined as the tendency to become aroused easily physiologically (by ready activation of the autonomic nervous system) and especially to experience frequent and intense negative emotions such as anger, fear, and distress (Buss & Plomin, 1984). Not all researchers agree with this definition, however. They find that the intensity with which an individual experiences emotions is independent of how often he or she has such feelings (Larsen, Diener, &

Emmons, 1986). That suggests that these two components of emotionality need to be considered separately, and that both need to be taken into account. For example, if Jane rarely experiences fear but becomes unbearably fearful in some situations, her emotional life would be quite different from someone who is often moderately fearful but never intensely afraid. Further, positive and negative emotions can function independently and they need to be measured separately. For example, people who often experience positive emotions may or may not also experience negative emotions often.

In spite of these variations, there is agreement that emotionality is an important aspect of temperament. There also is some evidence that emotional reactivity at high levels may be a long-term risk factor for emotional disorders, and for some somatic diseases. Studies by Strelau and Zawadzki (2005), for example, suggest that this temperament trait may increase the individual’s risk to react to life stressors by depression, and may in turn lead to increased likelihood of physical disorders.

Sociability refers to the degree to which the person seeks to interact with others and to be with people. (As such, it overlaps with the concept of extraversion versus introversion, introduced in Chapter 3.)

Activity may be defined with regard both to the vigor or intensity of responses and their tempo or speed. It refers to stable individual differences on a dimension that ranges from hyperactivity to extreme inactivity (e.g., Thomas & Chess, 1977).

In these dispositions genetic endowment seems to have a significant part and the evidence is increasingly strong (e.g., Clark & Watson, 1999; Plomin, 1990; Rowe, 1997). Figure 5.1 illustrates these types of results more concretely. It shows that on the dimension of emotionality, identical twins are rated as much more similar by their mothers than are fraternal twins.

Although the results are impressive, they are not easy to interpret. Some of the greater similarity found may reflect that the mothers themselves may treat the identical

Twin Studies 䉳 103

1.0 .9 .8 .7 .6 .5 .4 .3 .2 .1 0.0

Correlations

Boys Girls

.63

.20 .73

0 Monozygotic Dizygotic

Figure 5.1 Similarity of emotionality: mother’s ratings of monozygotic and dizygotic twin pairs.

Source: Correlation coefficients for degree of similarity from data in Buss, Plomin, and Willerman (1973).

twins more similarly, as might other people in the environment. The mothers also may have been influenced in their ratings not only by the twins’ behavior but by their own expectations and preconceptions for identical versus fraternal twins. Nevertheless, results like these tend to be obtained so consistently that they suggest a significant genetic role in personality with regard to the temperaments of emotionality, activity, and sociability (Buss & Plomin, 1984; Plomin et al., 1997). A comprehensive review of this research concluded that ‘‘. . . one-third to one-half of individual differences in temperamental traits can be attributed to genetic variation among children’’ (Rowe, 1997, p. 378).

Genetic researchers investigating differences between children in temperament could not use self-report questionnaires with their young participants and therefore used other measures, such as direct ratings of the children’s behavior by observers (e.g., Cherney et al., 1994; Goldsmith & Campos, 1986; Saudino & Plomin, 1996). With few exceptions (confined to the first few days of life), observational studies of young twins show genetic influence for diverse characteristics. These characteristics include the degree to which the child’s behavior is inhibited—an aspect of fearfulness (e.g., Robinson, Kagan, Reznick, & Corley, 1992); shyness both when observed at home and in the laboratory (Cherney et al., 1994); activity level (Saudino & Eaton, 1991); and empathy (Zahn-Waxler, Robinson, & Emde, 1992). Findings from these studies are especially valuable because they go beyond those that rely on self-reports—which are by far the most common—to closely observe what the child is doing. A notable example of research on temperaments that goes much beyond self-reports comes from Jerome Kagan’s long-term research on inhibition and shyness (In Focus 5.2).

5.7 In twin studies, how strongly do genetic factors seem to influence temperament? Why are these results difficult to interpret?

IN FOCUS 5.2

INHIBITED CHILDREN: KAGAN’S SHYNESS RESEARCH

Jerome Kagan (2006) argues that inhibition is one of the major dimensions of temperament, and finds early individ-ual differences exist on this dimension and endure in the course of development. The inhibition construct is similar to the neuroticism component of the Big Five. It refers to the extent to which infants are wary of novelty and unfamiliarity. Inhibited or ‘‘shy’’ children react to unfamil-iar people or situations with fear. They try to avoid those situations, and take much longer to relax when they are in them. They tend to have phobias when they grow up.

Kagan’s research program, spanning over four decades, examined how early these individual differences emerge, how stable they are over time, and whether or not they are biologically based. In one study, he observed identical and fraternal twins when they were 4 months old. The twins were videotaped in response to novel stimuli such as a stranger or a balloon popping. They were also exposed to familiar stimuli such as a mother’s face or her voice.

To measure how fearfully reactive children are to unfa-miliarity, the videotapes were coded for such behaviors as the arching of the back, unhappy facial expressions, and crying. Kagan and colleagues found that in their sample 20% of children were inhibited, 40% were low reactive or uninhibited, and 40% were mixed.

These children were followed up at 14, 24, and 54 months of age. At later follow-up, the highly reactive chil-dren showed greater fearful behavior, heart acceleration, and higher blood pressure when they were put in an unfamiliar situation. At age 54 months, the inhibited chil-dren talked less with unfamiliar adults who were trying to interact with them in the lab and tried to avoid them.

Furthermore, a moderate heritability influence was found (using the heritability index discussed in In Focus 5.3) at 14 months and at 24 months of age. In short, temperamen-tal differences in inhibition emerged as early as 4 months of age, were stable across the first 4 years of life, and showed moderate heritability, suggesting a biological-genetic com-ponent in its development (Robinson, Kagan, Reznick, &

Corley, 1992).

Although this research has demonstrated considerable stability in inhibition, the inhibited child under some conditions can change over time. For example, inhibited infants grew up to be less inhibited preschoolers if they had mothers who were not overly protective and placed reasonable demands on them (Fox, Henderson, Rubin, Calkins, & Schmidt, 2001). Still, complete and consistent crossover transformations from one extreme to the other were unlikely.

Attitudes and Beliefs

Genetic influences also seem to play a role in individual differences in attitudes and

5.8 How did Kagan use a twin study to evaluate the heritability of

inhibition? beliefs. Results come from a number of twin studies (Eaves et al., 1989), including one of twins who were adopted and reared apart (Tellegen et al., 1988). To illustrate, a substantial genetic influence was found on traditionalism, a general orientation that taps conservative (as opposed to liberal) attitudes on diverse topics and many other attitudes also seem to show genetic influence (Eaves et al., 1989; see Plomin et al., 1997).

Attitudes that are more heritable may differ from those that are less herita-ble systematically, and examining such differences sheds some light on the nature of genetic influences. For example, in one study the researcher separated many specific attitudes into two sets (Tesser, 1993). One set contained those that twin studies had found were more heritable (such as attitudes about the death penalty and about jazz). The other set consisted of those that were less heritable (such as attitudes about coeducation, straightjackets, and the truth of the Bible). Then the researcher set up experimental situations designed to change these attitudes in col-lege students. He found that the more heritable attitudes were harder to influence and also more important in determining the person’s judgments of interpersonal attraction.

Twin Studies 䉳 105 Aggressive and Altruistic Tendencies

Research with adult twins also points to the influence of genes on other aspects of social behavior. For example, self-reports on aggressiveness questionnaires were obtained from a large number of twin pairs in England (Rushton, Fulker, Neale, Nias, & Eysenck, 1986). The twins answered such questions as: ‘‘I try not to give people a hard time’’

and ‘‘Some people think I have a violent temper.’’ The altruism questionnaire asked for the frequency of such behaviors as ‘‘I have donated blood’’ and ‘‘I have given directions to a stranger.’’ Within the identical (monozygotic) twin pairs, the answers were more similar than would be expected by chance whereas between fraternal twins, the correlation was merely at a chance level. These results occurred both for the males and for the females. Using sophisticated statistical techniques, the researchers estimated that genetics accounted for approximately 50% of the individual differences in test answers.

Romantic Love and Marriage

Although genes seem to directly or indirectly influence individual differences on most measures of personality, as well as on social attitudes, including peoples’ self-esteem (e.g., McGuire, Neiderheiser, Reiss, Hetherington, & Plomin, 1994), one area that seems to be beyond DNA is romantic love. A behavior genetic twin family study focused on the genetic versus environmental influences on individual differences in adult romantic love styles. The participants were drawn from 890 adult twins and 172 spouses from the California Twin Registry married for an average of a dozen years (Waller & Shaver, 1994). Six different love styles were measured, ranging from one that values passion, excitement, intimacy, self-disclosure, and ‘‘being in love from the start,’’ to one that values a relationship that is affectionate and reliable and has companionship and friendship (with items like ‘‘It is hard for me to say exactly when our friendship turned into love’’).

The findings showed that how people love is almost completely due to the envi-ronment and essentially unaffected by genetic influences. In fact, this is one domain in

How people love is mostly influenced by their environment, not their genetics.

(Source: LWA/Dan Tardd/Getty Images, Inc)

which it is the family environment that turned out to be particularly important (Waller &

Shaver, 1994). As the researchers noted ‘‘. . . love styles may be learned during early familial or shared extra-familial interactions and subsequently played out in romantic relationships’’ (pp. 272–273).

But while love styles at least so far seem not heritable, the propensity to get married is. Johnson and colleagues (2004) studied over 7,000 women and men, including many complete twin pairs. According to their findings, the tendency to get married is itself genetically influenced.

Twins Reared Apart

To try to separate the role of genetics and environment, it is especially informative to assess identical twins who have been reared apart in different families. Reports have come from two large-scale studies of twins reared apart in Minnesota (Bouchard et al., 1990; Tellegen et al., 1988) and in Sweden (Pedersen, Plomin, McClearn, & Friberg, 1988; Plomin et al., 1988). The results surprised even many of the researchers who have long been convinced that genes affect personality.

5.10 What is the special value of comparing twins raised together and apart?

In this research, for more than a decade, Bouchard, Tellegen, and their associates studied a sample of identical (monozygotic) twin pairs reared apart who were separated early in life (on average before the end of the second month). They grew up in different families, but mostly in English-speaking countries. As adults, their responses were assessed on many medical and psychological measures, including personality questionnaire scales and intelligence tests (as seen in Table 5.4). Most had not seen each other for an average of about 30 years, although some had contact over the years.

Comparisons were made with a larger sample of twin pairs who had been reared together and grew up in Minnesota.

There were instances of dramatic psychological similarities within the twin pairs, even for twins who grew up in radically different environments for 30 years or more in many cases. These twins seemed to share some quite distinctive mannerisms, postures, attitudes, and interests. For example, in some cases they posed alike for photos. Some turned out to have virtually the same height, the same weight, the same number of marriages and children, the same drinking and smoking habits, the same mannerisms, the same clothes, food, and jewelry preferences, similar physical symptoms—and similar

TABLE 5.4 Names of Personality Scales in Studies of Twins Reared Apart and Together

Well-being Control

Social Potency Harm Avoidance

Achievement Traditionalism

Social Closeness Absorption

Stress Reaction Positive Emotionality

Alienation Negative Emotionality

Aggression Constraint

Note: On most scales, identical twins reared apart were as similar to each other as those reared together. The main exception was ‘‘social closeness’’

(on which those raised together were more similar).

Source: Tellegen, A., Lykken, D., Bouchard, T., Wilcox, K., Segal, N., &

Rich, S. (1988). Personality similarity in twins reared apart. Journal of Personality and Social Psychology, 54, 1035 (Table 3).

Twin Studies 䉳 107 scores on personality tests (e.g., Segal, 1999). Many also quickly felt a close emotional

connection with each other even though they had spent their entire lives apart.

There also were strong similarities in many of the test results of the identical twins.

Especially interesting, the similarity was almost as high for the monozygotic twins who grew up in different homes as it was for those raised within the same family (as was seen in Table 5.2). Bouchard and colleagues (1990) attributed approximately 70% of the individual differences found in intelligence to heredity. They interpreted the effects of heredity on personality (as assessed on their questionnaires) to be approximately 50%

and the effects of family environment to be trivial (Tellegen et al., 1988). Likewise, twin studies of the Big Five factors suggest ‘‘The shared genes, not the shared experiences, mainly determine the family resemblance of ‘blood relatives’’’ (Rowe, 1997, p. 380).

Beyond Self-Report Measures

One of the most surprising findings from genetic research on self-report personality questionnaires is that of the many traits that have been studied, virtually all show genetic influence. That may be because all these traits in fact reflect genetic predispositions. But at least some of the similarity may lie in the eyes of the beholder. For example, identical twins who have a higher level of frustration tolerance may think and honestly say that they experience less negative emotions than others, worry less, and rarely feel anxious, not because they experience these feelings less than others, but because they are less bothered by them. It is therefore particularly important to use measures of personality other than self-report questionnaires to investigate whether or not this result is somehow due to biases in the self-report measures themselves.

5.11 Describe the difficulties that attend the use of self-report measures to assess

5.11 Describe the difficulties that attend the use of self-report measures to assess