Co-recovery of physical size and cognitive ability from infancy to adolescence: A twin study.

This study tested phenotypic and biometric associations between physical and cognitive catch-up growth in a community sample of twins (n = 1285, 51.8% female, 89.3% White). Height and weight were measured at up to 17 time points between birth and 15 years, and cognitive ability was assessed at up to 16 time points between 3 months and 15 years. Weight and length at birth were positively associated with cognitive abilities in infancy and adolescence (r's = .16-.51). More rapid weight catch-up growth was associated with slower, steadier cognitive catch-up growth. Shared and nonshared environmental factors accounted for positive associations between physical size at birth and cognitive outcomes. Findings highlight the role of prenatal environmental experiences in physical and cognitive co-development.

Nonlinear Catch-Up Growth in Height, Weight, and Head Circumference from Birth to Adolescence: A Longitudinal Twin Study.

Owing to high rates of prenatal complications, twins are, on average, substantially smaller than population norms on physical measurements including height, weight, and head circumference at birth. By early childhood, twins are physically average. This study is the first to explore the process of catch-up growth by fitting asymptotic growth models to age-standardized height, weight, and head circumference measurements in a community sample of twins (n = 1281, 52.3% female) followed at up to 17 time points from birth to 15 years. Catch-up growth was rapid over the first year and plateaued around the population mean by early childhood. Shared environmental factors accounted for the majority of individual differences in initial physical size (57.7-65.5%), whereas additive genetic factors accounted for the majority of individual differences in the upper asymptotes of height, weight, and head circumference (73.4-92.6%). Both additive genetic and shared environmental factors were associated with variance in how quickly twins caught up. Gestational age and family SES emerged as important environmental correlates of physical catch-up growth.

A Longitudinal Analysis of Gene x Environment Interaction on Verbal Intelligence Across Adolescence and Early Adulthood.

The Scarr-Rowe hypothesis proposes that the heritability of intelligence is higher in more advantaged socioeconomic contexts. An early demonstration of this hypothesis was Rowe and colleagues (Rowe et al., Child Dev 70:1151-1162, 1999), where an interaction between the heritability of verbal intelligence and parental education was identified in adolescent siblings in Wave I of the National Longitudinal Study of Adolescent to Adult Health. The present study repeated their original analysis at Wave I using contemporary methods, replicated the finding during young adulthood at Wave III, and analyzed the interaction longitudinally utilizing multiple measurements. We examined parental education, family income, and peer academic environment as potential moderators. Results indicated increased heritability and decreased shared environmental variance of verbal intelligence at higher levels of parental education and peer academic environment in adolescence. Moreover, moderation by peer academic environment persisted into adulthood with its effect partially attributable to novel gene-environment interactions that arose in the process of cognitive development.

Simulated nonlinear genetic and environmental dynamics of complex traits.

Genetic studies of complex traits often show disparities in estimated heritability depending on the method used, whether by genomic associations or twin and family studies. We present a simulation of individual genomes with dynamic environmental conditions to consider how linear and nonlinear effects, gene-by-environment interactions, and gene-by-environment correlations may work together to govern the long-term development of complex traits and affect estimates of heritability from common methods. Our simulation studies demonstrate that the genetic effects estimated by genome wide association studies in unrelated individuals are inadequate to characterize gene-by-environment interaction, while including related individuals in genome-wide complex trait analysis (GCTA) allows gene-by-environment interactions to be recovered in the heritability. These theoretical findings provide an explanation for the "missing heritability" problem and bridge the conceptual gap between the most common findings of GCTA and twin studies. Future studies may use the simulation model to test hypotheses about phenotypic complexity either in an exploratory way or by replicating well-established observations of specific phenotypes.

On the big list of causes.

The methodological shift from twin studies to genome-wide association studies (GWASs) diminished estimates of true genetic causation underlying statistical heritability of behavioral differences. The sum total of causal genetic influence on behavior is not zero, but, (a) no one cited in the target article ever thought this was the case, and (b) there is still little known about concrete instances of genetic causation.

Genetically informed, multilevel analysis of the Flynn Effect across four decades and three WISC versions.

This study investigated the systematic rise in cognitive ability scores over generations, known as the Flynn Effect, across middle childhood and early adolescence (7-15 years; 291 monozygotic pairs, 298 dizygotic pairs; 89% White). Leveraging the unique structure of the Louisville Twin Study (longitudinal data collected continuously from 1957 to 1999 using the Wechsler Intelligence Scale for Children [WISC], WISC-R, and WISC-III ed.), multilevel analyses revealed between-subjects Flynn Effects-as both decrease in mean scores upon test re-standardization and increase in mean scores across cohorts-as well as within-child Flynn Effects on cognitive growth across age. Overall gains equaled approximately three IQ points per decade. Novel genetically informed analyses suggested that individual sensitivity to the Flynn Effect was moderated by an interplay of genetic and environmental factors.

Socioeconomic status impacts genetic influences on the longitudinal dynamic relationship between temperament and general cognitive ability in childhood: The Louisville Twin Study.

The current analysis investigates genetic and environmental influences on the bidirectional relationships between temperament and general cognitive ability (GCA). Measures of GCA and three temperament factors (persistence, approach, and reactivity) were collected from 486 children ages 4-9 years (80% white, 50% female) from the Louisville Twin Study from 1976 to 1998. The results indicated a bidirectional dynamic model of temperament influencing subsequent GCA and GCA influencing subsequent temperament. The dynamic relationship between temperament and GCA arose primarily from shared genetic variance, particularly in families with higher socioeconomic status, where input from temperament contributed on average 20% to genetic variance in GCA versus 0% in lower SES families.

What Do We Know About the Genetic Architecture of Psychopathology?

In the second half of the twentieth century, twin and family studies established beyond a reasonable doubt that all forms of psychopathology are substantially heritable and highly polygenic. These conclusions were simultaneously an important theoretical advance and a difficult methodological obstacle, as it became clear that heritability is universal and undifferentiated across forms of psychopathology, and the radical polygenicity of genetic effects limits the biological insight provided by genetically informed studies at the phenotypic level. The paradigm-shifting revolution brought on by the Human Genome Project has recapitulated the great methodological promise and the profound theoretical difficulties of the twin study era. We review these issues using the rubric of genetic architecture, which we define as a search for specific genetic insight that adds to the general conclusion that psychopathology is heritable and polygenic. Although significant problems remain, we see many promising avenues for progress.

The pillars of health: influence of multiple lifestyle behaviors on body mass index and depressive symptoms in adult twins.

BACKGROUND: Guidelines promoting healthy lifestyles are cornerstones of chronic disease prevention and treatment. The purpose of this study is to investigate independent and joint associations of five key health behaviors with health outcomes (body mass index (BMI kg/m2) and depressive symptoms) in adult twins. METHODS: We included 6,048 twin pairs from a community-based registry. Five key health behaviors were: (1) ≥ 8 h of sleep per night, (2) ≥ 5 servings of fruits and vegetables daily, (3) ≤ 2 h sedentary time per day, (4) ≥ 150 min of moderate-to-vigorous physical activity (MVPA) per week, and (5) no smoking. We analyzed phenotypic associations between behaviors and outcomes; whether phenotypic associations were confounded by additive genetic and shared environmental factors within twin pairs ("quasi-causal" associations); and which behaviors, considered simultaneously, had the largest associations with outcomes. RESULTS: We found negative phenotypic associations between number of behaviors achieved with BMI and depressive symptoms score (ps < 0.05). Associations remained significant, though attenuated, when controlling for genetic and shared environmental factors, and demographics, for depressive symptoms score but not BMI (p < 0.05). Quantitative variable importance measures derived from regression tree models showed sedentary time and MVPA were the most important variables in partitioning twins with different BMI, and smoking and sedentary time for partitioning twins with different depressive symptoms score. CONCLUSIONS: Achievement of commonly endorsed health behaviors is associated with lower BMI (especially sedentary and MVPA targets) and depressive symptoms score (especially sedentary and smoking targets). This provides further support of health behavior promotion to improve health outcomes.

This time I mean it: The nature-nurture debate is over.

The target article is skeptical of the heritability concept while maintaining an old-fashioned point of view about it. As a descriptive statistic, it is to be expected that heritability goes up and down in different circumstances, but the relationship between heritability coefficients and the biological processes that underlie them is difficult to specify, and may be impossible in humans.

Three legs of the missing heritability problem.

The so-called 'missing heritability problem' is often characterized by behavior geneticists as a numerical discrepancy between alternative kinds of heritability. For example, while 'traditional heritability' derived from twin and family studies indicates that approximately ∼50% of variation in intelligence is attributable to genetics, 'SNP heritability' derived from genome-wide association studies indicates that only ∼10% of variation in intelligence is attributable to genetics. This 40% gap in variance accounted for by alternative kinds of heritability is frequently referred to as what's "missing." Philosophers have picked up on this reading, suggesting that "dissolving" the missing heritability problem is merely a matter of closing the numerical gap between traditional and molecular kinds of heritability. We argue that this framing of the problem undervalues the severity of the many challenges to scientific understanding of the "heritability" of human behavior. On our view, resolving the numerical discrepancies between alternative kinds of heritability will do little to advance scientific explanation and understanding of behavior genetics. Thus, we propose a new conceptual framework of the missing heritability problem that comprises three independent methodological and explanatory challenges: the numerical gap, the prediction gap, and the mechanism gap.

Galton's Quincunx: Probabilistic causation in developmental behavior genetics.

In what sense are associations between particular markers and complex behaviors made by genome-wide association studies (GWAS) and related techniques discoveries of, or entries into the study of, the causes of those behaviors? In this paper, we argue that when applied to individuals, the kinds of probabilistic 'causes' of complex traits that GWAS-style studies can point towards do not provide the kind of causal information that is useful for generating explanations; they do not, in other words, point towards useful explanations of why particular individuals have the traits that they do. We develop an analogy centered around Galton's "Quincunx" machine; while each pin might be associated with outcomes of a certain sort, in any particular trial, that pin might be entirely bypassed even if the ball eventually comes to rest in the box most strongly associated with that pin. Indeed, in any particular trial, the actual outcome of a ball hitting a pin might be the opposite of what is usually expected. While we might find particular pins associated with outcomes in the aggregate, these associations will not provide causally relevant information for understanding individual outcomes. In a similar way, the complexities of development likely render impossible any moves from population-level statistical associations between genetic markers and complex behaviors to an understanding of the causal processes by which individuals come to have the traits that they in fact have.

Midlife Study of the Louisville Twins: Connecting Cognitive Development to Biological and Cognitive Aging.

The Louisville Twin Study (LTS) began in 1958 and became a premier longitudinal twin study of cognitive development. The LTS continuously collected data from twins through 2000 after which the study closed indefinitely due to lack of funding. Now that the majority of the sample is age 40 or older (61.36%, N = 1770), the LTS childhood data can be linked to midlife cognitive functioning, among other physical, biological, social, and psychiatric outcomes. We report results from two pilot studies in anticipation of beginning the midlife phase of the LTS. The first pilot study was a participant tracking study, in which we showed that approximately 90% of the Louisville families randomly sampled (N = 203) for the study could be found. The second pilot study consisted of 40 in-person interviews in which twins completed cognitive, memory, biometric, and functional ability measures. The main purpose of the second study was to correlate midlife measures of cognitive functioning to a measure of biological age, which is an alternative index to chronological age that quantifies age as a function of the breakdown of structural and functional physiological systems, and then to relate both of these measures to twins' cognitive developmental trajectories. Midlife IQ was uncorrelated with biological age (- .01) while better scores on episodic memory more strongly correlated with lower biological age (- .19 to - .31). As expected, midlife IQ positively correlated with IQ measures collected throughout childhood and adolescence. Additionally, positive linear rates of change in FSIQ scores in childhood significantly correlated with biological age (- .68), physical functioning (.71), and functional ability (- .55), suggesting that cognitive development predicts lower biological age, better physical functioning, and better functional ability. In sum, the Louisville twins can be relocated to investigate whether and how early and midlife cognitive and physical health factors contribute to cognitive aging.

Psychometric and Classification Properties of the Peas in a Pod Questionnaire.

We examined the item properties of the Two Peas Questionnaire (TPQ) among a sample of same-sex twin pairs from the Washington State Twin Registry. With the exception of the 'two peas' item, three of the mistakenness items showed differential item functioning. Results showed that the monozygotic (MZ) and dizygotic (DZ) pairs may differ in their responses on these items, even among those with similar latent traits of similarity and confusability. Upon comparing three classification methods to determine the zygosity of same-sex twins, the overall classification accuracy rate was over 90% using the unit-weighted pair zygosity sum score, providing an efficient and sufficiently accurate zygosity classification. Given the inherent nature of twin-pair similarity, the TPQ is more accurate in the identification of MZ than DZ pairs. We conclude that the TPQ is a generally accurate, but by no means infallible, method of determining zygosity in twins who have not been genotyped.

The Louisville Twin Study: Past, Present and Future.

The Louisville Twin Study (LTS) is nationally recognized as one of the largest and most comprehensive studies of child development related to multiple birth status. The LTS is unique because of the extensive longitudinal face-to-face assessments, the frequency of data collection, the inclusion of data on additional family members (i.e., parents, siblings, grandparents; and later, twins' own spouses and children), and the variety of data collection methods used. Data preservation efforts began in 2008 and are largely complete, although efforts are ongoing to obtain funding to convert the electronic data to a newer format. A pilot study was completed in the summer of 2018 to bring the twins, who are now middle-aged, back for testing. A grant is currently under review to extend the pilot study to include all former participants who are now ≥40 years of age. Opportunities for collaboration are welcome.

The Washington State Twin Registry: 2019 Update.

It has been over 5 years since the last special issue of Twin Research and Human Genetics on 'Twin Registries Worldwide: An Important Resource for Scientific Research' was published. Much progress has been made in the broad field of twin research since that time, and the current special issue is a follow-up to update the scientific community about twin registries around the globe. The present article builds upon our 2013 Registry description by summarizing current information on the Washington State Twin Registry (WSTR), including history and construction methods, member characteristics, available data, and major research goals. We also provide a section with brief summaries of recently completed studies and discuss the future research directions of the WSTR. The Registry has grown in terms of size and scope since 2013; highlights include recruitment of youth pairs under 18 years of age, extensive geocoding work to develop environmental exposures that can be linked to survey and administrative health data such as death records, and expansion of a biobank with specimens collected for genotyping, DNA methylation, and microbiome based-studies.

Gender Differences in the Structure of Marital Quality.

Marriages consist of shared experiences and interactions between husbands and wives that may lead to different impressions of the quality of the relationship. Few studies, unfortunately, have tested gender differences in the structure of marital quality, and even fewer studies have evaluated whether genetic and environmental influences on marital quality differ across gender. In this study, we evaluated gender differences in the structure of marital quality using independent samples of married male (n = 2406) and married female (n = 2215) participants from the National Survey of Midlife Development in the United States who provided ratings on twenty-eight marital quality items encompassing six marital quality constructs. We further explored gender differences in genetic and environmental influences on marital quality constructs in a subsample of 491 pairs of twins. Results suggest partial metric invariance across gender but structural variability in marital quality constructs. Notably, correlations between constructs were stronger in women than men. Results also support gender differences in the genetic and environmental influences on different aspects of marital quality. We discuss that men and women may approach and react to marriage differently as the primary reason why we observed differences in the structure of marital quality.

Rereading 'Biogenetics of Race and Class' 50 Years Later.

In 1968, long before the publication of Stephen J. Gould's The Mismeasure of Man, Herrnstein and Murray's The Bell Curve, or Arthur Jensen's Bias in Mental Testing, Irving Gottesman published a book chapter that addressed head-on the issues that would define the relationship between the genetics of social behavior and large-scale social theory for the next 50 years. That he could do so with his characteristic scholarly thoroughness and scientific tough-mindedness without once lapsing into regressive hereditarianism is a testimony to the scope of his scientific knowledge and the generosity of his intellectual spirit.

Family environment and the malleability of cognitive ability: a Swedish national home-reared and adopted-away cosibling control study.

Cognitive ability strongly aggregates in families, and prior twin and adoption studies have suggested that this is the result of both genetic and environmental factors. In this study, we used a powerful design--home-reared and adopted-away cosibling controls--to investigate the role of the rearing environment in cognitive ability. We identified, from a complete national Swedish sample of male-male siblings, 436 full-sibships in which at least one member was reared by one or more biological parents and the other by adoptive parents. IQ was measured at age 18-20 as part of the Swedish military service conscription examination. Parental educational level was rated on a 5-point scale. Controlling for clustering of offspring within biological families, the adopted siblings had an IQ 4.41 (SE = 0.75) points higher than their nonadopted siblings. Each additional unit of rearing parental education was associated with 1.71 (SE = 0.44) units of IQ. We replicated these results in 2,341 male-male half-sibships, in which, controlling for clustering within families, adoption was associated with a gain of IQ of 3.18 (SE = 0.34) points. Each additional unit of rearing parental education was associated with 1.94 (SE = 0.18) IQ units. Using full- and half-sibling sets matched for genetic background, we found replicated evidence that (i) rearing environment affects IQ measured in late adolescence, and (ii) a portion of the IQ of adopted siblings could be explained by the educational level of their adoptive parents.

Interaction between Parental Education and Twin Correlations for Cognitive Ability in a Norwegian Conscript Sample.

We examine a sample of Norwegian twin conscripts for evidence of an interaction between parental education and the heritability of general cognitive ability (GA). Ability scores were obtained on 1706 pairs of twins who were conscripted into the Norwegian Armed Forces between 1931and 1960. Education scores were available for mothers and fathers; the majority of the parents had less than a high school education. GA scores were heteroscadistic with respect to mid-parent education, with reduced variability at higher levels of education. Both MZ and DZ twin correlations for GA were linearly and negatively related to mid-parent education, DZ twins substantially more so. When the model was extended to an ACE model consisting of standardized positive ACE variance components, the modification appeared to disappear. Further analysis revealed that this occurred because the steep decline of DZ twin correlations with increasing mid-parent education resulted in a violation of the classical twin model for much of the parameter space. Other phenomena that might result in large declines in DZ twin correlations are considered, along with implications for other studies of socioeconomic interactions with the heritability of GA in European samples.