A Bivariate Path Analysis of Fear Data on Twins and Their Parents

Incorporating Polygenic Risk Scores in the ACE Twin Model to Estimate A-C Covariance

The assumption in the twin model that genotypic and environmental variables are uncorrelated is primarily made to ensure parameter identification, not because researchers necessarily think that these variables are uncorrelated. Although the biasing effects of such correlations are well understood, a method to estimate these parameters in the twin model would be useful. Here we explore the possibility of relaxing this assumption by adding polygenic scores to the (univariate) twin model. We demonstrate that this extension renders the additive genetic (A)—common environmental (C) covariance (σ_AC) identified. We study the statistical power to reject σ_AC = 0 in the ACE model and present the results of simulations.

The utility of twins in developmental cognitive neuroscience research: How twins strengthen the ABCD research design

The ABCD twin study will elucidate the genetic and environmental contributions to a wide range of mental and physical health outcomes in children, including substance use, brain and behavioral development, and their interrelationship. Comparisons within and between monozygotic and dizygotic twin pairs, further powered by multiple assessments, provide information about genetic and environmental contributions to developmental associations, and enable stronger tests of causal hypotheses, than do comparisons involving unrelated children. Thus a sub-study of 800 pairs of same-sex twins was embedded within the overall Adolescent Brain and Cognitive Development (ABCD) design. The ABCD Twin Hub comprises four leading centers for twin research in Minnesota, Colorado, Virginia, and Missouri. Each site is enrolling 200 twin pairs, as well as singletons. The twins are recruited from registries of all twin births in each State during 2006-2008. Singletons at each site are recruited following the same school-based procedures as the rest of the ABCD study. This paper describes the background and rationale for the ABCD twin study, the ascertainment of twin pairs and implementation strategy at each site, and the details of the proposed analytic strategies to quantify genetic and environmental influences and test hypotheses critical to the aims of the ABCD study.

Moderating the covariance between family member's substance use behavior

Twin and family studies implicitly assume that the covariation between family members remains constant across differences in age between the members of the family. However, age-specificity in gene expression for shared environmental factors could generate higher correlations between family members who are more similar in age. Cohort effects (cohort × genotype or cohort × common environment) could have the same effects, and both potentially reduce effect sizes estimated in genome-wide association studies where the subjects are heterogeneous in age. In this paper we describe a model in which the covariance between twins and non-twin siblings is moderated as a function of age difference. We describe the details of the model and simulate data using a variety of different parameter values to demonstrate that model fitting returns unbiased parameter estimates. Power analyses are then conducted to estimate the sample sizes required to detect the effects of moderation in a design of twins and siblings. Finally, the model is applied to data on cigarette smoking. We find that (1) the model effectively recovers the simulated parameters, (2) the power is relatively low and therefore requires large sample sizes before small to moderate effect sizes can be found reliably, and (3) the genetic covariance between siblings for smoking behavior decays very rapidly. Result 3 implies that, e.g., genome-wide studies of smoking behavior that use individuals assessed at different ages, or belonging to different birth-year cohorts may have had substantially reduced power to detect effects of genotype on cigarette use. It also implies that significant special twin environmental effects can be explained by age-moderation in some cases. This effect likely contributes to the missing heritability paradox.

Are extended twin family designs worth the trouble? A comparison of the bias, precision, and accuracy of parameters estimated in four twin family models

The classical twin design (CTD) uses observed covariances from monozygotic and dizygotic twin pairs to infer the relative magnitudes of genetic and environmental causes of phenotypic variation. Despite its wide use, it is well known that the CTD can produce biased estimates if its stringent assumptions are not met. By modeling observed covariances of twins' relatives in addition to twins themselves, extended twin family designs (ETFDs) require less stringent assumptions, can estimate many more parameters of interest, and should produce less biased estimates than the CTD. However, ETFDs are more complicated to use and interpret, and by attempting to estimate a large number of parameters, the precision of parameter estimates may suffer. This paper is a formal investigation into a simple question: Is it worthwhile to use more complex models such as ETFDs in behavioral genetics? In particular, we compare the bias, precision, and accuracy of estimates from the CTD and three increasingly complex ETFDs. We find the CTD does a decent job of estimating broad sense heritability, but CTD estimates of shared environmental effects and the relative importance of additive versus non-additive genetic variance can be biased, sometimes wildly so. Increasingly complex ETFDs, on the other hand, are more accurate and less sensitive to assumptions than simpler models. We conclude that researchers interested in characterizing the environment or the makeup of genetic variation should use ETFDs when possible.

Genetic and cultural transmission of antisocial behavior: an extended twin parent model

Considerable evidence from twin and adoption studies indicates that both genetic and shared environmental factors play a substantial role in the liability to antisocial behavior. Although twin and adoption designs can resolve genetic and environmental influences, they do not provide information about assortative mating, parent-offspring transmission, or the contribution of these factors to trait variation. We examined the role of genetic and environmental factors for conduct disorder (CD) using a twin-parent design. This design allows the simultaneous estimation of additive genetic, shared and individual-specific environmental effects, as well as sex differences in the expression of genes and environment in the presence of assortative mating and combined genetic and cultural transmission. A retrospective measure of CD was obtained from twins and their parents or guardians in the Virginia Twin Study of Adolescent Behavior Development and its Young Adult Follow up sample. Both genetic and environmental factors play a significant role in the liability to CD. Major influences on individual differences appeared to be additive genetic (38%-40%) and unique environmental (39%-42%) effects, with smaller contributions from the shared environment (18%-23%), assortative mating (-2%), cultural transmission (approximately 2%) and resulting genotype-environment covariance. This study showed significant heritability, which is slightly increased by assortative mating, and significant effects of primarily nonparental shared environment on CD.

Genetic and environmental causes of the interrelationships between self-reported fears. A study of a non-clinical sample of Norwegian identical twins and their families

The present study reports results from a study of the self-reported fears of identical twins and their spouses and offspring. Factor analysis with oblique rotation of questionnaire responses yielded four correlated fear dimensions: situational fears, illness-injury fears, social fears, and fear of small animals. Models allowing for genetic and cultural transmission, together with specially correlated environments for twins, were fitted, both for separate fears and across fears. Simple models with only genetic and uncorrelated environments were sufficient to account for each the fear dimensions considered separately. The cross-dimensional analyses revealed a genetic and an environmental factor common to the four fear dimensions, together with fear-specific genetic and environmental factors. The impact of the common genetic and common environmental factor varied across dimensions. No evidence of cultural transmission or specially correlated twin environments of the cross-dimensional environments was detected. It is concluded that both common and fear-specific genes and (individual-specific) common and fear-specific environments are necessary to account for the data. The results are discussed in terms of the prepared learning hypothesis and the expectancy bias hypothesis.

The heritability of common phobic fear: a twin study of a clinical sample

The aim of this study was to investigate the genetic and environmental contribution to common phobic fears, and to relate the findings to contemporary theories about the etiology of common phobic fears. Self-reported common phobic fear was studied in a treatment sample of 23 monozygotic and 38 same-sex dizygotic twin pairs. Heritability of .47 was observed for common phobic fear of small animals and social fear, and a heritability of .30 in common agoraphobic fear. For common fear of nature phenomena and situational fear, the heritability was 0. The finding that common nature and situational fears were solely caused by environmental factors is in support of learning theory, whereas results for animal, social, and other common phobic fear are in support of an integrative theory of biological preparedness, learning history, and a cognitive style of fearful expectation.

Multivariate genetic analysis of twin-family data on fears: Mx models

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.

Major depression and phobias: the genetic and environmental sources of comorbidity

In a population based sample of 2163 personally interviewed female twins, substantial comorbidity was observed between DSM-III-R defined major depression (MD) and 4 subtypes of phobia: agoraphobia, social phobia, animal phobia and situational phobia. However, the level of comorbidity of MD with agoraphobia was much greater than that found with the other phobic subtypes. We concluded bivariate twin analyses to decompose the genetic and environmental sources of comorbidity between MD and the phobias. Our results suggest that a modest proportion of the genetic vulnerability to MD also influences the risk for all phobic subtypes, with the possible exception of situational phobias. Furthermore, the magnitude of comorbidity resulting from this shared genetic vulnerability is similar across the phobic subtypes. By contrast, the non-familial environmental experiences which predispose to depression substantially increase the vulnerability to agoraphobia, have a modest impact on the risk for social and situational phobias and no effect on the risk for animal phobias. The increased comorbidity between MD and agoraphobia results, nearly entirely, from individual-specific environmental risk factors for MD which also increase the risk for agoraphobia but not for other phobias.

A LISREL 8 model with constrained parameters for twin and adoptive families

Constrained optimization recently has been implemented in the LISREL software package, allowing formulation of parent-offspring transmission models in a simple and efficient manner. A reverse path model of parental transmission is described within the LISREL framework for application to twin and/or adoptive family data. The model incorporates genetic and environmental parameter constraints arising from assortative mating and cultural transmission. An illustration of the LISREL model is given using measures obtained from twins and parents involving fear of social criticism.

Genotype-Environment Covariance for Multiple Phenotypes: A Multivariate Test Using Adopted and Nonadopted Children

A bivariate generalization of the genotype-environment (GE) covariation is presented. This biometrical parameter measures the relation between genotypic influences in one attribute with environmental influences in another attribute. A multivariate procedure for detecting the presence of both univariate and bivariate GE covariation is also described, based on a test of homogeneity of observed variance-covariance matrices in adopted and nonadopted individuals. An illustrative application to measures of specific cognitive abilities in four-year-old children in the Colorado Adoption Project (CAP) is provided. The analyses provided evidence of bivariate but not univariate GE covariance among perceptual speed, memory, verbal and spatial abilities in these children. A comparison of the results from the presently described and other more elaborate model-fitting procedures is made, with a discussion of the merits and disadvantages of each approach.

Genetics of fear and anxiety disorders

From protozoa to mammals, organisms have been selectively bred for genetic differences in defensive behaviour which are accompanied by differences in brain and other biological functions. Studies of twins indicate some genetic control of normal human fear from infancy onwards, of anxiety as a symptom and as a syndrome, and of phobic and obsessive-compulsive phenomena. Anxiety disorders are more common among the relatives of affected probands than of controls, especially among female and first-degree relatives; alcoholism and secondary depression may also be over-represented. Familial influences have been found for panic disorder, agoraphobia, and obsessive-compulsive problems. Panic disorder in depressed probands increases the risk to their relatives of phobia as well as of panic disorder, major depression, and alcoholism. The strongest family history of all anxiety disorders is seen in blood-injury phobia; even though it can be successfully treated by exposure, its roots may lie in a genetically determined specific autonomic susceptibility. Some genetic effects can be modified by environmental means.

Multivariate Familial Analysis of Cognitive Measures in the Colorado Family Reading Study

Multivariate path analysis is employed to examine the etiologies of variation and covariation of three composite cognitive measures in the Colorado Family Reading Study: reading ability, symbol-processing speed, and spatial/reasoning. Measures of phenotypic assortative and cross-assortative mating are incorporated in a multivariate analysis of familial resemblance within nuclear families. Phenotypic variances and covariances are partitioned into components due to familial (genetic and/or family environmental) influences and to specific, nontransmissible environmental influences in families with a reading-disabled child as well as families with children of normal reading ability. Comparable moderate familial influences are found across family type for all three composites and the phenotypic correlations between traits are largely due to familial influences.

Multivariate path analysis of familial resemblance

A method is presented for applying path analysis to general multivariate models of familial resemblance. In path models formulated using this approach, variables are defined as column vectors, path coefficients are defined as matrices of path coefficients, and correlations are defined as matrices of correlations. By applying a few simple rules for multivariate path analysis, general multivariate expected correlations can be derived from a path diagram which is essentially as simple as a univariate diagram and which can be used to analyze any number of variables. Multivariate expected correlations for three models of familial resemblance are derived, with particular attention given to the modeling of assortative mating: nuclear families with a phenotypic homogamy model of assortative mating, nuclear families with a social homogamy model of assortative mating, and twins and their parents with phenotypic homogamy. These models are applicable to many of the types of studies commonly undertaken in genetic epidemiology. The simplicity of the technique facilitates analyses of the etiology of variation and covariation among variables measured in such studies.