The genetic analysis of repeated measures. I. Simplex models

The stability and change of wellbeing across the lifespan: a longitudinal twin-sibling study

BACKGROUND

Wellbeing is relatively stable over the life span. However, individuals differ in this stability and change. One explanation for these differences could be the influence of different genetic or environmental factors on wellbeing over time.

METHODS

To investigate causes of stability and change of wellbeing across the lifespan, we used cohort-sequential data on wellbeing from twins and their siblings of the Netherlands Twin Register (NTR) (total N = 46.885, 56% females). We organized wellbeing data in multiple age groups, from childhood (age 5), to adolescence, up to old age (age 61+). Applying a longitudinal genetic simplex model, we investigated the phenotypic stability of wellbeing and continuity and change in genetic and environmental influences.

RESULTS

Wellbeing peaked in childhood, decreased during adolescence, and stabilized during adulthood. In childhood and adolescence, around 40% of the individual differences was explained by genetic effects. The heritability decreased toward old adulthood (35-24%) and the contribution of unique environmental effects increased to 76%. Environmental innovation was found at every age, whereas genetic innovation was only observed during adolescence (10-18 years). In childhood and adulthood, the absence of genetic innovation indicates a stable underlying set of genes influencing wellbeing during these life phases.

CONCLUSION

These findings provide insights into the stability and change of wellbeing and the genetic and environmental influences across the lifespan. Genetic effects were mostly stable, except in adolescence, whereas the environmental innovation at every age suggests that changing environmental factors are a source of changes in individual differences in wellbeing over time.

Midlife cumulative deficit frailty predicts Alzheimer's disease-related plasma biomarkers in older adults

BACKGROUND

The study explores whether frailty at midlife predicts mortality and levels of biomarkers associated with Alzheimer's disease and related dementias (ADRD) and neurodegeneration by early old age. We also examine the heritability of frailty across this age period.

METHODS

Participants were 1,286 community-dwelling men from the Vietnam Era Twin Study of Aging at average ages 56, 62 and 68, all without ADRD at baseline. The cumulative deficit frailty index (FI) comprised 37 items assessing multiple physiological systems. Plasma biomarkers at age 68 included beta-amyloid (Aβ40, Aβ42), total tau (t-tau) and neurofilament light chain (NfL).

RESULTS

Being frail doubled the risk of all-cause mortality by age 68 (OR = 2.44). Age 56 FI significantly predicted age 68 NfL (P = 0.014), Aβ40 (P = 0.001) and Aβ42 (P = 0.023), but not t-tau. Age 62 FI predicted all biomarkers at age 68: NfL (P = 0.023), Aβ40 (P = 0.002), Aβ42 (P = 0.001) and t-tau (P = 0.001). Age 68 FI scores were associated with age 68 levels of NfL (P = 0.027), Aβ40 (P < 0.001), Aβ42 (P = 0.001) and t-tau (P = 0.003). Genetic influences accounted for 45-48% of the variance in frailty and significantly contributed to its stability across 11 years.

CONCLUSIONS

Frailty during one's 50s doubled the risk of mortality by age 68. A mechanism linking frailty and ADRD may be through its associations with biomarkers related to neurodegeneration. Cumulative deficit frailty increases with age but remains moderately heritable across the age range studied. With environmental factors accounting for about half of its variance, early interventions aimed at reducing frailty may help to reduce risk for ADRD.

A Traitlike Dimension of Subjective Memory Concern Over 30 Years Among Adult Male Twins

Importance

Subjective memory concern has long been considered a state-related indicator of impending cognitive decline or dementia. The possibility that subjective memory concern may itself be a heritable trait is largely ignored, yet such an association would substantially confound its use in clinical or research settings.

Objective

To assess the heritability and traitlike dimensions of subjective memory concern and its clinical correlates.

Design, Setting, and Participants

This longitudinal twin cohort study was conducted from 1967 to 2019 among male adults with a mean (SD) age of 37.75 (2.52) years to follow-up at mean ages of 56.15 (2.72), 61.50 (2.43), and 67.35 (2.57) years (hereafter, 38, 56, 62, and 67 years, respectively) in the Vietnam Era Twin Study of Aging. The study included a national community-dwelling sample with health, education, and lifestyle characteristics comparable to a general sample of US men in this age cohort. Participants were monozygotic and dizygotic twins randomly recruited from the Vietnam Era Twin Registry. Data were analyzed from May 2021 to December 2022.

Main Outcomes and Measures

Measures included subjective memory concern at 4 time points; objective memory, depressive symptoms, and anxiety at the last 3 time points; negative emotionality (trait neuroticism) at age 56 years; polygenic risk scores (PRSs) for neuroticism, depression, and Alzheimer disease; APOE genotype; and parental history of dementia. Primary outcomes were heritability and correlations between subjective memory concern and other measures.

Results

The sample included 1555 male adults examined at age 38 years, 520 at age 56 years (due to late introduction of subjective memory concern questions), 1199 at age 62 years, and 1192 at age 67 years. Phenotypically, subjective memory concerns were relatively stable over time. At age 56 years, subjective memory concern had larger correlations with depressive symptoms (r, 0.32; 95% CI, 0.21 to 0.42), anxiety (r, 0.36; 95% CI, 0.18 to 0.51), and neuroticism (r, 0.34; 95% CI, 0.26 to 0.41) than with objective memory (r, -0.24; 95% CI, -0.33 to -0.13). Phenotypic results were similar at ages 62 and 67 years. A best-fitting autoregressive twin model indicated that genetic influences on subjective memory concern accumulated and persisted over time (h2 = 0.26-0.34 from age 38-67 years). At age 56 years, genetic influences for subjective memory concern were moderately correlated with genetic influences for anxiety (r, 0.36; 95% CI, 0.18 to 0.51), negative emotionality (r, 0.51; 95% CI, 0.44-0.57), and depressive symptoms (r, 0.20; 95% CI, 0.10 to 0.29) as well as objective memory (r, -0.22; 95% CI, -0.30 to -0.14). Similar genetic correlations were seen at ages 62 and 67 years. The neuroticism PRS was associated with subjective memory concern at age 38 years (r, 0.10; 95% CI, 0.03. to 0.18) and age 67 years (r, 0.09; 95% CI, 0.01 to 0.16). Subjective memory concern was not associated with any Alzheimer disease risk measures.

Conclusions and Relevance

This cohort study found stable genetic influences underlying subjective memory concern dating back to age 38 years. Subjective memory concern had larger correlations with affect-related measures than with memory-related measures. Improving the utility of subjective memory concern as an indicator of impending cognitive decline and dementia may depend on isolating its statelike component from its traitlike component.

The Development of Disgust and Its Relationship to Adolescent Psychosocial Functioning

Experiences of excessive disgust have been implicated in several psychopathologies. Research, however, has rarely examined disgust and its role in psychosocial functioning from a developmental standpoint. This study examines the relationship between disgust expression in early life and subsequent adolescent psychosocial functioning. Data from 165 children were collected as part of a longitudinal study. Disgust propensity in infancy and childhood was assessed using a facial expressivity task and food aversion task, respectively. Adolescent psychosocial functioning was measured through several self-report measures. Results suggest that there exists a degree of consistency in disgust expression within the first year of life, and that childhood disgust propensity may be related to impairment in early adolescent psychosocial functioning. These findings highlight the potential importance of identifying early disgust expression as a marker for later psychopathology.

Determining the stability of genome-wide factors in BMI between ages 40 to 69 years

Genome-wide association studies (GWAS) have successfully identified common variants associated with BMI. However, the stability of aggregate genetic variation influencing BMI from midlife and beyond is unknown. By analysing 165,717 men and 193,073 women from the UKBiobank, we performed BMI GWAS on six independent five-year age intervals between 40 and 72 years. We then applied genomic structural equation modeling to test competing hypotheses regarding the stability of genetic effects for BMI. LDSR genetic correlations between BMI assessed between ages 40 to 73 were all very high and ranged 0.89 to 1.00. Genomic structural equation modeling revealed that molecular genetic variance in BMI at each age interval could not be explained by the accumulation of any age-specific genetic influences or autoregressive processes. Instead, a common set of stable genetic influences appears to underpin genome-wide variation in BMI from middle to early old age in men and women alike.

The Impact of Genes and Environment on Brain Ageing in Males Aged 51 to 72 Years

Magnetic resonance imaging data are being used in statistical models to predicted brain ageing (PBA) and as biomarkers for neurodegenerative diseases such as Alzheimer's Disease. Despite their increasing application, the genetic and environmental etiology of global PBA indices is unknown. Likewise, the degree to which genetic influences in PBA are longitudinally stable and how PBA changes over time are also unknown. We analyzed data from 734 men from the Vietnam Era Twin Study of Aging with repeated MRI assessments between the ages 51-72 years. Biometrical genetic analyses "twin models" revealed significant and highly correlated estimates of additive genetic heritability ranging from 59 to 75%. Multivariate longitudinal modeling revealed that covariation between PBA at different timepoints could be explained by a single latent factor with 73% heritability. Our results suggest that genetic influences on PBA are detectable in midlife or earlier, are longitudinally very stable, and are largely explained by common genetic influences.

The Augmented Classical Twin Design: Incorporating Genome-Wide Identity by Descent Sharing Into Twin Studies in Order to Model Violations of the Equal Environments Assumption

The Classical Twin Method (CTM) compares the similarity of monozygotic (MZ) twins with that of dizygotic (DZ) twins to make inferences about the relative importance of genes and environment in the etiology of individual differences. The design has been applied to thousands of traits across the biomedical, behavioral and social sciences and is arguably the most widely used natural experiment known to science. The fundamental assumption of the CTM is that trait relevant environmental covariation within MZ pairs is the same as that found within DZ pairs, so that zygosity differences in within-pair variance must be due to genetic factors uncontaminated by the environment. This equal environments assumption (EEA) has been, and still is hotly contested, and has been mentioned as a possible contributing factor to the missing heritability conundrum. In this manuscript, we introduce a new model for testing the EEA, which we call the Augmented Classical Twin Design which uses identity by descent (IBD) sharing between DZ twin pairs to estimate separate environmental variance components for MZ and DZ twin pairs, and provides a test of whether these are equal. We show through simulation that given large samples of DZ twin pairs, the model provides unbiased estimates of variance components and valid tests of the EEA under strong assumptions (e.g. no epistatic variance, IBD sharing in DZ twins estimated accurately etc.) which may not hold in reality. Sample sizes in excess of 50,000 DZ twin pairs with genome-wide genetic data are likely to be required in order to detect substantial violations of the EEA with moderate power. Consequently, we recommend that the Augmented Classical Twin Design only be applied to datasets with very large numbers of DZ twin pairs (> 50,000 DZ twin pairs), and given the strong assumptions relating to the absence of epistatic variance, appropriate caution be exercised regarding interpretation of the results.

How nonshared environmental factors come to correlate with heredity

Conventional longitudinal behavioral genetic models estimate the relative contribution of genetic and environmental factors to stability and change of traits and behaviors. Longitudinal models rarely explain the processes that generate observed differences between genetically and socially related individuals. We propose that exchanges between individuals and their environments (i.e., phenotype-environment effects) can explain the emergence of observed differences over time. Phenotype-environment models, however, would require violation of the independence assumption of standard behavioral genetic models; that is, uncorrelated genetic and environmental factors. We review how specification of phenotype-environment effects contributes to understanding observed changes in genetic variability over time and longitudinal correlations among nonshared environmental factors. We then provide an example using 30 days of positive and negative affect scores from an all-female sample of twins. Results demonstrate that the phenotype-environment effects explain how heritability estimates fluctuate as well as how nonshared environmental factors persist over time. We discuss possible mechanisms underlying change in gene-environment correlation over time, the advantages and challenges of including gene-environment correlation in longitudinal twin models, and recommendations for future research.

Psychometric Modelling of Longitudinal Genetically Informative Twin Data

The often-used A(C)E model that decomposes phenotypic variance into parts due to additive genetic and environmental influences can be extended to a longitudinal model when the trait has been assessed at multiple occasions. This enables inference about the nature (e.g., genetic or environmental) of the covariance among the different measurement points. In the case that the measurement of the phenotype relies on self-report data (e.g., questionnaire data), often, aggregated scores (e.g., sum–scores) are used as a proxy for the phenotype. However, earlier research based on the univariate ACE model that concerns a single measurement occasion has shown that this can lead to an underestimation of heritability and that instead, one should prefer to model the raw item data by integrating an explicit measurement model into the analysis. This has, however, not been translated to the more complex longitudinal case. In this paper, we first present a latent state twin A(C)E model that combines the genetic twin model with an item response theory (IRT) model as well as its specification in a Bayesian framework. Two simulation studies were conducted to investigate 1) how large the bias is when sum–scores are used in the longitudinal A(C)E model and 2) if using the latent twin model can overcome the potential bias. Results of the first simulation study (e.g., AE model) demonstrated that using a sum–score approach leads to underestimated heritability estimates and biased covariance estimates. Surprisingly, the IRT approach also lead to bias, but to a much lesser degree. The amount of bias increased in the second simulation study (e.g., ACE model) under both frameworks, with the IRT approach still being the less biased approach. Since the bias was less severe under the IRT approach than under the sum–score approach and due to other advantages of latent variable modelling, we still advise researcher to adopt the IRT approach. We further illustrate differences between the traditional sum–score approach and the latent state twin A(C)E model by analyzing data of a two-wave twin study, consisting of the answers of 8,016 twins on a scale developed to measure social attitudes related to conservatism.

The gene-environmental architecture of the development of adolescent substance use

BACKGROUND

Using a longitudinal twin design and a latent growth curve/autoregressive approach, this study examined the genetic-environmental architecture of substance use across adolescence.

METHODS

Self-reports of substance use (i.e. alcohol, marijuana) were collected at ages 13, 14, 15, and 17 years from 476 twin pairs (475 boys, 477 girls) living in the Province of Quebec, Canada. Substance use increased linearly across the adolescent years.

RESULTS

ACE modeling revealed that genetic, as well as shared and non-shared environmental factors explained the overall level of substance use and that these same factors also partly accounted for growth in substance use from age 13 to 17. Additional genetic factors predicted the growth in substance use. Finally, autoregressive effects revealed age-specific non-shared environmental influences and, to a lesser degree, age-specific genetic influences, which together accounted for the stability of substance use across adolescence.

CONCLUSIONS

The results support and expand the notion that genetic and environmental influences on substance use during adolescence are both developmentally stable and developmentally dynamic.

The stability of educational achievement across school years is largely explained by genetic factors

Little is known about the etiology of developmental change and continuity in educational achievement. Here, we study achievement from primary school to the end of compulsory education for 6000 twin pairs in the UK-representative Twins Early Development Study sample. Results showed that educational achievement is highly heritable across school years and across subjects studied at school (twin heritability ~60%; SNP heritability ~30%); achievement is highly stable (phenotypic correlations ~0.70 from ages 7 to 16). Twin analyses, applying simplex and common pathway models, showed that genetic factors accounted for most of this stability (70%), even after controlling for intelligence (60%). Shared environmental factors also contributed to the stability, while change was mostly accounted for by individual-specific environmental factors. Polygenic scores, derived from a genome-wide association analysis of adult years of education, also showed stable effects on school achievement. We conclude that the remarkable stability of achievement is largely driven genetically even after accounting for intelligence.

Sum Scores in Twin Growth Curve Models: Practicality Versus Bias

To study behavioral or psychiatric phenotypes, multiple indices of the behavior or disorder are often collected that are thought to best reflect the phenotype. Combining these items into a single score (e.g. a sum score) is a simple and practical approach for modeling such data, but this simplicity can come at a cost in longitudinal studies, where the relevance of individual items often changes as a function of age. Such changes violate the assumptions of longitudinal measurement invariance (MI), and this violation has the potential to obfuscate the interpretation of the results of latent growth models fit to sum scores. The objectives of this study are (1) to investigate the extent to which violations of longitudinal MI lead to bias in parameter estimates of the average growth curve trajectory, and (2) whether absence of MI affects estimates of the heritability of these growth curve parameters. To this end, we analytically derive the bias in the estimated means and variances of the latent growth factors fit to sum scores when the assumption of longitudinal MI is violated. This bias is further quantified via Monte Carlo simulation, and is illustrated in an empirical analysis of aggression in children aged 3-12 years. These analyses show that measurement non-invariance across age can indeed bias growth curve mean and variance estimates, and our quantification of this bias permits researchers to weigh the costs of using a simple sum score in longitudinal studies. Simulation results indicate that the genetic variance decomposition of growth factors is, however, not biased due to measurement non-invariance across age, provided the phenotype is measurement invariant across birth-order and zygosity in twins.

Contributions of Genes and Environment to Developmental Change in Alcohol Use

The precise nature of how genetic and environmental risk factors influence changes in alcohol use (AU) over time has not yet been investigated. Therefore, the aim of the present study is to examine the nature of longitudinal changes in these risk factors to AU from mid-adolescence through young adulthood. Using a large sample of male twins, we compared five developmental models that each makes different predictions regarding the longitudinal changes in genetic and environmental risks for AU. The best-fitting model indicated that genetic influences were consistent with a gradual growth in the liability to AU, whereas unique environmental risk factors were consistent with an accumulation of risks across time. These results imply that two distinct processes influence adolescent AU between the ages of 15-25. Genetic effects influence baseline levels of AU and rates of change across time, while unique environmental effects are more cumulative.

A Note on the Scope of Developmental Behaviour Genetics

The behaviour genetic decomposition of individual differences has been presented as being irrelevant to the study of human behavioural ontogeny. This introduces two problems. First, the analysis of systematic differences constitutes the basis for most statistical models used in the social sciences. If, generally speaking, this type of analysis is uninformative regarding development, how then can one empirically investigate human development? Second, behaviour genetic analyses are the only way to arrive at meaningful statements regarding the contributions of heredity and environment to human development. If results thus obtained are irrelevant, it is impossible to say anything on the subject of heredity, environment, and human ontogeny that is both meaningful and informative.

It is argued that developmental behaviour genetics should not be viewed as a theory of development, but rather as a method of testing certain well-defined hypotheses regarding the contributions of genetic and environmental influences to human development.

Individual differences assessed at any point in time reflect developmental processes prior to that time-gene-environment models are in a very basic sense inherently developmental... (Loehlin, 1975, p.41).

Obviously the finding of innate differences in behaviour does not illuminate the development of that behaviour in any way (Johnston, 1988, p. 623).

Person-Specific Non-shared Environmental Influences in Intra-individual Variability: A Preliminary Case of Daily School Feelings in Monozygotic Twins

Most behavioural genetic studies focus on genetic and environmental influences on inter-individual phenotypic differences at the population level. The growing collection of intensive longitudinal data in social and behavioural science offers a unique opportunity to examine genetic and environmental influences on intra-individual phenotypic variability at the individual level. The current study introduces a novel idiographic approach and one novel method to investigate genetic and environmental influences on intra-individual variability by a simple empirical demonstration. Person-specific non-shared environmental influences on intra-individual variability of daily school feelings were estimated using time series data from twenty-one pairs of monozygotic twins (age = 10 years, 16 female pairs) over two consecutive weeks. Results showed substantial inter-individual heterogeneity in person-specific non-shared environmental influences. The current study represents a first step in investigating environmental influences on intra-individual variability with an idiographic approach, and provides implications for future behavioural genetic studies to examine developmental processes from a microscopic angle.

Widowhood and the Stability of Late Life Depressive Symptomatology in the Swedish Adoption Twin Study of Aging

Although the Swedish Adoption Twin of Aging (SATSA) has been used to investigate phenotypic stability of late life depressive symptoms, the biometric processes underlying this stability have not been studied. Under a reciprocal effects modeling framework, we used SATSA twins' Center for Epidemiologic Studies Depression (CES-D) Scale data across 5 waves (from 1987-2007) to test whether the reciprocal exchange between twins within a family and their nonshared environments (P<=>E) promote the accumulation of gene-environment correlation (rGE) over time. The model generates increasing rGE that produces subsequent stable environmental differences between twins within a family-a process hypothesized to explain stability in chronic late life depressive symptoms. Widowhood is included as a stressful life experience that may introduce an additional nonshared source of variability in CES-D scores. Genetic effects and nonshared environmental effects are primary sources of stability of late life depressive symptoms without evidence of underlying rGE processes. Additionally, widowhood explained stable differences in CES-D scores between twins within a family up to 3 years after spousal loss.

Familial influences on the full range of variability in attention and activity levels during adolescence: A longitudinal twin study

To investigate familial influences on the full range of variability in attention and activity across adolescence, we collected maternal ratings of 339 twin pairs at ages 12, 14, and 16, and estimated the transmitted and new familial influences on attention and activity as measured by the Strengths and Weaknesses of Attention-Deficit/Hyperactivity Disorder Symptoms and Normal Behavior Scale. Familial influences were substantial for both traits across adolescence: genetic influences accounted for 54%-73% (attention) and 31%-73% (activity) of the total variance, and shared environmental influences accounted for 0%-22% of the attention variance and 13%-57% of the activity variance. The longitudinal stability of individual differences in attention and activity was largely accounted for by familial influences transmitted from previous ages. Innovations over adolescence were also partially attributable to familial influences. Studying the full range of variability in attention and activity may facilitate our understanding of attention-deficit/hyperactivity disorder's etiology and intervention.

Genetic and Environmental Stability of Neuroticism From Adolescence to Adulthood

Longitudinal studies of neuroticism have shown that, on average, neuroticism scores decrease from adolescence to adulthood. The heritability of neuroticism is estimated between 0.30 and 0.60 and does not seem to vary greatly as a function of age. Shared environmental effects are rarely reported. Less is known about the role of genetic and environmental influences on the rank order stability of neuroticism in the period from adolescence to adulthood. We studied the stability of neuroticism in a cohort sequential (classical) twin design, from adolescence (age 14 years) to young adulthood (age 32 years). A genetic simplex model that was fitted to the longitudinal neuroticism data showed that the genetic stability of neuroticism was relatively high (genetic correlations between adjacent age bins >0.9), and increased from adolescence to adulthood. Environmental stability was appreciably lower (environmental correlations between adjacent age bins were between 0.3 and 0.6). This low stability was largely due to age-specific environmental variance, which was dominated by measurement error. This attenuated the age-to-age environmental correlations. We constructed an environmental covariance matrix corrected for this error, under the strong assumption that all age-specific environmental variance is error variance. The environmental (co)variance matrix corrected for attenuation revealed highly stable environmental influences on neuroticism (correlations between adjacent age bins were between 0.7 and 0.9). Our results indicate that both genetic and environmental influences have enduring effects on individual differences in neuroticism.

Testing Models for the Contributions of Genes and Environment to Developmental Change in Adolescent Depression

We tested two models to identify the genetic and environmental processes underlying longitudinal changes in depression among adolescents. The first assumes that observed changes in covariance structure result from the unfolding of inherent, random individual differences in the overall levels and rates of change in depression over time (random growth curves). The second assumes that observed changes are due to time-specific random effects (innovations) accumulating over time (autoregressive effects). We found little evidence of age-specific genetic effects or persistent genetic innovations. Instead, genetic effects are consistent with a gradual unfolding in the liability to depression and rates of change with increasing age. Likewise, the environment also creates significant individual differences in overall levels of depression and rates of change. However, there are also time-specific environmental experiences that persist with fidelity. The implications of these differing genetic and environmental mechanisms in the etiology of depression are considered.

Stability in symptoms of anxiety and depression as a function of genotype and environment: a longitudinal twin study from ages 3 to 63 years

BACKGROUND

The influence of genetic factors on major depressive disorder is lower than on other psychiatric disorders. Heritability estimates mainly derive from cross-sectional studies, and knowledge on the longitudinal aetiology of symptoms of anxiety and depression (SxAnxDep) across the lifespan is limited. We aimed to assess phenotypic, genetic and environmental stability in SxAnxDep between ages 3 and 63 years.

METHOD

We used a cohort-sequential design combining data from 49 524 twins followed from birth to age ⩾20 years, and from adolescence into adulthood. SxAnxDep were assessed repeatedly with a maximum of eight assessments over a 25-year period. Data were ordered in 30 age groups and analysed with longitudinal genetic models.

RESULTS

Over age, there was a significant increase during adolescence in mean scores with sex differences (women>men) emerging. Heritability was high in childhood and decreased to 30-40% during adulthood. This decrease in heritability was due to an increase in environmental variance. Phenotypic stability was moderate in children (correlations across ages ~0.5) and high in adolescents (r = 0.6), young adults (r = 0.7), and adults (r = 0.8). Longitudinal stability was mostly attributable to genetic factors. During childhood and adolescence there was also significant genetic innovation, which was absent in adults. Environmental effects contributed to short-term stability.

CONCLUSIONS

The substantial stability in SxAnxDep is mainly due to genetic effects. The importance of environmental effects increases with age and explains the relatively low heritability of depression in adults. The environmental effects are transient, but the contribution to stability increases with age.

Longitudinal stability in reading comprehension is largely heritable from grades 1 to 6

Reading comprehension is a foundational academic skill and significant attention has focused on reading development. This report is the first to examine the stability and change in genetic and environmental influences on reading comprehension across Grades 1 to 6. This developmental range is particularly important because it encompasses the timespan in which most children move from learning how to read to using reading for learning. Longitudinal simplex models were fitted separately for two independent twin samples (N = 706; N = 976). Results suggested that the shared environment contributed to variance in early but not later reading. Instead, stability in reading development was largely mediated by continuous genetic influences. Thus, although reading is clearly a learned skill and the environment remains important for reading development, individual differences in reading comprehension appear to be also influenced by a core of genetic stability that persists through the developmental course of reading.

Genetic and Environmental Stability of Intelligence in Childhood and Adolescence

The present study examined the genetic and environmental contributions to the temporal stability of verbal, non-verbal and general intelligence across a developmental period spanning childhood and adolescence (5–18 years). Longitudinal twin data collected in four different studies on a total of 1,748 twins, comprising 4,641 measurement points in total, were analyzed using genetic adaptations of the simplex model. The heterogeneity in the type of instrument used to assess psychometric intelligence across the different subsamples and ages allowed us to address the auxiliary question of how to optimally utilize the existing longitudinal data in the context of gene-finding studies. The results were consistent across domains (verbal, non-verbal and general intelligence), and indicated that phenotypic stability was driven primarily by the high stability of additive genetic factors, that the stability of common environment was moderate, and that the unique environment contributed primarily to change. The cross-subscale stability was consistently low, indicating a small overlap between different domains of intelligence over time. The high stability of additive genetic factors justifies the use of a linear combination of scores across the different ages in the context of gene-finding studies.

GE covariance through phenotype to environment transmission: an assessment in longitudinal twin data and application to childhood anxiety

We considered identification of phenotype (at occasion t) to environment (at occasion t + 1) transmission in longitudinal model comprising genetic, common and unique environmental simplex models (autoregressions). This type of transmission, which gives rise to genotype-environment covariance, is considered to be important in developmental psychology. Having established identifying constraints, we addressed the issue of statistical power to detect such transmission given a limited set of parameter values. The power is very poor in the ACE simplex, but is good in the AE model. We investigated misspecification, and found that fitting the standard ACE simplex to covariance matrices generated by an AE simplex with phenotype to E transmission produces the particular result of a rank 1 C (common environment) covariance matrix with positive transmission, and a rank 1 D (dominance) matrix given negative transmission. We applied the models to mother ratings of anxiety in female twins (aged 3, 7, 10, and 12 years), and obtained support for the positive effect of one twin's phenotype on the other twin's environment.

The Behavioural Genetics of Personality Development in Adulthood—Classic, Contemporary, and Future Trends

Behavioural genetic research has led to important advances in the field of personality psychology. When carried out on longitudinal data, behavioural genetic studies also offer promising ways to examine the genetic and environmental origins of personality stability and change. Here, we review the findings of longitudinal twin studies, discuss their implications for our understanding of adult personality development, and point out open questions that need to be addressed by future research. Three general conclusions stand out. First, there is a strong and relatively stable genetic foundation of individual differences in personality throughout the adult life span; second, environmental influences become more important and contribute to an increasing rank–order stability of personality traits from early to middle adulthood; and third, both genetic and nonshared environmental influences contribute to both stability and change in personality traits. Equipped with this knowledge, the most urgent tasks for the next generation of behavioural genetic studies on personality development will be to (i) identify measurable environmental factors that matter and (ii) to capture the interplay between genetic and environmental influences on personality stability and change throughout adulthood. Copyright © 2014 European Association of Personality Psychology

Exploring how nature and nurture affect the development of reading: an analysis of the Florida Twin Project on reading

Research on the development of reading skills through the primary school years has pointed to the importance of individual differences in initial ability as well as the growth of those skills. Additionally, it has been theorized that reading skills develop incrementally. The present study examined the genetic and environmental influences on 2 developmental models representing these parallel ideas, generalizing the findings to explore the processes of reading development. Participants were drawn from the Florida Twin Project on Reading, with a total of 2,370 pairs of twins representative of the state of Florida. Twins' oral reading fluency scores from school progress monitoring records collected in the fall of Grades 1-5 were used to model development. Results suggested that genetic influences on the development of reading are general, shared across the early school years, as well as novel, with new genetic influences introduced at each of the first 3 years of school. The shared environment estimates suggest a pattern of general influences only, suggesting environmental effects that are moderate and stable across development.

Genetic and environmental influences on chest circumference during infancy: a longitudinal study of japanese twins

BACKGROUND

Chest circumference (CC) is suggested to be a good indicator of early life nutrition, but little is known on the heritability of CC. Thus we analysed the effects of genetic and environmental factors on the development of CC in Japanese infants.

METHODS

CC was measured longitudinally from birth until 1 year of age in a cohort of 211 monozygotic and 160 dizygotic complete Japanese twin pairs born in 1989-2002. The data were analysed using applications of structural linear equation modelling for twin data.

RESULTS

No sex-specific differences in the variance components were found. Environmental factors unique to each twin explained the major part of the variation of CC at birth whereas environmental factors shared by co-twins were more important at 1-3 months of age. From 3 months of age, the effect of genetic factors become steadily stronger and they explained the majority of the variation at 1 year of age. Strong genetic continuity in the development of CC was found, but also new sets of genes were activated during the first year of life. The origin of the environmental part of the variation could be tracked before 3 months of age. A substantial part of common and specific environmental factors affecting CC affected also birthweight.

CONCLUSIONS

CC is sensitive to intrauterine environmental factors, but these effects diminish during the first year of life, at least if postnatal environment is good. CC can be a useful indicator when identifying newborns who have suffered suboptimal pre-natal conditions.

Genetic and environmental influences on personality profile stability: unraveling the normativeness problem

The present study is the first to disentangle the genetic and environmental influences on personality profile stability. Spanning a period of 10 years, we analyzed the etiology of 3 aspects of profile stability (overall profile stability, distinctive profile stability, and profile normativeness) using self- and peer reports from 539 identical and 280 fraternal twins reared together. This 3-wave multirater twin design allowed us to estimate the genetic and environmental effects on latent true scores of the 3 aspects of profile stability while controlling for method effects and random error. Consistent biometric results were only found for profile normativeness, whereas overall and distinctive profile stability scores turned out to be biased. Over time, we found personality profile normativeness to be relatively stable. This stability was due to both stable genetic and nonshared environmental effects, whereas innovative variance was completely explained by nonshared environmental effects. Our findings emphasize the importance of distinguishing between the different aspects of profile stability, since overall and distinctive stability scores are likely biased due to the normativeness problem. Yet indicating a person's similarity to the average person, the normativeness of a personality profile itself has a psychological meaning beyond socially desirable responding.

Models for the longitudinal genetic analysis of same-age twins: application to HDL cholesterol

Models are presented for the analysis of longitudinal data from same-age twins which permit the exploration of a remarkably diverse array of alternative explanations for continuity and change during development. Data of this type permit the detection of new sources of genetic or environmental covariation during development that are not expressed at earlier ages and, because they include the effects of age-specific genes, the resulting heritability estimates are more reliable than those obtained from relatives who differ in age. The proposed models were applied to measurements of HDL cholesterol obtained on 81 pairs of monozygotic (MZ) twins and 69 dizygotic (DZ) pairs at 11, 12.5 and 14 years of age. All three MZ co-twin correlations were substantially higher than the self correlations across occasions, suggesting that new sources of genetic or environmental covariation must be expressed during early adolescence. This interpretation was confirmed by analysis of the full covariance matrices which showed that only models which assumed the expression of new or age-specific genes could explain the observed pattern of covariation. Because they include the effects of age-specific genes, the resulting heritabilities (0.80–0.83) were substantially higher than many previous estimates.

Heritability of blood pressure increases during mental stress

We studied the influence of mental stress on the contributions of genes and environment to individual variation in systolic (SBP) and diastolic (DBP) blood pressure by structural equation modelling in 320 adolescent male and female twins. Blood pressure data were collected during rest and during a reaction time and a mental arithmetic task. Univariate analyses of SBP and DBP showed familial aggregation for blood pressure. A genetic explanation for this resemblance was most likely, although during rest conditions a model that attributed familial resemblance to shared environmental factors, also fitted the data. There was no evidence for sex differences in heritabilities. Multivariate analyses showed significant heterogeneity between sexes for the intercorrelations of the blood pressure data measured under different rest and task conditions. Multivariate genetic analyses were therefore carried out separately in males and females. For SBP and DBP in females and for SBP in males an increase in heritabilities was seen for blood pressure measured during stress, as compared to rest measurements. The influence of shared environ-mental factors decreased during stress. For DBP in males no significant contributions of shared environment were found. The multivariate analyses indicated that the same genetic and environmental influences are expressed during rest and stress conditions.

Genetic Factors Underlie Stability of Obsessive–Compulsive Symptoms

The contribution of genetic and environmental factors to the stability of obsessive–compulsive (OC) symptoms has not yet been established in adult population based samples. We obtained the Young Adult Self Report Obsessive–Compulsive Subscale in mono- and dizygotic twins from the population-based Netherlands Twin Register in 1991, 1995 and 1997 and the Padua Inventory Revised Abbreviated in 2002. Stability of OC symptoms was analyzed as a function of genetic and environmental components. Heritability of OC behavior was around 40% at each time-point, independent of the instrument used. OC behavior was moderately stable with correlations ranging between r = .2 (for 11-year intervals), .4 (for 4–5 year intervals) and .6 (for 2 year intervals). Genetic correlations across time were higher, varying between .4 and .9, indicating that the stability of OC symptoms is mainly due to stable genetic factors. This study showed a moderate heritability and stability for OC behavior in adults. Genetic stability across time is high.

Stable genetic effects on symptoms of alcohol abuse and dependence from adolescence into early adulthood

Relatively little is known about how genetic influences on alcohol abuse and dependence (AAD) change with age. We examined the change in influence of genetic and environmental factors which explain symptoms of AAD from adolescence into early adulthood. Symptoms of AAD were assessed using the four AAD screening questions of the CAGE inventory. Data were obtained up to six times by self-report questionnaires for 8,398 twins from the Netherlands Twin Register aged between 15 and 32 years. Longitudinal genetic simplex modeling was performed with Mx. Results showed that shared environmental influences were present for age 15-17 (57%) and age 18-20 (18%). Unique environmental influences gained importance over time, contributing 15% of the variance at age 15-17 and 48% at age 30-32. At younger ages, unique environmental influences were largely age-specific, while at later ages, age-specific influences became less important. Genetic influences on AAD symptoms over age could be accounted for by one factor, with the relative influence of this factor differing across ages. Genetic influences increased from 28% at age 15-17 to 58% at age 21-23 and remained high in magnitude thereafter. These results are in line with a developmentally stable hypothesis that predicts that a single set of genetic risk factors acts on symptoms of AAD from adolescence into young adulthood.

Genetics of head circumference in infancy: a longitudinal study of Japanese twins

OBJECTIVES

Previous studies have shown strong genetic influence to head circumference (HC), but still little is known on the development of genetic etiology of HC in infancy, especially in non-Caucasian populations. Thus, we decided to analyze the genetics of HC growth in Japanese infants.

METHODS

Longitudinal measures of HC were available from birth to 13 months of age in 206 monozygotic and 156 dizygotic complete twin pairs. Genetic modeling for twin data was used.

RESULTS

We found only little evidence for sex-specific differences in the genetics of HC and thus analyzed boys and girls together. After 5 months of age the heritability of HC was high, but before that age also a substantial common environmental component was present. Not only strong genetic persistence for HC was found but also a new genetic variation emerged. New environmental variation shared by co-twins affecting HC was found until 3 months of age, and this effect was further transmitted until 1 year of age.

CONCLUSIONS

HC and its growth are strongly genetically regulated. Largely, the same genetic factors affect the variation of HC at different ages, and new genetic variation emerged during the first year of life. Knowledge on the genetic component in the variation of HC may help to design tools for defining abnormal growth of HC in population-based screenings for related disorders.

Genetic association in multivariate phenotypic data: power in five models

This article concerns the power of various data analytic strategies to detect the effect of a single genetic variant (GV) in multivariate data. We simulated exactly fitting monozygotic and dizygotic phenotypic data according to single and two common factor models, and simplex models. We calculated the power to detect the GV in twin 1 data in an ANOVA of phenotypic sum scores, in a MANOVA, and in exploratory factor analysis (EFA), in which the common factors are regressed on the genetic variant. We also report power in the full twin model, and power of the single phenotype ANOVA. The results indicate that (1) if the GV affects all phenotypes, the sum score ANOVA and the EFA are most powerful, while the MANOVA is less powerful. Increasing phenotypic correlations further decreases the power of the MANOVA; and (2) if the GV affects only a subset of the phenotypes, the EFA or the MANOVA are most powerful, while sum score ANOVA is less powerful. In this case, an increase in phenotypic correlations may enhance the power of MANOVA and EFA. If the effect of the GV is modeled directly on the phenotypes in the EFA, the power of the EFA is approximately equal to the power of the MANOVA.

Stability of genetic influences on pulmonary function in a longitudinal study of octogenarian twins

Using data from the first four waves of the OCTO-Twin study (twins 80 + years), the present study investigated the stability and change of genetic and environmental contributions to pulmonary function. Using a genetic simplex model, variance in peak expiratory flow (PEF) at each wave was decomposed into additive genetic and nonshared (specific) environmental factors. Additionally, this analysis distinguished the source of these influences, either from previous waves (transmissions) or from novel influences at each wave (innovations). At each time point (except wave 1), the genetic variance was due to genetic transmissions from prior time points. Conversely, the specific environmental variance in PEF at each time point was mainly due to environmental innovations. These results imply that genetic factors contribute to the stability of pulmonary function over time whereas environmental factors contribute to its change.

The developmental etiology of high IQ

The genetic and environmental trends in IQ development were assessed in 483 same-sex twin pairs in the Colorado longitudinal twin study using maximum-likelihood model-fitting analysis. The twins were assessed periodically from ages 1 to 16. Results show a decreasing influence of shared environment and an increasing influence of heritability across development, with large and increasing age to age stability of genetic influences. Non-shared environment contributes almost exclusively to age to age change. Similar analyses were conducted designating the top 15% of the sample as having high IQ at each age. The developmental etiology of high IQ did not significantly differ from that found for the continuous measure in this relatively novel analysis. These results demonstrate early stability in etiological influences on IQ and have potential implications for gene-finding efforts, suggesting that samples selected for high IQ can be used to find genetic variation that will be applicable to the full range of the IQ distribution, although conclusive demonstration that the same genes are indeed involved was beyond the scope of this study.

A genome-wide scan for Eysenckian personality dimensions in adolescent twin sibships: psychoticism, extraversion, neuroticism, and lie

We report the first genome-wide scan of adolescent personality. We conducted a genome-wide scan to detect linkage for measures of adolescent Psychoticism, Extraversion, Neuroticism, and Lie from the Junior Eysenck Personality Questionnaire. Data are based on 1,280 genotyped Australian adolescent twins and their siblings. The highest linkage peaks were found on chromosomes 16 and 19 for Neuroticism, on chromosomes 1, 7, 10, 13 m, and 18 for Psychoticism, and on chromosomes 2 and 3 for Extraversion.

Variance components linkage analysis with repeated measurements

BACKGROUND

When subjects are measured multiple times, linkage analysis needs to appropriately model these repeated measures. A number of methods have been proposed to model repeated measures in linkage analysis. Here, we focus on assessing the impact of repeated measures on the power and cost of a linkage study.

METHODS

We describe three alternative extensions of the variance components approach to accommodate repeated measures in a quantitative trait linkage study. We explicitly relate power and cost through the number of measures for different designs. Based on these models, we derive general formulas for optimal number of repeated measures for a given power or cost and use analytical calculations and simulations to compare power for different numbers of repeated measures across several scenarios. We give rigorous proof for the results under the balanced design.

RESULTS

Repeated measures substantially improve power and the proportional increase in LOD score depends mostly on measurement error and total heritability but not much on marker map, the number of alleles per marker or family structure. When measurement error takes up 20% of the trait variability and 4 measures/subject are taken, the proportional increase in LOD score ranges from 38% for traits with heritability of approximately 20% to 63% for traits with heritability of approximately 80%. An R package is provided to determine optimal number of repeated measures for given measurement error and cost. Variance component and regression based implementations of our methods are included in the MERLIN package to facilitate their use in practical studies.

Genetic and environmental influences on the stability of withdrawn behavior in children: a longitudinal, multi-informant twin study

We examined the contribution of genetic and environmental influences on the stability of withdrawn behavior (WB) in childhood using a longitudinal multiple rater twin design. Maternal and paternal ratings on the withdrawn subscale of the Child Behavior Checklist (CBCL) were obtained from 14,889 families when the twins were 3, 7, 10 and 12 years old. A longitudinal psychometric model was fitted to the data and the fit of transmission and common factor models were evaluated for each variance component. WB showed considerable stability throughout childhood, with correlation coefficients ranging from about .30 for the 9-year time interval to .65 for shorter time intervals. Individual differences in WB as observed by the mother and the father were found to be largely influenced by genetic effects at all four time points, in both boys (50-66%) and girls (38-64%). Shared environmental influences explained a small to modest proportion (0-24%) of the variance at all ages and were slightly more pronounced in girls. Non-shared environmental influences were of moderate importance to the variance and slightly increased with age, from 22-28% at age 3 to 35-41% at age 12 years. The stability of WB was largely explained by genetic effects, accounting for 74% of stability in boys and 65% in girls. Shared environmental effects explained 7% (boys) and 17% (girls) of the behavioral stability. Most shared environmental effects were common to both raters, suggesting little influence of rater bias in the assessment of WB. The shared environmental effects common to both raters were best described by a common factor model, indicating that these effects are stable and persistent throughout childhood. Non-shared environmental effects accounted for the remaining covariance over time.

Familial resemblance and heritability

Familial resemblance arises when family members are more similar than unrelated pairs of individuals, and may be estimated in terms of correlations or covariances among family members. Multifactorial heritability (or generalized heritability) quantifies the strength of the familial resemblance and represents the percentage of variance that is due to all additive familial effects including additive genetic and those of the familial environment. However, the traditional concept of heritability, which may be more appropriately called the genetic heritability, represents only the percentage of phenotypic variance due to additive genetic effects. Resolving the various sources of familial resemblance entails other issues. For example, there may be major gene effects that are largely or entirely nonadditive, temporal or developmental trends, and gene-gene (epistasis) and gene-environment interactions. The design of a family study determines which of these sources are resolvable. For example, in nuclear families consisting of parents and offspring, the genetic and familial environmental effects are not resolvable because these relatives share both genes and environments. However, extended pedigree and twin and adoption designs allow separation of the heritable effects and, possibly, more complex etiologies, including interactions. Various factors affect the estimation and interpretability of heritabilities, for example, assumptions regarding linearity and additivity, assortative mating, and the underlying distribution of the data. Nonnormality of the data can lead to errors in hypothesis testing, although it yields reasonably unbiased estimates. Fortunately, these and other complications can be directly modeled in many of the sophisticated software packages available today in genetic epidemiology.

Genetic and environmental determination of tracking in subcutaneous fat distribution during adolescence

BACKGROUND

The distribution of fat and adipose tissue is an important predictor of disease risk. Variation in fat distribution during adolescence is correlated with fat distribution in adulthood.

OBJECTIVE

The objective was to gain insight into the relative contribution of genes and environment to the stability of subcutaneous fat distribution from early adolescence into young adulthood.

DESIGN

Ratio of trunk to extremity skinfold thickness (TER) data from the Leuven Longitudinal Twin Study (n = 105 Belgian twin pairs followed from 10 to 18 y of age) was entered into a longitudinal path analysis.

RESULTS

The best-fitting model included additive genetic sources of variance and nonshared environment. Heritabilities ranged between 84.3% (95% CI: 63.9-92.3%) and 88.6% (95% CI: 76.5-94.1%) in boys and between 78.4% (95% CI: 59.3-88.3%) and 88.3% (95% CI: 77.0-93.8%) in girls. The majority of the phenotypic tracking (boys: 0.40-0.78; girls: 0.38-0.72) could be attributed to the moderate-to-high genetic correlations (rG) (between 0.27-0.84 and 0.38-0.80 for the various age intervals in boys and girls, respectively). This rG could be attributed to both genetic sources of variance, which are the same throughout adolescence, as well as genetic sources of variance that are "switched-on" at a certain age, the effect of which is then transmitted to subsequent observations. Environmental correlations (rE) in boys ranged between 0.51 and 0.70 but contributed relatively little to phenotypic tracking because the amount of variance explained by the environment was low (11.4-15.7%). In girls rE was low to moderate at best (0.09-0.48).

CONCLUSION

Phenotypic tracking in subcutaneous fat distribution during adolescence is predominantly explained by additive genetic sources of variance.

Genes and Developmental Stabiltiy of Aggressive Behavior Problems at Home and School in a Community Sample of Twins Aged 7–12

Though behavioral genetic studies of aggression have implicated heritable and environmental factors, there is limited understanding of how these factors influence aggression across different settings and over time. Ratings for 732 twins were collected from parents and teachers during middle childhood and early adolescence. Total aggression scores on the Child Behavioral Checklist (CBCL) and Teacher Report Form (TRF) were examined at each age, across both settings, and developmentally. In this sample, aggressive behavior was moderately to largely heritable at each age within the home (.76-.84) and school (.42-.61). Across each age, ratings by parents and teachers were moderately correlated (.19-.36). Genetic and environmental effects that were limited to a particular setting were important etiological factors for aggressive behavior consistently within each setting, while only genetic factors influenced levels of aggression across both settings. Stability during these ages was due to genetic effects common to each age and the persistence of child-specific environmental experiences within each setting. These results suggest that genetic and environmental influences on children's aggressive behavior are largely setting specific. Levels of aggression seen consistently across both settings are due to genetic influences. Developmentally stable levels of aggressive behavior result from genetic influences common to all ages and individual environmental influences whose effects persist across ages.

Genetic time-series analysis identifies a major QTL for in vivo alcohol metabolism not predicted by in vitro studies of structural protein polymorphism at the ADH1B or ADH1C loci

After ingestion of a standardized dose of ethanol, alcohol concentrations were assessed, over 3.5 hours from blood (six readings) and breath (10 readings) in a sample of 412 MZ and DZ twins who took part in an Alcohol Challenge Twin Study (ACTS). Nearly all participants were subsequently genotyped on two polymorphic SNPs in the ADH1B and ADH1C loci known to affect in vitro ADH activity. In the DZ pairs, 14 microsatellite markers covering a 20.5 cM region on chromosome 4 that includes the ADH gene family were assessed, Variation in the timed series of autocorrelated blood and breath alcohol readings was studied using a bivariate simplex design. The contribution of a quantitative trait locus (QTL) or QTL's linked to the ADH region was estimated via a mixture of likelihoods weighted by identity-by-descent probabilities. The effects of allelic substitution at the ADH1B and ADH1C loci were estimated in the means part of the model simultaneously with the effects sex and age. There was a major contribution to variance in alcohol metabolism due to a QTL which accounted for about 64% of the additive genetic covariation common to both blood and breath alcohol readings at the first time point. No effects of the ADH1B*47His or ADH1C*349Ile alleles on in vivo metabolism were observed, although these have been shown to have major effects in vitro. This implies that there is a major determinant of variation for in vivo alcohol metabolism in the ADH region that is not accounted for by these polymorphisms. Earlier analyses of these data suggested that alcohol metabolism is related to drinking behavior and imply that this QTL may be protective against alcohol dependence.

Genetic and environmental determination of tracking in static strength during adolescence

The purpose of this study was to determine whether the observed phenotypic stability in static strength during adolescence, as measured by interage correlations in arm pull, is mainly caused by genetic and/or environmental factors. Subjects were from the Leuven Longitudinal Twin Study ( n = 105 pairs, equally divided over 5 zygosity groups). Arm-pull data were aligned on age at peak height velocity to attenuate the temporal fluctuations in interage correlations caused by differences in timing of the adolescent growth spurt. Developmental genetic models were fitted using structural equation modeling. After the data were aligned on age at peak height velocity, the annual interage correlations conformed to a quasi-simplex structure over a 4-yr interval. The best-fitting models included additive genetic and unique environmental sources of variation. Additive genetic factors that already explained a significant amount of variation at previous measurement occasions explained 44.3 and 22.5% of the total variation at the last measurement occasion in boys and girls, respectively. Corresponding values for unique environmental sources of variance are 31.2 and 44.5%, respectively. In conclusion, the observed stability of static strength during adolescence is caused by both stable genetic influences and stable unique environmental influences in boys and girls. Additive genetic factors seem to be the most important source of stability in boys, whereas unique environmental factors appear to be more predominant in girls.

Genetic and Environmental Causes of Tracking in Explosive Strength during Adolescence

PURPOSE

To determine whether the observed phenotypic stability in explosive strength during adolescence, as measured by inter-age correlations in vertical jump (VTJ), is mainly caused by genetic and/or environmental factors.

METHODS

Subjects are from the Leuven Longitudinal Twin Study (LLTS) (n = 105 pairs, equally divided over five zygosity groups). VTJ data were aligned on age at peak height velocity (APHV) to attenuate the temporal fluctuations in inter-age correlations caused by differences in timing of the adolescent growth spurt. Simplex models were fitted using structural equation modelling.

RESULTS

After aligning the data on APHV, the annual inter-age correlations show a clear simplex structure over a 4 year interval. The best fitting models included additive genetic and unique environmental sources of variation. Heritability estimates ranged between 60.8% (CI 37.7%-77.2%) and 87.3% (CI 74.2%-94.0%) for boys and between 76.5% (CI 56.7%-89.0%) and 88.6% (CI 77.8%-94.1%) for girls. Up to 56.4% and 62.8% of the total variation at the last measurement occasion is explained by additive genetic factors that already explained a significant amount of variation at previous measurement occasions in boys and girls respectively. It thus can be concluded that the observed stability of explosive strength during adolescence is mainly caused by a stable genetic influence in boys and girls.

CONCLUSIONS

Additive genetic factors seem to be the main cause of the observed phenotypic stability in VTJ performance in boys and girls during adolescence.