Genetic Factors in Temperamental Individuality

Transcription

1 Genetic Factors in Temperamental Individuality A Longitudinal Study of Same-Sexed Twins from Two Months to Six Years of Age Anne Mari Torgersen, Cando Psychol. Abstract. A previous publication reported a study of the genetic aspects of temperamental differences in a sample of 53 same-sexed infant twin pairs. In all of the 9 temperamental categories the MZ twin pairs were more alike than the DZ same-sexed twins at both 2 and 9 months of age. New temperamental data have been collected and analyzed on this same sample at 6 years of age, and the findings have been compared with those at 2 and 9 months of age. Ratings of distractibility could not be reliably obtained at 6 years, and this category is not included. For the categories of activity, approach-withdrawal, intensity and attention span/persistence, the differences in level of intrapair similarity (the measure of genetic influence) are higher at 6 years than in infancy. Adaptability and mood are similarat both age periods, and regularity and threshold are lower at 6 years. The implications of these findings are discussed. Journal ofthe American Academy ofchild Psychiatry, 20: , In a previous publication (Torgersen and Kringlen, 1978), a study of the genetic aspects of temperamental differences in a sample of 53 same-sexed infant twin pairs was reported. The parentinterview protocol and item-scoring method for the infancy period developed by Thomas, Chess, and their co-workers to rate the 9 categories of temperament were utilized (Thomas et al., 1963). Temperamental data were obtained by maternal interviews at 2 and 9 months of age. In all 9 temperamental variables measured, the monozygous (MZ) twins were more alike than the dizygous (DZ) same-sexed twins, both at 2 and 9 months. The greater similarity in the MZ as compared with the DZ pairs was statistically significant for 3 temperamental categories (rhythmicity, threshold, and intensity) at 2 A. M. Torgersen is a Psychologist at the Department of Child Psychiatry, Ullevaal Hospital, (Ullevaal sykehus, avd. 18, Oslo 1, Norway), where reprints may be requested /81/ $ American Academy of Child Psychiatry. 702

2 Genetic Factors in Temperament 703 months, and for all 9 categories at age 9 months. These findings did not appear to reflect differences in maternal perception of the MZ versus DZ twin pairs or any influence of the degree of closeness in physical appearance on the extent of temperamental similarity. It was concluded that there is a significant but not exclusive genetic role in the development of temperamental characteristics. New temperamental data have been collected and analyzed on this same sample at 6 years of age, and the findings have been compared with those at 2 and 9 months of age. This report is concerned with the presentation of these findings and their implications. SAMPLE AND METHOD Follow-up data at 6 years of age were obtained on 32 of the MZ twin pairs and 16 ofthe DZ pairs. The original infancy zygosity ratings were rechecked at 6 years, using eye color and questionnaires, which have been found to have a high degree of accuracy, approaching blood and serum typing (Cohen et ai., 1973). The original ratings were confirmed, except that one twin pair originally rated as MZ was changed to DZ, and one pair originally rated as uncertain was changed to MZ. These minimal shifts do not affect the findings of the infancy study. One DZ twin pair and two MZ pairs were lost to this follow-up study. The two twin pairs still rated as of uncertain zygosity were not included. Data on temperament were obtained in a semistructured interview with the mother. As in the New York Longitudinal Study (NYLS) protocols, detailed, objective descriptions of the child's behavior in different routine situations of daily life were obtained. The interviews were tape-recorded and transcribed. One hundred forty-seven items relating to the 9 categories of temperament and scorable on a five point scale were identified. The items were grouped under each of the 9 categories according to the criteria of the NYLS (Thomas et al., 1963; Thomas, Chess, and Birch, 1968). Twenty of the maternal interviews were then rescored by the author to determine interscorer reliabilities. Items with an interscorer reliability of less than 80% agreement within one scalepoint were excluded. Items that were scorable in less than 70% of the interviews were also eliminated. The items within each temperamental category were then factor analyzed by means of Principal Component Analysis. Items on the first factors which had factorial

3 704 Anne Mari Torgersen loadings lower than 0.30 were excluded, and the remaining 51 items were utilized for the further analyses. The internal consistency (Cronbach Alpha) of the temperamental categories were determined. Table 1 shows the number of items and the internal consistency for each category. As can be seen in table 1, there were at least four items for each category except distractibility, which had three items. The internal consistency of the distractibility category was also very low (0.20), and this category was therefore excluded from further analyses in this study. The remaining 8 categories showed internal consistency ranging from 0.64 to 0.73, except regularity, which had a consistency of Regularity was retained for the further analyses because of the high interscorer reliability of each item and the close correspondence to the definition of regularity in the NYLS. The correlations among the 8 categories were also determined. They were found to be quite independent of each other, except for a positive relationship between high intensity and high activity (0.57), and a slight relationship between high intensity and both low adaptability (-0.34) and low threshold (-0.34). Following these preliminary analyses, the genetic issue was explored. The degree to which the MZ twins were alike and the degree to which the DZ twins were alike were determined for each temperamental category and compared. These findings were also compared with those at 2 and 9 months of age. Statistical Procedures Several statistical methods have been used to evaluate the significance of heritability of the temperamental categories in this Table I Numbers of Items and Inner Consistency (a) within Each Temperamental Category Temperamental Category Activity Rcgularitv Approach-Withdrawal Adaptability Intensity Threshold Mood Distractibility Attention Span/Persistence Number of Items a

4 Genetic Factors in Temperament 705 study. The intraclass correlations, measures of the degree of closeness of the twin pairs within the MZ and DZ groups, were calculated. A common method used to determine whether the differences between the MZ and DZ twins are statistically significant is to calculate the F-value proposed by Vandenberg (1966). This is the relationship between the intrapair variances (var.) within the two twin groups. The formula is: F= varidz with the degree of freedom: NDZ and NMZ' The intrapair variances were calculated according to this formula: var! = where N is the number of pairs and X is a score observed for twin A or B in a pair. Recently, however, Christian and co-workers (1974) have proposed a more conservative measure of genetic influence, which they have called the genetic component estimate (F'). This estimate should be used when the probability of no difference between the total variances (among-pair variance plus intrapair variance, or var; + var.) of the two zygosity groups is less than If the total variances within a temperamental category are more similar (p ~ 0.20) between the two groups, this F' will give estimates that are too low to be significant, and will not be calculated for this category. The formula for F' is as follows: F' = varamz + varidz varadz + VariMZ Degrees of freedom are calculated after Cochran (1951). RESULTS The mean intrapair differences and the intracless correlations for each temperamental category are presented in table 2.

5 706 Anne Mari Torgersen Table 2 Mean Intrapair Differences and the Intraclass Correlations of the Twin Variances in Temperament at 6 Years Temperamental Category Activity Regularity Approach-Withdrawal Adaptability Intensity Threshold Mood Attention Span/Persistence Mean Intrapair Differences MZ DZ Intraclass Correlations MZ DZ As can be seen in table 2, the MZ twin pairs are more alike within all the 8 categories than are the DZ pairs. The MZ twins are very much alike in 7 categories (0.73 to 0.95 levels), and much less alike in mood (0.37 level). The DZ twins, on the other hand, are moderately alike only for adaptability (0.68 level), somewhat alike for regularity, approach-withdrawal, intensity, and threshold (0.23 to 0.54 levels), and not alike as a group for activity, mood, and attention span/persistence. Thus, only in adaptability do both sets of twins show a high intraclass correlation. The F-values (Vandenberg, 1966) were calculated to determine the degree to which the differences between the MZ and DZ twins are statistically significant. These calculations are presented in table 3. As can be seen in table 3, the F-values are statistically significant for all temperamental categories, varying from highly significant levels for activity and intensity to the 0.05 level for adaptability. Table 3 also shows that there are significant (p < 0.20) differences in total variance (F T ) between the two twin pair groups in the categories of regularity, threshold, and mood. Consequently, the alternative F' proposed by Christian et al. (1974) was calculated for each of these 3 categories, thus reducing the p-values of the three categories to a nonsignificant level for regularity and mood but not for threshold. Overall, table 3 indicates that the differences in the degree of intrapair similarity between the MZ and DZ twin pairs at 6 years (in all categories the MZ twins are more alike) are significant at the level for activity, approach-withdrawal, intensity, and attention span/persistence; at the 0.01 level for threshold; at the 0.05 level for adaptability; and are not significant for regularity and mood.

6 Genetic Factors in Temperament 707 Table 3 The F-Ratios of the Twin Variances in Temperament at 6 Years Temperamental Category Activity Regularity Approach-Withdrawal Adaptability Intensity Threshold Mood Attention Span /Persistence * p < two-tailed test Intrapair Variances F : I'-Ratios of: Total Variances r, * !J* 1.!J7" 1.07 Component Estimates I" :1 lao p<.001 ~s.001.0:; ~S.001 A final analysis involved the comparison of the F-values at 2 months, 9 months, and 6 years. This comparison is presented graphically in figure 1. The F-values in this figure are those proposed by Vandenberg (1966). Consequently, the F-values are too high for regularity and mood at 6 years as calculated with Christian's F'-value. When the F' -values were calculated for the 2- and 9-month data, modifications were required with mood and threshold at 2 months and with persistence at 9 months. The F' -values are indicated on the figure and do not change the tendencies of the figure. (F-values and F' -values are not directly comparable on a graph because the two values rely on different statistical formulae.) As can be seen from figure 1, for the categories of activity, approach-withdrawal, intensity, and attention span/persistence, the differences in level of intrapair similarity are higher at 6 years than in infancy (data for attention span/persistence category in infancy were only available at 9 months). The F-values in the other categories are either very similar at infancy and 6 years (adaptability and mood) or are lower at 6 years (regularity and threshold). DISCUSSION This study of same-sexed twins has explored the genetic factor in temperamental individuality. The comparison of intrapair differences in temperamental traits in the monozygotic and dizygotic twin groups has been used as the presumptive index of such a genetic influence.

7 708 Anne Mari Torgersen F - Values of the Twin Variances in Temperament at 2 Months, 9 Months, and 6 Years months ~ 9 months D 6 yeors Activity Regulority Approach- Adaptability Intensity Threshold Withdrawal -=F' Temperamental Figure 1 Category Mood Allention Span / Persistence One might speculate that identical twins may be treated more similarly by parents than are nonidentical twins and that this treatment could lead to intrapair differences between the two twin groups on a nongenetic basis. Cohen et al. (1977) have tested this hypothesis on a number of behavioral measures. They found that monozygotic twins whose parents thought they were dizygotic or were not certain of zygosity were as alike as known monozygotic twins. This negative finding for a nongenetic, environmental factor affirms the value of the comparison of intrapair differences in temperamental traits in the monozygotic and dizygotic twin groups as a presumptive index of genetic influence. Using this criterion, the findings indicate a significant genetic influence for all 9 temperamental categories at 9 months,

8 Genetic Factors in Temperament 709 (Torgersen and Kringlen, 1978). The data of the present study, at 6 years, permit the following classification for the 8 temperamental categories studied: 1. F-values higher at 6 years than at 9 months. This is true for activity, approach-withdrawal, intensity, and attention span/persistence. The statistical significance of the F-values for these categories at 6 years is also very high, with p-values of less than F-values low at both 9 months and 6 years. This is true for adaptability and mood. Compared with the other categories, the F-values at 6 years are also much lower, barely at the 0.05 level for adaptability, and even lower for mood, with the p-value being nonsignificant. 3. F-values lower at 6 years that at 9 months. This is true for regularity and threshold. For threshold, the F-value remains statistically significant at 6 years (p < 0.01) but for regularity it becomes nonsignificant. It is of interest that these 2 categories showed the highest F-values of all 9 categories at 9 months. It is possible that methodological problems, such as the chance that the same category was not rated functionally in infancy and at 6 years (Thomas and Chess, 1977, p. 158), may have affected the findings of this study. It is also possible that as a child grows older the same behaviors may be more likely to be multifactorally determined than at an earlier age. These problems are complex, and adequate methods for their solution remain to be developed. The following conclusions are drawn on the assumption that these issues are at least partially resolved by maintaining the definitional identity of each category over time, and utilizing different ageappropriate behavioral criteria at succeeding age periods (Thomas and Chess, 1977, P: 159). This procedure has been followed in the present study, and it will be assumed that the F-value findings as a measure of genetic influence can be compared between infancy and 6 years. If this assumption is correct, several inferences can be drawn from the findings. In the infancy period, temperamental individuality reflects a strong genetic component for most of the 9 categories. Other organismic and/or environmental factors have, of course, also been influential, but could not be analyzed from the data of this study (Torgersen and Kringlen, 1978). At 6 years of age, genetic influence was highly significant for the categories of activity, approach-withdrawal, intensity, and attention span / persistence. This suggests that either these traits were relatively

9 710 Anne Mari Torgersen uninfluenced by environmental factors in the preschool years, or that the child-environment interactional process reinforced the early individuality, or both. For adaptability and mood, which had among the lowest F values in infancy, the genetic influence at 6 years was also modest and even nonsignificant. For regularity and threshold, the genetic influence at 6 years was much lower than in infancy. That regularity was not even statistically significant suggests an appreciable environmental influence on these traits through the preschool years. The sequential course for distractibility could not be reliably evaluated. These findings can be interpreted within an interactionist framework, in which behavioral development and the expression of any psychological attribute over time reflect a constantly evolving process of organism-environment interaction (Thomas and Chess, 1977). This approach is also emphasized by Sameroff (1975). "The child is in a perpetual state of active reorganization and cannot properly be regarded as maintaining inborn characteristics as static qualities. In this view, the constants in development are not some sets of traits but rather the processes by which these traits are maintained between organism and environment" (p. 281). For those temperamental traits that show a significant genetic factor at 6 years, it is likely that the process of organism-environment interaction serve to maintain or even heighten this genetic influence. Where the genetic factor remains modest or is even decreased by 6 years, the opposite effect of the interactional process is likely. The findings, therefore, suggest that a study of the differential interactional patterns for the different temperament categories could be fruitful. In this regard, it is of interest that the three temperamental traits with the least evidence for a genetic factor at 6 years -regularity, adaptability, and mood-have been identified as risk factors for the development of behavior disorder. Irregularity, low adaptability, and negative mood, together with withdrawal and high intensity, have been identified in the NYLS (Thomas, Chess, and Birch, 1968) as constituting the constellation of the difficult child, at high risk for behavior disturbance. Graham, Rutter, and George (1973) have also found a correlation between the categories of irregularity, low malleability (similar to low adaptability), negative mood, and fastidiousness (not measured in this study or the

10 Genetic Factors in Temperament 711 NYLS) and behavior disorder development. This suggests a strong, though not exclusive, environmental influence in the development of behavior disorders in early and middle childhood. REFERENCES CHRISTIAN, J. C., KANG, K. W., & NORTON, j. A., JR. (1974), Choice of an estimate of genetic variance from twin data. Am. J. Hum. Genet., 26: COCHRAN, W. G. (1951), Testing a linear relation among variances. Biometrics J., (March). COHEN, D. j., DIBBLE, E., & GRAWE, j. M. (1973), Separating identical from fraternal twins. Arch. Gen. Psychiat. 29: (1977), Fathers' and mothers' perceptions of children's personality. Arch. Gen. Psychiat. 34: GRAHAM, P., RUTTER, M., & GEORGE, S. (1973), Temperamental characteristics as predictors of behavior disorders in children. Amer. J. Orthopsychiat., 43: SAMEROFF, A. j. (1975), Early influences on development: "fact or fancy?" Merrill Palmer Quarterly, 21 : THOMAS, A., & CHESS, S. (1977), Temperament and Development, New York: Brunner/Mazel, ---- & BIRCH, H. G. (1968), Temperament and Behavior Disorders, New York: New York University Press HERTZIG, M. E., & KORN, S. (1963), Behavioral Individuality in Early Childhood, New York: New York University Press. TORGERSEN, A. M. (1980), Temperamental categories in 6-year-old children: a factorial analysis. To be published. Available by request. --& KRINGLEN, E. (1978), Genetic aspects of temperamental differences in infants, This Journal, 17: VANDENBERG, S. G. (1966), Contributions of twin research to psychology. In: Behavioral Genetics, ed. M. Manosevitz, C. Lindzeg, & D. D. Thiessen. New York: Appelton-Century Crofts.