Genetic Aspects Of Temperament Research Paper

1. Temperament

The concept of temperament as a part of personality that is primarily biologically based, observable already in infancy, and stable throughout the life span, is deeply rooted in prescientific views of human nature. For example, the Hippocratic typology of four temperamental types (sanguine, melancholic, choleric, phlegmatic) was highly influential throughout the Middle Ages and the Enlightenment. In the twentieth century, three main strands of temperament research emerged that later became interconnected (see Strelau and Angleitner 1991).

Soviet and Eastern European research began in the 1930s with Pavlov’s observation of differences among dogs in his conditioning experiments. Animals with a ‘strong nervous system’ responded with increased intensity to increases in stimulation whereas animals with a ‘weak nervous system’ failed to respond above some threshold. The Pavlov school also introduced the concepts of strength of inhibition, balance between activation and inhibition, and mobility (the flexibility of the adjustment to changing conditions). This approach was developed further in England by Gray (1982) and has influenced particularly physiological and clinical research on adults. According to Gray, individuals differ on two main temperamental dimensions—inhibition and impulsivity. Inhibited individuals react more strongly to conditioned signals of novelty and punishment than uninhibited individuals. Impulsive persons react more strongly to conditioned signals of reward than unimpulsive persons.

A second research tradition was based on self- descriptions of temperament in questionnaires. British factor analytic studies, which were begun in the 1930s by Burt, were taken further by Eysenck who identified two major dimensions of temperamental differences, extraversion–introversion and emotional stability–emotional lability (or neuroticism). Eysenck (1967) was of the opinion that these two dimensions of self- described temperament directly reflected physiological differences in arousability. According to this highly influential theory, extraverts need more stimulation than introverts for optimal arousal, and introverts are sooner overstimulated than extraverts (Pavlov’s idea of the weak nervous system). Eysenck attributed these assumed physiological differences to an arousal system in the brain stem. A similar, less developed theory attributed differences in emotional stability to the differential arousability of the limbic system. However, attempts to test this theory in psychological experiments have met with only limited success (see Strelau and Angleitner 1991).

A third research tradition is the study of temperamental differences in infants and young children (Rothbart and Bates 1998). The pediatricians Thomas and Chess initiated the highly influential New York Longitudinal Study on early temperamental differences (Thomas et al. 1963). They repeatedly interviewed parents about their infant’s behavior in specific contexts. These descriptions were sorted into nine dimensions of temperament: activity level, approach withdrawal, adaptability, mood, threshold, intensity, distractibility, rhythmicity, and persistence. Subsequent research showed that some of these nine dimensions were strongly correlated, and parent or teacher questionnaires were designed that assess fewer, less correlated dimensions of infants’ or children’s temperament. For example, the Colorado Childhood Temperament Inventory assesses five dimensions: (negative) emotionality, activity, persistence, shyness, and sociability.

In recent years, these three traditions have become increasingly interrelated. Gray (1987) showed that his model of inhibition–impulsivity can be translated into Eysenck’s model of extraversion–neuroticism at the level of self-description. Windle and Lerner (1986) developed a questionnaire for the assessment of temperamental dimensions from childhood through adulthood. Halverson et al. (1994) pointed out how temperamental dimensions are related to the more general five-factor model of personality description. By and large, temperament refers to the first two factors of this model, extraversion and emotional stability, which were already identified by the British factor analytic tradition, with age-specific additions particularly for infants and younger children.

Therefore the working definition of temperament for this research paper refers to individual differences in basic psychological processes that constitute the affective, activational, and attentional core of personality (Rothbart and Bates 1998), and which are mainly related to the first two factors of the five-factor model of personality description, extraversion and neuroticism.

2. Estimation Of Heritability

The genetic influence on temperamental differences in a human population can be studied both directly and indirectly. The direct method tries to establish a statistical relation between the frequency of variants of a gene in the population (its alleles) and temperamental traits. This method provides estimates of the influence of specific genes on specific traits and requires molecular biological knowledge about which genes and alleles can be distinguished. This direct method became feasible towards the end of the twentieth century as the relevant knowledge became increasingly available.

The present article reviews results from familial analyses conducted in the field of behavioral genetics, a subdiscipline of population genetics (see Plomin et al. 1997 for an overview). This indirect method estimates the overall genetic influence on a specific dimension of temperament relative to the overall environmental influence on this dimension through analyses of the temperamental similarity of members of the same family such as twins, adoptive siblings, and their parents. This method requires only basic knowledge about rules of inheritance, not molecular biological knowledge about genes and their alleles. However, the method is based on certain assumptions that may not always be valid.

The indirect method is based on the fact that the genetic similarity of family members varies from 0 percent shared alleles in the case of adopted siblings or adopted children and their nonbiological parents, to 50 percent shared alleles in the case of biologically related siblings, nonidentical twins (dizygotic twins), and their biological parents, and 100 percent shared alleles in the case of identical twins (monozygotic twins). Siblings, nonidentical twins, and their parents share 50 percent of their alleles on average because half of children’s alleles stem from the mother and the other half from the father. Identical twins are an exception because they develop from the same original cell that only later splits into two different individuals; they are therefore genetically identical.

Two basic assumptions of the indirect method are: (a) influences on temperamental differences in the population can be either environmental or genetic, and (b) the overall impact of the environment, relative to genes, is identical for siblings, adoptees, and twins. Therefore, differences in their similarity can be fully interpreted as genetic effects. Thus, if identical twins are more similar in a specific temperamental trait than nonidentical twins, or biologically related siblings are more similar than adopted siblings, this difference in similarity can be genetically interpreted. The larger the difference in similarity, the stronger the genetic influence on the temperamental trait. Thus, there exist two main indirect methods of estimating genetic influences: comparing identical twin pairs with nonidentical twin pairs (twin method), and comparing biologically related sibling pairs with adopted sibling pairs (adoption method). As a result it is possible to crosscheck the results of these two methods with one another. Other indirect methods compare the similarity between identical twins that are reared apart as opposes to those reared together, or the similarity between parents and their biological children as opposed to their adopted children.

The strength of the genetic influence on temperamental differvences can be quantified by the heritability coefficient h². This coefficient can be interpreted as the proportion of variance in a specific temperamental trvait in the population that is due to genetic differences: h² = genetic variance total variance. It can be estimated on the basis of differences between intraclass correlations. In the twin method, the intraclass correlation between nonidentical twins (a measure of their similarity) is subtracted from the intraclass correlation between identical twins. Because this difference estimates 100 – 50 percent = 50 percent of the genetic influence (identical twins share 100 percent of their alleles, and nonidentical twin s 50 percent), it is doubled; this double difference is h . In the adoption method, the difference between the sibling correlation and the adopted sibling correlation is doubled because it estimates 50 – 0 percent = 50 percent of the genetic influence (adopted siblings do not share alleles).

The heritability coefficient can vary between 0 and 100 percent. Zero heritability would be found if the variation of the temperamental trait in the population is only due to environmental differences. In this case, adopted and biologically related siblings are equally similar in this trait, as are identical and nonidentical twins. A heritability of 100 percent would be found if the variation of the trait in the population is only due to genetic differences. In this case, adopted siblings would be as dissimilar as children from different families, and identical twins would perfectly match each other in the trait; normal siblings would be moderately similar.

The concept of heritability is a relativistic concept because it estimates the amount of genetic influences relative to environmental influences. Therefore, heritability can vary between populations and between different traits within the same population. Thus, the heritability of adolescent shyness can be higher in the USA than in African cultures, and adolescent shyness can be more heritable than adolescent sociability in the USA. Furthermore, heritability depends on the variability of genes and environments in a given culture. If the genetic variability is increased (e.g., through genetically dissimilar immigrants), or if the environmental variance is decreased (e.g., through compensatory education), the genetic influence can increase; if the genetic variance is decreased (e.g., through a virus that kills people with particular alleles), or if the environmental variance is increased (e.g., through an unjust political system), the genetic influence can decrease.

The heritability of a trait depends not only on the trait, the population, and its culture, but also on age. The genetic influence on a trait may decrease with age because more and more environmental influences accumulate. On the other hand the genetic influence may also increase with age because more and more genes become relevant for the trait or because older children and adults are more able than young children to select their environment according to their genetically influenced interests and motives (such selection would decrease the environmental variance). All in all, heritability is a highly relativistic concept with regard to the target trait, culture, and age.

3. Heritability Of Temperamental Traits

Empirical studies on the heritability of temperamental traits are nearly exclusively restricted to North American and Western European samples. Therefore it is unknown whether the following findings can be generalized to other cultures. Because heritability can vary with age, the findings are discussed separately for different age groups. Because the findings also vary considerably with the estimation method used, twin studies and adoption studies are discussed separately.

In twin studies of older infants and children, two groups of traits can be distinguished with regard to their heritability (see Goldsmith et al. 1997). A first group of traits (activity, emotionality, sociability, impulsivity, approach withdrawal) is characterized by a high similarity of identical twins (intraclass correlations approximately 0.60) and a dissimilarity of nonidentical twins (correlations close to zero), suggesting the odd result of more than 100 percent heritability for some of these traits. A second group of traits (inhibition to strangers, adaptability, mood, distractibility, rhythmicity, intensity, persistence, and threshold) is characterized by an equally or even higher similarity of identical twins along with a moderate similarity of nonidentical twins, suggesting 40–60 percent heritability. Adoption studies for this age range have resulted in zero or even negative correlations for adopted children and somewhat more positive correlations for biologically related siblings, suggesting 10–50 percent heritability.

Whereas in all these studies a moderate to strong genetic influence on temperamental traits was found, one twin study found no genetic influence for 5 different observed temperamental traits in 4-day-old infants (Riese 1990). Together with findings of a low stability of temperamental differences over the first months of life, this result can be attributed to strong environmental influences around birth that obscure genetic effects (e.g., perinatal and postnatal difficulties that transitionally influence the infants’ emotional and attentional reactivity).

For adolescents and adults, most studies have focused on extraversion and emotional stability (or the reverse trait, neuroticism). Loehlin (1992) reviewed these studies. For twin studies, the results were similar for both traits: approximately 65 percent genetic influence, based on moderately high correlations between identical twins and low correlations between nonidentical twins. For adoption studies, extraversion showed a higher heritability of approximately 50 percent than neuroticism (only approximately 15 percent heritable).

In a large study of Finnish twins, Viken et al. (1994) found a decrease in the heritability of both extraversion and neuroticism between late adolescence and the late twenties and no change in heritability thereafter. It should be noted that this study relied on crosssectional analyses that compared the results for different birth cohorts. Therefore, the age effects may reflect cultural changes rather than age-related changes. Only longitudinal studies that follow the same persons over a long time can disentangle agerelated changes from historical changes.

Overall, there is clear evidence for genetic influence on temperamental traits at all ages except close to birth, but the findings are far from being consistent between the twin and the adoption method. Possible reasons for these inconsistencies are discussed in Sect. 4. When the results are compared to the findings for other individual differences such as intelligence, conscientiousness, or agreeableness, the heritabilities are not higher (see Loehlin 1992, Plomin et al. 1997). Thus, variations in temperament do not seem to be more genetically influenced than other variations of personality and intelligence.

4. Methodological Problems

Both the twin method and the adoption method are affected by multiple problems that can bias their results. For temperament-related traits, the main problems seem to be contrast effects and non-additive genetic influences. Contrast effects make twins and siblings more different, and can occur in two different forms. First, they can be due to sibling rivalry (e.g., one becomes more dominant and the other more submissive in their interaction, and these tendencies generalize to other relationships). In this case, twins and siblings are more different than one would expect on the basis of their genes and other environmental influences. Such contrast effects are expected to decrease with increasing age differences between the siblings; thus, they should operate particularly in the case of twins.

Second, contrast effects can affect personality judgments by family members because the parents or the siblings contrast a child with his or her siblings. For example, the parents may overestimate differences between two siblings’ aggressiveness if one is moderately aggressive relative to peers and the other one is not aggressive because they perceive one as ‘the aggressive one’ and the other as ‘the non-aggressive one.’ These judgmental contrast effects are expected to increase with increasing dissimilarity of the siblings. Therefore, they are expected to underestimate most strongly the similarity of adopted children, and moderately strongly the similarity of biologically related siblings; for identical twins they should be particularly small.

Saudino et al. (1995) studied contrast effects for parental ratings of temperament. They compared the similarity of identical twins, nonidentical twins, siblings, and full, half, and genetically unrelated siblings in stepfamilies. This approach enabled estimation of the strength of contrast effects. They found significant contrast effects for all temperamental traits. These contrast effects may explain both the odd finding of more than 100 percent genetic variance for many temperamental traits in twin studies (the difference between identical and nonidentical twins is overestimated because the nonidentical twins are judged to be less similar than they really are) and the very low or even negative correlations for adopted siblings (they are judged to be less similar than they really are). It should be noted, however, that after controlling for contrast effects, significant genetic effects remained for all traits in this study.

The second main methodological problem is that the influences of different genes on a temperamental trait may not necessarily be additive. For example, the mere presence of the alleles A and B may not in itself increase the risk for high emotionality but the simultaneous presence of both alleles may. Because identical twins share all alleles, they share 100 percent of all genetic effects, including such non-additive effects, and adopted siblings share neither additive nor non-additive effects. However, nonidentical twins and siblings share 50 percent of the additive effects but a smaller percentage of the non-additive effects because these require that they share combinations of alleles. For example, the probability that they share alleles A and B is only 25 percent. If non-additive effects exist, the difference between identical and nonidentical twins estimates more than 50 percent of the genetic effects. Therefore, the heritability coefficient overestimates genetic influences. Similarly, the adoption method underestimates genetic differences because the difference between biologically related and adopted siblings estimates less than 50 percent of the genetic effects.

In the case of non-additive effects, the genetic estimates from the twin method and the adoption method are biased in opposite directions (overestimation vs. underestimation). This principle applies also to other problems such as the genetic similarity of the parents due to partner preferences (their homogamy), or a genetic similarity between adopted children and their nonbiological parents due to nonrandom assignment to families by the adoption agencies (selective placement). Therefore, it is possible to control these problems by the simultaneous analysis of similarity data from both twins and adopted children. This combination method requires complex statistical models (Neale and Cardon 1992) but circumvents most methodological problems of both the twin and the adoption method. It is even possible to estimate the strength of the various biases (e.g., separate estimations for additive and non-additive genetic effects). Thus, although there are many problems with these indirect estimations of genetic influences on temperament, most of them can be controlled.

5. Misinterpretations Of Heritability Estimates

Often heritability estimates are misinterpreted as general estimates of ‘the’ genetic effect on temperament. In contrast, it was pointed out above that heritability estimates depend on (a) the specific temperamental trait, (b) the population of interest, (c) the historical period in its cultural development, and (d) the age of the participants. Another serious misinterpretation is the assumption that genetic effects on temperament can be altered only by changing the genes themselves through gene technology. This widely shared view is not correct because genes and environmental influences often interact in affecting personality. For example, the effects of pathological alleles such as the one that causes phenylketonuria can be minimized through environmental intervention (in this case: keeping a particular diet in the first years of life; see Plomin et al. 1997). Although the alleles that are relevant for temperament-related traits are as yet largely unknown, it is possible that their effects on temperament might be modifiable through diet, medication, specific parenting, or education. Heritability does not necessarily imply non-modifiability.

6. Conclusion

Indirect estimations of the overall genetic influence on temperament indicate significant genetic effects. The many methodological problems of these estimations can be controlled to a great extent through adequate study designs and statistical models. The estimates inform us whether temperamental differences in temperament among peers of a particular population are partly due to genetic differences. Because all temperament-related traits show significant genetic influence, at least in Western cultures beginning with later infancy, it seems promising to search for the specific genes that are responsible for these genetic influences.

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