The Florida State Twin Registry: Research Aims and Design

The Florida State Twin Registry

The Florida State Twin Registry has existed for nearly two decades, and it grew primarily over 10 years as part of the Florida Learning Disability Research Center at Florida State University. The registry contains 2753 sets of twins and other multiples in Florida. The mean age of the sample currently is 17.96 years (SD = 2.05). The registry is diverse in terms of race and ethnicity (49% White, 19% African American, 24% Hispanic and 8% mixed or other). The characteristics of the samples and measures in the studies that served to build the registry are outlined.

Examining Associations Among ADHD, Homework Behavior, and Reading Comprehension: A Twin Study

Previous literature has indicated an important association between reading comprehension and both attention-deficit/hyperactivity disorder (ADHD) and homework habits. This investigation sought to extend previous knowledge by providing information about how ADHD and homework behavior (i.e., completing homework regularly) may jointly influence reading comprehension. Using a genetically sensitive design, this study examined the genetic and environmental influences on and between ADHD, homework behavior and reading comprehension. Participants for this study included 691 twin pairs (351 monozygotic, 340 same-sex dizygotic) from the Florida Twin Project on Behavior and Environment (FTP-BE) and 2647 twin pairs (865 monozygotic, 1782 dizygotic) from the larger Florida Twin Project on Reading (FTP-R) in Grades 3 through 7. Three separate models, each representing a different definition of ADHD (full ADHD, inattention only, and hyperactivity/impulsivity only), showed similar patterns of results; therefore, results of the full ADHD model are discussed. Overlapping genetic influences were found between ADHD, homework behavior, and reading comprehension, but no shared environmental influences among all three. However, shared environmental influences overlapped between homework behavior and reading comprehension. Although the sources of this environmental overlap are unknown, these results have implications for improving homework practices and their subsequent influence on literacy skills through homework environments.

Examining the etiological associations among higher-order temperament dimensions

A multivariate independent pathway model was used to examine the shared and unique genetic and environmental influences of Positive Affect (PA), Negative Affect (NA), and effortful control (EC) in a sample of 686 twin pairs (M age = 10.07, SD = 1.74). There were common genetic influences and nonshared environmental influences shared across all three temperament dimensions and shared environmental influences in common to NA and EC. There were also significant independent genetic influences unique to PA and NA and significant independent shared environmental influences unique to PA. This study demonstrates that there are genetic and environmental influences that affect the covariance among temperament dimensions as well as unique genetic and environmental influences that influence the dimensions independently.

Negative affect shares genetic and environmental influences with symptoms of childhood internalizing and externalizing disorders

The co-occurrence of internalizing and externalizing disorders suggests that they may have common underlying vulnerability factors. Research has shown that negative affect is moderately positively correlated with both internalizing and externalizing disorders in children. The present study is the first to provide an examination of negative affect in relation to a wide spectrum of childhood internalizing and externalizing problems using a biometric model. This study extends prior findings of more narrowly focused associations by using a factor approach including multiple disorders. The sample for this study included families of 691 same-sex 7- to 13-year old twin pairs. A multifactorial independent pathway model was used to examine the genetic and environmental influences underlying the covariation of parent-reported negative affect, internalizing symptoms, and externalizing symptoms. Results of the current study suggest that negative affect shares genetic and environmental influences with both internalizing and externalizing disorders in childhood. These common influences may partially explain their comorbidity. Understanding that negative affect is at least one contributor to the covariation among these disorders may highlight avenues for early risk assessment, intervention, and perhaps prevention.

Common genetic and nonshared environmental factors contribute to the association between socioemotional dispositions and the externalizing factor in children

BACKGROUND

Childhood behavioral disorders including conduct disorder (CD), oppositional defiant disorder (ODD), and attention-deficit/hyperactivity disorder (ADHD) often co-occur. Prior twin research shows that common sets of genetic and environmental factors are associated with these various disorders and they form a latent factor called Externalizing. The developmental propensity model posits that CD develops in part from socioemotional dispositions of prosociality, negative emotionality, and daring; and recent research has supported the expected genetic and environmental associations between these dispositions and CD. This study examined the developmental propensity model in relation to the broader Externalizing factor that represents the covariance among behavior disorders in children.

METHODS

Parents of 686 six- to twelve-year-old twin pairs rated them on symptoms of CD, ADHD, and ODD using the disruptive behavior disorder scale and on prosociality, negative emotionality, and daring using the child and adolescent dispositions scale. A latent factor multivariate Cholesky model was used with each disposition latent factor comprised of respective questionnaire items and the Externalizing factor comprised of symptom dimensions of CD, ADHD inattention, ADHD hyperactivity/impulsivity, and ODD.

RESULTS

Results supported the hypothesis that the socioemotional dispositions and the Externalizing factor have genetic factors in common, but there was not a single genetic factor associated with all of the constructs. As expected, nonshared environment factors were shared by the dispositions and externalizing factor but, again, no single nonshared environmental factor was common to all constructs. A shared environmental factor was associated with both negative emotionality and externalizing.

CONCLUSIONS

The developmental propensity model was supported and appears to extend to the broader externalizing spectrum of childhood disorders. Socioemotional dispositions of prosociality, negative emotionality, and (to a lesser extent) daring may contribute to the covariation among behavioral disorders and perhaps to their comorbid expression through common sets of primarily genetic but also environmental factors.

An update on the Florida State Twin Registry

The Florida State Twin Registry began in 2002 through a pilot study of personality disorders and executive cognitive functioning in adult twins. Since 2006, the registry has grown substantially as part of the Learning Disability Research Center at Florida State University that recently began its second funding cycle through the National Institute of Child Health and Development. An update on the Florida State Twin Registry sample, focus, and measures is provided, as well as future directions.

Heritability of Wisconsin Card Sorting Test (WCST) and Stroop Color-Word Test Performance in Normal Individuals: Implications for the Search for Endophenotypes

Asurge in the search for endophenotypes for psychiatric disorders has occurred in the past several years. An important criterion of an endophenotype is that it is heritable. Two of the most widely used executive cognitive functioning measures are the Wisconsin Card Sorting Test (WCST) and the Stroop Color-Word Test. Each has been considered as a possible endophenotype. However, research on the heritability of each of these measures is sparse, and in the case of the WCST, mixed. As part of a pilot twin study examining cognitive functioning and personality in adults, the WCST and the Stroop were administered to 80 monozygotic and 29 dizygotic twin pairs screened for absence of neurological disease and head injury. Results replicated and extended previous findings for moderate heritability of Stroop performance. However, the WCST showed little evidence of genetic influence, suggesting that it might not meet one of the criteria for an endophenotype.

The Estimated Probability of Dizygotic Twins: A Comparison of Two Methods

This study presents a general model of two binary variables and applies it to twin sex pairing data from 21 twin data sources to estimate the frequency of dizygotic twins. The purpose of this study is to clarify the relationship between maximum likelihood and Weinberg's differential rule zygosity estimation methods. We explore the accuracy of these zygosity estimation measures in relation to twin ascertainment methods and the probability of a male. Twin sex pairing data from 21 twin data sources representing 15 countries was collected for use in this study. Maximum likelihood estimation of the probability of dizygotic twins is applied to describe the variation in the frequency of dizygotic twin births. The differences between maximum likelihood and Weinberg's differential rule zygosity estimation methods are presented as a function of twin data ascertainment method and the probability of a male. Maximum likelihood estimation of the probability of dizygotic twins ranges from 0.083 (95% approximate CI: 0.082, 0.085) to 0.750 (95% approximate CI: 0.749, 0.752) for voluntary ascertainment data sources and from 0.374 (95% approximate CI: 0.373, 0.375) to 0.987 (95% approximate CI: 0.959, 1.016) for active ascertainment data sources. In 17 of the 21 twin data sources differences of 0.01 or less occur between maximum likelihood and Weinberg zygosity estimation methods. The Weinberg and maximum likelihood estimates are negligibly different in most applications. Using the above general maximum likelihood estimate, the probability of a dizygotic twin is subject to substantial variation that is largely a function of twin data ascertainment method.

Genetic influence on literacy constructs in kindergarten and first grade: evidence from a diverse twin sample

Historically, twin research on reading has been conducted on older children and the generalizability of results across racial/ethnic/socioeconomic groups is unclear. To address these gaps, early literacy skills were examined among 1,401 twin pairs in kindergarten and 1,285 twin pairs in first grade (ages 5-7). A multi-group analysis was conducted separately for subsamples defined by neighborhood income while controlling for race/ethnicity within each grade. Substantial additive genetic and shared environmental effects were found for early literacy skills measured in kindergarten. In first grade, variance in early reading was associated with large additive genetic effects for middle and high neighborhood income twins, but shared environmental influence was substantial for low neighborhood income twins. Results suggest that the etiological architecture of some early literacy skills may differ across economic contexts.