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  • 1
    Electronic Resource
    Electronic Resource
    A test of the equal-environment assumption in twin studies of psychiatric illness (1993)
    Springer
    Behavior genetics 23 (1993), S. 21-27 
    Details
    ISSN: 1573-3297
    Keywords: Twin studies ; equal-environment assumption ; psychiatric disorders ; depression ; alcoholism ; anxiety disorders
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Psychology
    Notes: Abstract The traditional twin method is predicated on the equal-environment assumption (EEA)—that monozygotic (MZ) and dizygotic (DZ) twins are equally correlated in their exposure to environmental events of etiologic importance for the trait under study. In 1968, Scarr proposed a test of the EEA which examines the impact of phenotypic similarity in twins of perceived versus true zygosity. We apply this test for the EEA to five common psychiatric disorders (major depression, generalized anxiety disorder, phobia, bulimia, and alcoholism), as assessed by personal interview, in 1030 female-female twin pairs from the Virginia Twin Registry with known zygosity. We use a newly developed model-fitting approach which treats perceived zygosity as a form of specified familial environment. In 158 of the 1030 pairs (15.3%), one or both twins disagreed with the project-assigned zygosity. Model fitting provided no evidence for a significant influence of perceived zygosity on twin resemblance for any of the five disorders. Although limited in power, these results support the validity of the EEA in twin studies of psychiatric disorders.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01067551
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  • 2
    Electronic Resource
    Electronic Resource
    Estimating and controlling for the effects of volunteer bias with pairs of relatives (1993)
    Springer
    Behavior genetics 23 (1993), S. 271-277 
    Details
    ISSN: 1573-3297
    Keywords: Volunteer ; bias ; likelihood ; research design ; twin methodology ; family study
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Psychology
    Notes: Abstract If pairs of relatives correlate in their liability to participate in a research project, it is possible to test for the effects of volunteering on the criterion variable of interest. Much of the information for this test comes from a difference in criterion variable mean between individuals with and those without a cooperative relative. Also, if data are available from more than one class of relative, it may be possible to discriminate between (i) volunteering that occurs as a consequence of the criterion variable and (ii) volunteering as a cause of the criterion. Likelihood formulae are presented that permit quantification and significance testing of volunteer bias. If data are collected from a genetically informative design such as a twin study, it is possible to estimate genetic and environmental parameters independent of the contaminating effects of such bias. We describe some methods of reducing the computational burden of multidimensional integration to allow extension to multivariate data. Implications for research design and management are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01082466
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  • 3
    Electronic Resource
    Electronic Resource
    Multivariate genetic analysis of twin-family data on fears: Mx models (1994)
    Springer
    Behavior genetics 24 (1994), S. 119-139 
    Details
    ISSN: 1573-3297
    Keywords: Multivariate genetic analysis ; twin-family data ; fears ; Mx models ; cultural transmission ; genetic dominance ; assortative mating
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Psychology
    Notes: Abstract We describe the implementation of multivariate models of familial resemblance with the Mx package. The structural equation models allow for the effects of assortative mating, additive and dominant genes, common and specific environment, and both genetic and cultural transmission between generations. Two approaches are compared: a correlational one based on Fulker and a factor model described by Phillips and Fulker. Both are illustrated by application to published data on social fears and fear of leadership measured in monozygotic and dizygotic twins and their parents. In the example data, genetic dominance yields a more parsimonious explanation of the data than does cultural transmission, although neither is needed to obtain a good fit to the data. A model of reduced genetic correlation between generations also fits the data but has inherent limitations in this sample. Extensions to sex-limitation and more complex models are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01067816
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  • 4
    Electronic Resource
    Electronic Resource
    Human parental behavior: Evidence for genetic influence and potential implication for gene-culture transmission (1994)
    Springer
    Behavior genetics 24 (1994), S. 327-335 
    Details
    ISSN: 1573-3297
    Keywords: Child-rearing ; socialization ; coevolution ; sociobiology ; twin design ; Parental Bonding Instrument
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Psychology
    Notes: Abstract A large sample of adult twins (1117 pairs), who were concordant for having had children were asked to report on their child-rearing practices. A 14-item version of the Parental Bonding Instrument (PBI) was used to assess rearing practices of parent twins. The two factors of Care and Overprotection, commonly found in other studies, were recovered from this analysis of the PBI's parent form. Model-fitting analyses indicate that human parental behavior is under significant genetic influence. Findings further suggest that this influence is sex limited, with a higher heritability in mothers than in fathers and that it may result partly from the expression of dominant genes. For both PBI factors and both parents, the best-fitting models invariably assumed sex-limited genetic effects and unique environmental influences only. Broad heritability ranged from 19% (father overprotection) to 39% (mother care). These results are interpreted in the broader perspective of gene-culture theory.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01067533
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  • 5
    Electronic Resource
    Electronic Resource
    A twin-family study of self-report symptoms of panic-phobia and somatization (1995)
    Springer
    Behavior genetics 25 (1995), S. 499-515 
    Details
    ISSN: 1573-3297
    Keywords: Twins ; twin kinships ; cultural inheritance ; twin environment ; anxiety ; panic ; phobia
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Psychology
    Notes: Abstract Self-report symptoms of anxiety are widely used in mental health and social science research as an index of current psychiatric state. Previous twin studies have suggested that genetic factors account for a significant proportion of the variance in these symptoms. To replicate and extend these findings, we examined self-report symtoms of panic-phobia and somatization in the “Virginia 30,000” twin-family sample. Model fitting applied to 80 unique relationships in the twin-family pedigree produced the following major results: (i) genetic effects were significant for both symptom factors, accounting for between 25 and 49% of the total variance, with the exception of symptoms of panic-phobia in females, where they accounted for 15–16% of the variance; (ii) familial environmental effects were absent for symptoms of somatization, while for symptoms of panic-phobia they accounted for a very small proportion of variance in males (≤1.2%) and a modest proportion in females (6–17%) (iii) spousal correlations were present for both factors, ranging from +0.05 to +0.20; (iv) genetic factors which influenced symptoms were generally the same in males and females, although their effect was greater in males; (v) heritability estimates were lower in the population-based than in the volunteer sample; and (vi) when test-retest reliability was included in the model, results suggest that genetic factors account for at least half of the stable variance for all symptom factors, except panic-phobia in females. Our results support the validity of previous twin studies of self-report symptoms of anxiety and suggest that genetic factors significantly influence these symptoms but familial-environmental factors play little or no etiologic role.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02327574
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  • 6
    Electronic Resource
    Electronic Resource
    Multivariate multipoint linkage analysis of quantitative trait loci (1996)
    Springer
    Behavior genetics 26 (1996), S. 519-525 
    Details
    ISSN: 1573-3297
    Keywords: Quantitative trait loci ; multivariate analysis ; multipoint linkage ; structural models ; sib pairs ; factor analysis, factor rotation ; Mx
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Psychology
    Notes: Abstract Resolution of the genetic components of complex disorders may require simultaneous analysis of the contribution of individual quantitative trait loci (QTLs) to multiple variables. A likelihood approach is used to illustrate how the complexities of multivariate data may be resolved with multipoint linkage analysis. Sibling pair data were simulated from a model in which two QTLs and trait-specific polygenic effects explained all the sibling resemblance within and between five variables. Multipoint linkage analysis was used to obtain individual pair probabilities of having zero, one, or two alleles identical by descent, and these probabilities were applied in a weighted maximum-likelihood fit function. The results were compared with those obtained using conventional linear structural equation modeling to estimate the contribution of latent genetic factors to the genetic covariance in the multiple measures. Both analyses were conducted using the Mx package. Relatively poor agreement was found between genetic factors defined in purely statistical terms by varimax rotation of the first two factors of the genetic covariance matrix and the structure obtained by fitting a model jointly to the phenotypic and the multipoint linkage data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02359757
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  • 7
    Electronic Resource
    Electronic Resource
    Analyzing twin resemblance in multisymptom data: Genetic applications of a latent class model for symptoms of conduct disorder in juvenile boys (1993)
    Springer
    Behavior genetics 23 (1993), S. 5-19 
    Details
    ISSN: 1573-3297
    Keywords: Adolescence ; age ; categorical data ; conduct disorder ; development ; etiological heterogeneity ; genotype × environment interaction ; latent class models ; major gene ; segregation analysis ; twins
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Psychology
    Notes: Abstract A model based on the latent class model is developed for the effects of genes and environment on multivariate categorical data in twins. The model captures many essential features of dimensional and categorical conceptions of complex behavioral phenotypes and can include, as special cases, a variety of major locus models including those that allow for etiological heterogeneity, differential sensitivity of latent classes to measured covariates, and genotype × environment interaction (G×E). Many features of the model are illustrated by an application to ratings on eight items relating to conduct disorder selected from the Rutter Parent Questionnaire (RPQ). Mothers rated their 8-to 16-year-old male twin offspring [174 monozygotic (MZ) and 164 dizygotic (DZ) pairs]. The impact of age on the frequency of reported symptoms was relatively slight. Preliminary latent class analysis suggests that four classes are required to explain the reported behavioral profiles of the individual twins. A more detailed analysis of the pairwise response profiles reveals a significant association between twins for membership of latent classes and that the association is greater in MZ than DZ twins, suggesting that genetic factors played a significant role in class membership. Further analysis shows that the frequencies of MZ pairs discordant for membership of some latent classes are close to zero, while others are definitely not zero. One possible explanation of this finding is that the items reflect underlying etiological heterogeneity, with some response profiles reflecting genetic categories and others revealing a latent environmental risk factor. We explore two “four-class” models for etiological heterogeneity which make different assumptions about the way in which genes and environment interact to produce complex disease phenotypes. The first model allows for genetic heterogeneity that is expressed only in individuals exposed to a high-risk (“predisposing”) environment. The second model allows the environment to differentiate two forms of the disorder in individuals of high genetic risk. The first model fits better than the second, but neither fits as well as the general model for four latent classes associated in twins. The results suggest that a single-locus/two-allele model cannot fit the data on these eight items even when we allow for etiological heterogeneity. The pattern of endorsement probabilities associated with each of the four classes precludes a simple “unidimensional” model for the latent process underlying variation in symptom profile in this population. The extension of the approach to larger pedigrees and to linkage analysis is briefly considered.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01067550
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  • 8
    Electronic Resource
    Electronic Resource
    A pilot Swedish twin study of affective illness including hospital- and population-ascertained subsamples: Results of model fitting (1995)
    Springer
    Behavior genetics 25 (1995), S. 217-232 
    Details
    ISSN: 1573-3297
    Keywords: Affective illness ; Swedish Twin Registry ; depression ; mania ; bipolar illness ; twin studies ; model fitting
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Psychology
    Notes: Abstract We investigated the heritability of liability to affective illness (AI) in twins ascertained through psychiatric hospitalization for AI from the Swedish Psychiatric Twin Registry and from the general population Swedish Twin Registry. Lifetime diagnoses were assessed by mailed questionnaire containing, in self-report format, DSM-III-R criteria for mania and major depression (MD). Jointly analyzing both subsamples using Mx, and assuming a multifactorial threshold model, the best-fitting twin model using narrow diagnostic criteria suggested that the liability to AI could be explained by additive genetic effects, with an estimated heritability of liability of 64%, and individual-specific environment. Using broad criteria, results were similar except that the estimated broad heritability of liability was higher (83%) and due largely to dominance genetic effects. Fitting sex-dependent models suggested that the same genetic and environmental factors influenced liability to AI in men and women to the same degree, although women had a lower threshold of manifestation. These results suggested that in Sweden, AI is a highly heritable syndrome and family resemblance is due largely or entirely to genetic factors.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02197180
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  • 9
    Electronic Resource
    Electronic Resource
    Physical similarity and the equal-environment assumption in twin studies of psychiatric disorders (1995)
    Springer
    Behavior genetics 25 (1995), S. 327-335 
    Details
    ISSN: 1573-3297
    Keywords: Twin studies ; equal-environment assumption ; psychiatric disorders
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Psychology
    Notes: Abstract The equal-environment assumption (EEA), upon which twin methodology is based, was examined for the impact of physical similarity on phenotypic resemblance in five common psychiatric disorders: major depression, generalized anxiety disorder, phobia, alcoholism, and bulimia. A population-based sample of 882 female-female twin pairs of known zygosity was rated for similarity of appearance by color photographs. Psychiatric diagnoses were made by clinical assessment of personal interviews of the twins. Structural equation modeling of the data using physical similarity as a form of specified common environment provided no evidence for a significant effect of physical resemblance on concordance for major depression, generalized anxiety disorder, phobia, and alcoholism, thereby supporting the validity of the EEA in twin studies of these disorders. Results for bulimia, on the other hand, suggest, within the limitations of this study, that physical similarity may significantly influence twin resemblance for this disorder.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02197281
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  • 10
    Electronic Resource
    Electronic Resource
    Assortative mating for relative weight: Genetic implications (1996)
    Springer
    Behavior genetics 26 (1996), S. 103-111 
    Details
    ISSN: 1573-3297
    Keywords: Assortative mating ; heritability ; relative body weight ; genetics ; mate selection ; phenotypic variation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Psychology
    Notes: Abstract Most work on the genetics of relative weight has not considered the role of assortative mating, i.e., mate selection based on similarity between mates. We investigated the extent to which engaged men and women in an archival longitudinal database were similar to each other in relative body weightprior to marriage and cohabitation. After controlling for age, a small but statistically significant mate correlation was found for relative weight (r=.13,p=.023), indicating some assortative mating. Furthermore, we examined whether mate similarity in relative weight prior to marriage predicts survival of the marriage. No significant effects were found. In sum, these results are consistent with those of other studies in suggesting that there is a small but significant intermate correlation for relative weight. However, they are unique in showing that these results cannot be explained on the basis of (a) cohabitation, (b) age similarity, or (c) selective survival of marriages between couples more similar in relative weight. The implications of these findings for heritability studies, linkage studies, and the estimation of shared environmental effects are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02359888
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