Copyright 1990 by the American Psychological Association, Inc. 0882-7974/90/W0.75
5, No. 1,25-30
Genetic Influence on Life Events During the Last Half of the Life Span Paul Liechtenstein
This document is copyrighted by the American Psychological Association or one of its allied publishers. This article is intended solely for the personal use of the individual user and is not to be disseminated broadly.
Robert Plomin
Center for Developmental and Health Genetics College of Health and Human Development Pennsylvania State University
Department of Environmental Hygiene Karolinska Institute Stockholm, Sweden
Nancy L. Pedersen
Gerald E. McClearn and John R. Nesselroade Center for Developmental and Health Genetics College of Health and Human Development Pennsylvania State University
Center for Developmental and Health Genetics College of Health and Human Development Pennsylvania State University and Department of Environmental Hygiene Karolinska Institute Stockholm, Sweden
Genetic influence on perceptions of major events later in life was assessed with a combination of twin and adoption designs as part of the Swedish Adoption/Twin Study of Aging (SATSA). The SATSA design includes 4 groups totaling 399 pairs of same-sex twins: identical and fraternal twins reared apart and matched twins reared together. The average age of the twins was 59 years. The results demonstrate significant genetic influence on reports of the occurrence of life events, especially for controllable events in which the individual can play an active role. Maximum likelihood modelfitting estimates of genetic influence indicate that 40% of the variance of the total life events score is due to genetic differences among individuals. How genetic factors can affect life experiences and - directions for future research are discussed.
A new direction in behavioral genetics research is to consider the possibility of genetic influence on measures that are apparently measures of the environment (Plomin, 1986). Genetic influence on measures of environment is not as paradoxical as it seems at first thought because genetically influenced characteristics, such as cognitive abilities and personality, might affect how individuals construct their environment and how they feel about and behave toward others. In addition, others might respond to individuals on the basis of such characteristics. Genetic influence seems especially likely for the most widely used measures that rely on self-reported perceptions of the environment because genetic effects can accrue as these perceptions filter through an individual's memories, feelings, and personality. The original research in this vein demonstrated genetic influence on adolescents' perceptions of their family environment (Rowe, 1981, 1983). Recent research has replicated and extended these findings by showing genetic influence on perceptions of childhood family environment when viewed some SO years later, in the last half of the life span (Plomin, McClearn,
Pedersen, Nesselroade, & Bergeman, 1988), and on adults' ratings of their current family environment (Plomin, McClearn, Pedersen, Nesselroade, & Bergeman, in press). Genetic influence on perceptions of the family environment may emerge because of the genetic concatenations among family members. This article extends this research beyond the family to another widely used type of environmental measure—life events. Older individuals experience many major life changes (e.g., Danish, Smyer, & Nowak, 1980), and such events are thought to be important correlates of mental health later in life (e.g., Renner & Birren, 1980). On the basis of previous research on perceptions of the family environment, we hypothesized that perceptions of life events during the last half of the life span would show significant genetic influence.
Method Sample The sample has been described previously in this journal (Plomin, Pedersen, et al., 1988). The twins are participating in the Swedish Adoption/Twin Study of Aging (SATSA; McClearn, Pedersen, Plomin, Nesselroade, & Friberg, 1988). In the fall of 1984, a life events questionnaire was sent to the SATSA twins. Responses from both members of a pair were complete for 49 pairs of identical twins reared apart (MZA), 98 pairs of identical twins reared together (MZT), 125 pairs of fraternal twins reared apart (DZA), and 127 pairs of fraternal twins reared together (DZT). Zygosity of the twins was determined with physical similarity criteria as described by McClearn et al. (1988). The average age of separation for the twins reared apart was 2.8 years; 48% of the pairs were separated during the first year of life, and 82% were separated by
The Swedish Adoption/Twin Study of Aging (SATSA) is an ongoing study conducted at the Department of Environmental Hygiene of the Karolinska Institute in Stockholm, in collaboration with the Center for Developmental and Health Genetics at Pennsylvania State University. SATSA is supported in part by grants from the National Institute on Aging (AG-04563) and the John D. and Catherine T. MacArthur Foundation Research Program on Successful Aging. Correspondence concerning this article should be addressed to Robert Plomin, Center for Developmental and Health Genetics, 211 Henderson Building South, Pennsylvania State University, University Park, Pennsylvania 16802.
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PLOMIN, LICHTENSTEIN, PEDERSEN, McCLEARN, NESSELROADE
Table 1 Life Events, Percentage of Occurrence, and Rated Importance
This document is copyrighted by the American Psychological Association or one of its allied publishers. This article is intended solely for the personal use of the individual user and is not to be disseminated broadly.
Item 1. Retirement 2, Major deterioration in financial status 3. Serious illness in child 4. Death of child 5. Serious conflicts with child 6. Somatic illness, self 7. Can't manage to look after oneself 8, Divorce 9. Home care of spouse 10. Getting married 1 1 . Deterioration in married life 12. Somatic illness, spouse 13. Death of spouse 14. Nursing home care, spouse IS. Mental illness, spouse 16. Improvement in married life 17, Home care, self 18. Forced change in residence with reduced contacts 19. Mental illness, self 20. Death of siblings or friends 21. Changes in relations with grandchildren 22. Loss of sexual ability or interest 23. Paying fine for minor violation of law 24. Major improvement in financial status 25. Making a new acquaintance
Percentage of individuals for whom event occurred
Mean value of rated importance of event
40.7
2.19
18.9
2.13 2.88 2.96 2.83 2.41
8.9 5.1 6,4 29.0
2.6 12.3
5.9 92.7 10.6 23.6 15.6
2.3 3.2 16.8
8,8 5.6 3.3
2.77 2.85 2.78 2.76 2.80 2.56 2.96 2.91 2.81 2.67 2.64
28.4
2,59 2.79 2.75
6.5
2.67
19.0
2.10
13.2
1.93
36.1 38.9
2.20 2.22
Note. Wording for items abbreviated. Importance rated ; importance; 2 = some importance; 3 = great importance.
I = little
the age of 5 years. Their average age at time of testing was 58.6 years (SD = 13.6 years), with a fairly normal distribution around the sixth decade of life: 2% of the sample were more than 80 years old, 18% were 71-80, 31% were 61-70, 20% were 51-60, 18% were 41-50, 9% were 31-40, and 2% were 27-30. In all, 60% of the twins were women, which conforms to gender expectations for the last half of the life span.
Measure Our measure of life events was based on the Social Readjustment Rating Scale (Holmes & Rahe, 1967) used in over a thousand studies (Holmes, 1979) and modified for older individuals as part of the largescale H-70 study in Gothenburg, Sweden (Persson, 1980), Considerable disagreement exists concerning the best way to assess life events, and there is dissatisfaction with traditional questionnaire measures such as the Social Readjustment Rating Scale (e.g., Paykel, 1983). However, the value of the present research does not rest on its use of a particular measure of life events. The goal of the research is to assess genetic influence on a reasonable measure of life events, with the knowledge that other measures might yield different results, a possibility that offers an obvious direction for future research in this area. We refer to our measure of life events in terms of perceptions of life events as a reminder that self-report measures of events do not necessarily correspond to reality.
The events, the percentage of the sample reporting that the events had occurred, and the mean rated importance of the events are listed in Table 1. Significant age differences occurred for Items 2 and 22 (younger higher) and for Item 5 (older higher). Significant gender differences were observed for Items 6, 11, 20, 24, and 25 (women higher) and Items 9 and 22 (men higher). A traditional total life events score was constructed by summing each reported event weighted by the average importance assigned to the event by all individuals who completed the questionnaire. We chose this procedure rather than using the usual consensus weights because the SATSA items are written in Swedish and worded somewhat differently. However, this procedure made little difference because various weighting schemes yielded life events scores that correlate in excess of .96. The total life events score showed a significant age effect (older higher) but no gender effect. In addition to the total score, scales were constructed to address recent concerns that distinctions between undesirable and desirable events and between uncontrollable and controllable events may be important (Thoits, 1983). Four scales were constructed: undesirable events (all items except 1, 10,16, 21, 24, and 25), desirable events (Items 10, 16,24, and 25), uncontrollable events (Items 3,4,9,12,13,14,15,18, and 20), and controllable events (Items 2, 5, 8, 10, 11, 16, 21, 23, 24, and 25). Items involving self-care (Items 6,7,17, and 19) were used to form an additional scale because of their ambiguous status in relation to control. Undesirable and desirable events correlated .37; their correlations with the total score were .94 and .62, respectively. Controllable and uncontrollable scores correlated .31; correlations with the total score were .73 and .81, respectively. Self-care correlated .66 with the total score. Because age and gender differences can inflate twin correlations, scores were corrected for linear effects of age and gender by using standardized residual scores from the regression of the total score and five scale scores on age and gender.
Model Fitting Maximum likelihood model-fitting analyses were used to analyze the data from the four twin groups simultaneously, to provide estimates and standard errors of parameters of genetic and environmental influences, and to test the goodness of fit of alternative models (Loehlin, 1987; Plomin, DeFries, & McCleam, 1990). The essence of the SATSA model is that evidence for genetic influence can be adduced if twins reared apart are significantly similar and if identical twin correlations significantly exceed fraternal twin correlations. Genetic variance is estimated as nonadditive genetic variance to the extent that the fraternal twin correlation is less than half the magnitude of the identical twin correlation. Additive genetic values refer to the sum of the average effects of all genes that influence a character and thus "breed true." In terms of additive genetic variance, identical twins are identical, whereas fraternal twins, like other first-degree relatives, resemble each other 50% on average for segregating genes. With regard to nonadditive genetic variance, however, fraternal twins share only a quarter of genetic variance due to dominance (nonadditive genetic effects between alleles at a locus) and relatively little genetic variance due to epistasis (nonadditive genetic effects across loci). In contrast, identical twins share all nonadditive genetic effects. Thus, if nonadditive genetic variance is important for a trait, the correlation for fraternal twins will be less than one half the correlation for identical twins. (For details, see Plomin, DeFries, & Mcdearn, 1990.) The influence of shared rearing factors can be assessed by comparing twins reared together with twins reared apart. Twin resemblance that cannot be explained by heredity or by shared rearing environment is attributed to correlated environmental factors that include selective placement and shared postrearing environments. 'Variance not explained by these genetic and environmental parameters is ascribed to a
27
This document is copyrighted by the American Psychological Association or one of its allied publishers. This article is intended solely for the personal use of the individual user and is not to be disseminated broadly.
GENETIC INFLUENCE ON LIFE EVENTS residual term, nonshared environment, that includes error of measurement. Figure 1 presents a path model representation of the study's design, a model that is slightly more elaborate than the SATSA model presented previously in this journal (Plomin, Pedersen, et al., 1988). The path model includes one observed variable (P) for members of a twin pair (Pi and PI) and five latent variables. The latent variables are additive genotypic values (
5, No. 1,25-30
Genetic Influence on Life Events During the Last Half of the Life Span Paul Liechtenstein
This document is copyrighted by the American Psychological Association or one of its allied publishers. This article is intended solely for the personal use of the individual user and is not to be disseminated broadly.
Robert Plomin
Center for Developmental and Health Genetics College of Health and Human Development Pennsylvania State University
Department of Environmental Hygiene Karolinska Institute Stockholm, Sweden
Nancy L. Pedersen
Gerald E. McClearn and John R. Nesselroade Center for Developmental and Health Genetics College of Health and Human Development Pennsylvania State University
Center for Developmental and Health Genetics College of Health and Human Development Pennsylvania State University and Department of Environmental Hygiene Karolinska Institute Stockholm, Sweden
Genetic influence on perceptions of major events later in life was assessed with a combination of twin and adoption designs as part of the Swedish Adoption/Twin Study of Aging (SATSA). The SATSA design includes 4 groups totaling 399 pairs of same-sex twins: identical and fraternal twins reared apart and matched twins reared together. The average age of the twins was 59 years. The results demonstrate significant genetic influence on reports of the occurrence of life events, especially for controllable events in which the individual can play an active role. Maximum likelihood modelfitting estimates of genetic influence indicate that 40% of the variance of the total life events score is due to genetic differences among individuals. How genetic factors can affect life experiences and - directions for future research are discussed.
A new direction in behavioral genetics research is to consider the possibility of genetic influence on measures that are apparently measures of the environment (Plomin, 1986). Genetic influence on measures of environment is not as paradoxical as it seems at first thought because genetically influenced characteristics, such as cognitive abilities and personality, might affect how individuals construct their environment and how they feel about and behave toward others. In addition, others might respond to individuals on the basis of such characteristics. Genetic influence seems especially likely for the most widely used measures that rely on self-reported perceptions of the environment because genetic effects can accrue as these perceptions filter through an individual's memories, feelings, and personality. The original research in this vein demonstrated genetic influence on adolescents' perceptions of their family environment (Rowe, 1981, 1983). Recent research has replicated and extended these findings by showing genetic influence on perceptions of childhood family environment when viewed some SO years later, in the last half of the life span (Plomin, McClearn,
Pedersen, Nesselroade, & Bergeman, 1988), and on adults' ratings of their current family environment (Plomin, McClearn, Pedersen, Nesselroade, & Bergeman, in press). Genetic influence on perceptions of the family environment may emerge because of the genetic concatenations among family members. This article extends this research beyond the family to another widely used type of environmental measure—life events. Older individuals experience many major life changes (e.g., Danish, Smyer, & Nowak, 1980), and such events are thought to be important correlates of mental health later in life (e.g., Renner & Birren, 1980). On the basis of previous research on perceptions of the family environment, we hypothesized that perceptions of life events during the last half of the life span would show significant genetic influence.
Method Sample The sample has been described previously in this journal (Plomin, Pedersen, et al., 1988). The twins are participating in the Swedish Adoption/Twin Study of Aging (SATSA; McClearn, Pedersen, Plomin, Nesselroade, & Friberg, 1988). In the fall of 1984, a life events questionnaire was sent to the SATSA twins. Responses from both members of a pair were complete for 49 pairs of identical twins reared apart (MZA), 98 pairs of identical twins reared together (MZT), 125 pairs of fraternal twins reared apart (DZA), and 127 pairs of fraternal twins reared together (DZT). Zygosity of the twins was determined with physical similarity criteria as described by McClearn et al. (1988). The average age of separation for the twins reared apart was 2.8 years; 48% of the pairs were separated during the first year of life, and 82% were separated by
The Swedish Adoption/Twin Study of Aging (SATSA) is an ongoing study conducted at the Department of Environmental Hygiene of the Karolinska Institute in Stockholm, in collaboration with the Center for Developmental and Health Genetics at Pennsylvania State University. SATSA is supported in part by grants from the National Institute on Aging (AG-04563) and the John D. and Catherine T. MacArthur Foundation Research Program on Successful Aging. Correspondence concerning this article should be addressed to Robert Plomin, Center for Developmental and Health Genetics, 211 Henderson Building South, Pennsylvania State University, University Park, Pennsylvania 16802.
25
26
PLOMIN, LICHTENSTEIN, PEDERSEN, McCLEARN, NESSELROADE
Table 1 Life Events, Percentage of Occurrence, and Rated Importance
This document is copyrighted by the American Psychological Association or one of its allied publishers. This article is intended solely for the personal use of the individual user and is not to be disseminated broadly.
Item 1. Retirement 2, Major deterioration in financial status 3. Serious illness in child 4. Death of child 5. Serious conflicts with child 6. Somatic illness, self 7. Can't manage to look after oneself 8, Divorce 9. Home care of spouse 10. Getting married 1 1 . Deterioration in married life 12. Somatic illness, spouse 13. Death of spouse 14. Nursing home care, spouse IS. Mental illness, spouse 16. Improvement in married life 17, Home care, self 18. Forced change in residence with reduced contacts 19. Mental illness, self 20. Death of siblings or friends 21. Changes in relations with grandchildren 22. Loss of sexual ability or interest 23. Paying fine for minor violation of law 24. Major improvement in financial status 25. Making a new acquaintance
Percentage of individuals for whom event occurred
Mean value of rated importance of event
40.7
2.19
18.9
2.13 2.88 2.96 2.83 2.41
8.9 5.1 6,4 29.0
2.6 12.3
5.9 92.7 10.6 23.6 15.6
2.3 3.2 16.8
8,8 5.6 3.3
2.77 2.85 2.78 2.76 2.80 2.56 2.96 2.91 2.81 2.67 2.64
28.4
2,59 2.79 2.75
6.5
2.67
19.0
2.10
13.2
1.93
36.1 38.9
2.20 2.22
Note. Wording for items abbreviated. Importance rated ; importance; 2 = some importance; 3 = great importance.
I = little
the age of 5 years. Their average age at time of testing was 58.6 years (SD = 13.6 years), with a fairly normal distribution around the sixth decade of life: 2% of the sample were more than 80 years old, 18% were 71-80, 31% were 61-70, 20% were 51-60, 18% were 41-50, 9% were 31-40, and 2% were 27-30. In all, 60% of the twins were women, which conforms to gender expectations for the last half of the life span.
Measure Our measure of life events was based on the Social Readjustment Rating Scale (Holmes & Rahe, 1967) used in over a thousand studies (Holmes, 1979) and modified for older individuals as part of the largescale H-70 study in Gothenburg, Sweden (Persson, 1980), Considerable disagreement exists concerning the best way to assess life events, and there is dissatisfaction with traditional questionnaire measures such as the Social Readjustment Rating Scale (e.g., Paykel, 1983). However, the value of the present research does not rest on its use of a particular measure of life events. The goal of the research is to assess genetic influence on a reasonable measure of life events, with the knowledge that other measures might yield different results, a possibility that offers an obvious direction for future research in this area. We refer to our measure of life events in terms of perceptions of life events as a reminder that self-report measures of events do not necessarily correspond to reality.
The events, the percentage of the sample reporting that the events had occurred, and the mean rated importance of the events are listed in Table 1. Significant age differences occurred for Items 2 and 22 (younger higher) and for Item 5 (older higher). Significant gender differences were observed for Items 6, 11, 20, 24, and 25 (women higher) and Items 9 and 22 (men higher). A traditional total life events score was constructed by summing each reported event weighted by the average importance assigned to the event by all individuals who completed the questionnaire. We chose this procedure rather than using the usual consensus weights because the SATSA items are written in Swedish and worded somewhat differently. However, this procedure made little difference because various weighting schemes yielded life events scores that correlate in excess of .96. The total life events score showed a significant age effect (older higher) but no gender effect. In addition to the total score, scales were constructed to address recent concerns that distinctions between undesirable and desirable events and between uncontrollable and controllable events may be important (Thoits, 1983). Four scales were constructed: undesirable events (all items except 1, 10,16, 21, 24, and 25), desirable events (Items 10, 16,24, and 25), uncontrollable events (Items 3,4,9,12,13,14,15,18, and 20), and controllable events (Items 2, 5, 8, 10, 11, 16, 21, 23, 24, and 25). Items involving self-care (Items 6,7,17, and 19) were used to form an additional scale because of their ambiguous status in relation to control. Undesirable and desirable events correlated .37; their correlations with the total score were .94 and .62, respectively. Controllable and uncontrollable scores correlated .31; correlations with the total score were .73 and .81, respectively. Self-care correlated .66 with the total score. Because age and gender differences can inflate twin correlations, scores were corrected for linear effects of age and gender by using standardized residual scores from the regression of the total score and five scale scores on age and gender.
Model Fitting Maximum likelihood model-fitting analyses were used to analyze the data from the four twin groups simultaneously, to provide estimates and standard errors of parameters of genetic and environmental influences, and to test the goodness of fit of alternative models (Loehlin, 1987; Plomin, DeFries, & McCleam, 1990). The essence of the SATSA model is that evidence for genetic influence can be adduced if twins reared apart are significantly similar and if identical twin correlations significantly exceed fraternal twin correlations. Genetic variance is estimated as nonadditive genetic variance to the extent that the fraternal twin correlation is less than half the magnitude of the identical twin correlation. Additive genetic values refer to the sum of the average effects of all genes that influence a character and thus "breed true." In terms of additive genetic variance, identical twins are identical, whereas fraternal twins, like other first-degree relatives, resemble each other 50% on average for segregating genes. With regard to nonadditive genetic variance, however, fraternal twins share only a quarter of genetic variance due to dominance (nonadditive genetic effects between alleles at a locus) and relatively little genetic variance due to epistasis (nonadditive genetic effects across loci). In contrast, identical twins share all nonadditive genetic effects. Thus, if nonadditive genetic variance is important for a trait, the correlation for fraternal twins will be less than one half the correlation for identical twins. (For details, see Plomin, DeFries, & Mcdearn, 1990.) The influence of shared rearing factors can be assessed by comparing twins reared together with twins reared apart. Twin resemblance that cannot be explained by heredity or by shared rearing environment is attributed to correlated environmental factors that include selective placement and shared postrearing environments. 'Variance not explained by these genetic and environmental parameters is ascribed to a
27
This document is copyrighted by the American Psychological Association or one of its allied publishers. This article is intended solely for the personal use of the individual user and is not to be disseminated broadly.
GENETIC INFLUENCE ON LIFE EVENTS residual term, nonshared environment, that includes error of measurement. Figure 1 presents a path model representation of the study's design, a model that is slightly more elaborate than the SATSA model presented previously in this journal (Plomin, Pedersen, et al., 1988). The path model includes one observed variable (P) for members of a twin pair (Pi and PI) and five latent variables. The latent variables are additive genotypic values (
