Psychological Reports, 1992, 71, 1115-1122. O Psychological Reports 1992
PERSONALITY FACTORS I N MONOZYGOTIC AND DIZYGOTIC TWINS: A COMPARATIVE STUDY' D. D E LEO
S. CAPODIECI
Department of Psychiatry University of Padzia
Department of Psychiatry University of Padua
A. VILLA Syracuse University Summary.-To explore similarities or differences on a number of psychological variables relating to personality for members of 16 pairs of monozygotic and 16 pairs of dizygotic twins and between the two groups, the Rorschach, Luscher, Cattell 16 PF, Bern Sex-role Inventory, and Symptom Distress Check List were given to 64 subjects. Analysis confirms data in the literature concerning significant similarities on some personality traits For monozygotic and dizygotic pairs, but with some differences between the groups tested. The particular similarity between the twins may be more attributable to the type of relationship developed early between the twins and the mother than to a greater genetic than environmental influence.
Characteristics associated with the particularity of the twin relationship have been studied by various authors. Burlingham (1946) and Leonard (1961) claimed that being a member of a pair of twins (monozygotes or dizygotes) has profound consequences on personahty development. Barron and Parisi (1976) maintained that, in the developmental stage of personality, the mirror image of themselves and the sight of their twin merges, which may lead to a delay and confusion in building a picture of the human body. Cacciaguerra (1975) disagreed, claiming that twins adequately build the model of the human body thanks to the constant presence of another person able to reinforce development of identity. ~ e o n a r d(1961) hypothesized that primary intertwin identification underlay the delay in the process of maturation and individuation was said not to occur before 3 or 4 years of age, as opposed to singly born subjects for whom it begins at the age of two-anda-half. Zazzo (1984, 1986), in his intensive research on twins, introduced a third factor (besides the hereditary and environmental ones) which was represented by that particular microworld, couple-life. This third element attenuates and counterbalances the power of heredity so that the component could explain how and why there are differences between components for one pair of twins. According to the literature, it appears that there is greater similarity in personality characteristics for monozygotic twins, not only because they have the same genetic make-up, but above all because of the type 'Address enquiries to Salvatore Capdieci, via Casona 12/B, 30173 Mestre, Venezia, Italia
1116
D. DE LEO, ETAL
of relationship established between mother and identical twins and the difficulties of these subjects in the individuation-differentiation process. There is disagreement on whether such treatment is associated with the greater monozygotic similarity in appearance (which may invoke similar reactions from others, so that it should be regarded as a type of genotypic environmental interaction) or as environmental in origin because the treatment might be relatively independent of the physical appearance of the twins. The aim of this study was to examine the similarities and differences on a number of psychological variables of personality within both monozygotic and dizygotic pairs of twins and between the pairs.
METHOD The sample consisted of 32 pairs (16 pairs of monozygotic and 16 dizygotic twins) or a total of 64 subjects. The 32 pairs were composed of 16 pairs of male (9 monozygotic and 7 dizygotic) twins and 16 pairs of female (7 monozygotic and 7 dizygotic) twins. The average age of the pairs of monozygotes was 21.7 yr. (range 18 to 29 yr.) and that of the dizygotic twins 21.6 yr. (range 18 to 31 yr.), without a significant difference between them. The subjects came from the same geographical area, insofar as 93.75% were residents in the municipality or province of Padua (only two pairs came from other provinces). All pairs were middle class, and 96.9% had senior high school diplomas (only one pair terminated their studies after 8 years of school). Of these subjects, 58% attended this university. The study was conducted by giving the pairs of twins the following tests and questionnaires: the Rorschach test (Passi Tognazzo, 1968), the Luscher test (Luscher, 1976), the Cattell 16 PF, Form C (Cattell, 1986), the Bern Sex-role Inventory (De Leo & Vdla, 1986), and the Symptom Distress Check List (Derogatis & Cleary, 1777). The members of the pairs were examined individually to avoid communication of information from one subject to the next. Then they were divided into monozygotes and dizygotes by collecting the health files relating to their births and case histories provided by the mothers and by examining similarities on the objective parameters. The scores for each individual were processed using the intraclass correlation coefficient r and Kendall's tau coefficient. The r coefficient has been expressly studied for pairs of twins and is a development of Pearson's product-moment coefficient. The significance of the within-class correlation coefficient was verified with Fisher's E Kendall's coefficient was used in processing the scores on the Luscher test to verify concordance or discordance in the choices of colours by members of each pair of monozygotic and dizygotic twins.
RESULTS Table 1 shows the means and standard deviations for the factors of the Cattell 16 PF for the two groups of twins (monozygotes and dizygotes). Four
PERSONALITY FACTORS: TWINS
1117
TABLE 1 MEANSAND STANDARD DEVIATIONS ON CATTELL 16 PF FORTwo GROUPSOF Twms 16 PF Factor
Monozygotes
M
SD
Dizygotes r'
MD
M
SD
4.09 2.36 .468t 3.19 1.57 4.78 1.62 5.06 1.32 ,226 5.13 1.74 5.84 1.46 -.729 -.I47 5.06 2.27 C 6.22 2.24 E 4.84 1.74 ,047 4.97 2.32 F 7.53 2.05 .478t 6.59 2.23 G 4.53 2.41 -.469 5.41 1.66 H 5.41 2.12 ,034 5.19 1.91 7.22 2.11 I 7.38 2.04 .310* 5.16 1.87 L 4.69 1.51 -.218 6.19 1.87 M 6.06 2.26 ,159 6.97 1.94 ,283 N 6.53 2.02 ,067 6.19 1.79 0 5.78 1.58 4.72 ,232" 1.90 1.75 Q1 4.38 4.00 1.92 2.18 -.448 Q2 4.25 3.03 1.60 Q3 3.28 2.02 -.519 Q4 6.31 2.26 ,132 7.41 1.93 Note.-r' = intraclass correlation coefficient. MD = Motivation Distortion. '.01
PERSONALITY FACTORS I N MONOZYGOTIC AND DIZYGOTIC TWINS: A COMPARATIVE STUDY' D. D E LEO
S. CAPODIECI
Department of Psychiatry University of Padzia
Department of Psychiatry University of Padua
A. VILLA Syracuse University Summary.-To explore similarities or differences on a number of psychological variables relating to personality for members of 16 pairs of monozygotic and 16 pairs of dizygotic twins and between the two groups, the Rorschach, Luscher, Cattell 16 PF, Bern Sex-role Inventory, and Symptom Distress Check List were given to 64 subjects. Analysis confirms data in the literature concerning significant similarities on some personality traits For monozygotic and dizygotic pairs, but with some differences between the groups tested. The particular similarity between the twins may be more attributable to the type of relationship developed early between the twins and the mother than to a greater genetic than environmental influence.
Characteristics associated with the particularity of the twin relationship have been studied by various authors. Burlingham (1946) and Leonard (1961) claimed that being a member of a pair of twins (monozygotes or dizygotes) has profound consequences on personahty development. Barron and Parisi (1976) maintained that, in the developmental stage of personality, the mirror image of themselves and the sight of their twin merges, which may lead to a delay and confusion in building a picture of the human body. Cacciaguerra (1975) disagreed, claiming that twins adequately build the model of the human body thanks to the constant presence of another person able to reinforce development of identity. ~ e o n a r d(1961) hypothesized that primary intertwin identification underlay the delay in the process of maturation and individuation was said not to occur before 3 or 4 years of age, as opposed to singly born subjects for whom it begins at the age of two-anda-half. Zazzo (1984, 1986), in his intensive research on twins, introduced a third factor (besides the hereditary and environmental ones) which was represented by that particular microworld, couple-life. This third element attenuates and counterbalances the power of heredity so that the component could explain how and why there are differences between components for one pair of twins. According to the literature, it appears that there is greater similarity in personality characteristics for monozygotic twins, not only because they have the same genetic make-up, but above all because of the type 'Address enquiries to Salvatore Capdieci, via Casona 12/B, 30173 Mestre, Venezia, Italia
1116
D. DE LEO, ETAL
of relationship established between mother and identical twins and the difficulties of these subjects in the individuation-differentiation process. There is disagreement on whether such treatment is associated with the greater monozygotic similarity in appearance (which may invoke similar reactions from others, so that it should be regarded as a type of genotypic environmental interaction) or as environmental in origin because the treatment might be relatively independent of the physical appearance of the twins. The aim of this study was to examine the similarities and differences on a number of psychological variables of personality within both monozygotic and dizygotic pairs of twins and between the pairs.
METHOD The sample consisted of 32 pairs (16 pairs of monozygotic and 16 dizygotic twins) or a total of 64 subjects. The 32 pairs were composed of 16 pairs of male (9 monozygotic and 7 dizygotic) twins and 16 pairs of female (7 monozygotic and 7 dizygotic) twins. The average age of the pairs of monozygotes was 21.7 yr. (range 18 to 29 yr.) and that of the dizygotic twins 21.6 yr. (range 18 to 31 yr.), without a significant difference between them. The subjects came from the same geographical area, insofar as 93.75% were residents in the municipality or province of Padua (only two pairs came from other provinces). All pairs were middle class, and 96.9% had senior high school diplomas (only one pair terminated their studies after 8 years of school). Of these subjects, 58% attended this university. The study was conducted by giving the pairs of twins the following tests and questionnaires: the Rorschach test (Passi Tognazzo, 1968), the Luscher test (Luscher, 1976), the Cattell 16 PF, Form C (Cattell, 1986), the Bern Sex-role Inventory (De Leo & Vdla, 1986), and the Symptom Distress Check List (Derogatis & Cleary, 1777). The members of the pairs were examined individually to avoid communication of information from one subject to the next. Then they were divided into monozygotes and dizygotes by collecting the health files relating to their births and case histories provided by the mothers and by examining similarities on the objective parameters. The scores for each individual were processed using the intraclass correlation coefficient r and Kendall's tau coefficient. The r coefficient has been expressly studied for pairs of twins and is a development of Pearson's product-moment coefficient. The significance of the within-class correlation coefficient was verified with Fisher's E Kendall's coefficient was used in processing the scores on the Luscher test to verify concordance or discordance in the choices of colours by members of each pair of monozygotic and dizygotic twins.
RESULTS Table 1 shows the means and standard deviations for the factors of the Cattell 16 PF for the two groups of twins (monozygotes and dizygotes). Four
PERSONALITY FACTORS: TWINS
1117
TABLE 1 MEANSAND STANDARD DEVIATIONS ON CATTELL 16 PF FORTwo GROUPSOF Twms 16 PF Factor
Monozygotes
M
SD
Dizygotes r'
MD
M
SD
4.09 2.36 .468t 3.19 1.57 4.78 1.62 5.06 1.32 ,226 5.13 1.74 5.84 1.46 -.729 -.I47 5.06 2.27 C 6.22 2.24 E 4.84 1.74 ,047 4.97 2.32 F 7.53 2.05 .478t 6.59 2.23 G 4.53 2.41 -.469 5.41 1.66 H 5.41 2.12 ,034 5.19 1.91 7.22 2.11 I 7.38 2.04 .310* 5.16 1.87 L 4.69 1.51 -.218 6.19 1.87 M 6.06 2.26 ,159 6.97 1.94 ,283 N 6.53 2.02 ,067 6.19 1.79 0 5.78 1.58 4.72 ,232" 1.90 1.75 Q1 4.38 4.00 1.92 2.18 -.448 Q2 4.25 3.03 1.60 Q3 3.28 2.02 -.519 Q4 6.31 2.26 ,132 7.41 1.93 Note.-r' = intraclass correlation coefficient. MD = Motivation Distortion. '.01
