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J K Hewitt

Publications and source records attributed to J K Hewitt.

At least 19 recordsLinked to original sources
PubMed1992

The analysis of parental ratings of children's behavior using LISREL.

A common procedure for assessing children's behavior is to obtain parental ratings of the child. Since the ratings obtained are a function of both parent and child, disentangling the child's phenotype from that of the rater becomes an important methodological problem. For the analysis of genetic and environmental contributions to children's behavior, solutions to this are available when multiple raters, e.g., two parents, rate multiple children, e.g., twins. This paper describes and illustrates simple LISREL models for the analysis of parental ratings of children's behavior. We show how the assumption that mothers and fathers are rating the same behavior in children can be contrasted with the weaker alternative that parents are rating correlated behaviors. Given the stronger assumption, which appears adequate for ratings of children's internalizing behavior problems, the contribution of rater bias and unreliability may be separated from the shared and nonshared environmental components of variation in a behavior genetic analysis.

Adolescent↗
PubMed1992

Childhood behavior problems: a comparison of twin and non-twin samples.

This study compares standardized measures of childhood behavior problems in a community-based twin sample with those for normative samples from the general population. Maternal parent ratings on the Child Behavior Checklist (CBCL) for 1824 twins were compared with the CBCL normative sample. The results indicated that twins showed small but consistently higher levels of problem behaviors. These elevations were significant for older children on both internalizing and externalizing behaviors; for younger children the elevations were significant for externalizing but not internalizing behaviors.

Adolescent↗
PubMed1992

Multivariate genetic analysis of blood pressure and body size. The Medical College of Virginia Twin Study.

BACKGROUND: In subjects of all ages, those who weigh the most often have the highest blood pressure. Thus, in epidemiological studies, weight is the most important correlate of blood pressure. Using the data from the Medical College of Virginia Twin Study, we asked these questions: 1) Do the same genetic paths that regulate body size also regulate systolic and diastolic blood pressure? 2) Are there distinct genetic pathways that regulate each of these variables? 3) Does environment play a major regulatory role? 4) Are the correlations among these variables mainly due to genetic or environmental effects? 5) Do genetic paths that regulate body size mediate the correlation between systolic blood pressure and diastolic blood pressure? METHODS AND RESULTS: We ascertained 253 Caucasian twin pairs living in the Commonwealth of Virginia. The average age was 11.2 +/- 0.2 years. We used multivariate path analyses to investigate the genetic relations among systolic blood pressure, diastolic blood pressure, and body size. We found that there was a highly significant genetic relation between systolic blood pressure and body size and between systolic and diastolic blood pressure. There are genetic paths that are shared within these two sets of variables, but in each case, the paths for each pair appear to be separate from one another. CONCLUSIONS: These analyses provide a method to partition correlation coefficients found in epidemiological studies into genetic and environmental components. The correlations found among these three variables are in large part due to these genetic relations. We found no genetic relation between diastolic blood pressure and body size.

Blood Pressure↗
PubMed1992

Evidence for independent genetic influences on obesity in middle age.

The National Heart, Lung, and Blood Institute (NHLBI) Twin Study provided longitudinal data on a cohort of 514 pairs of adult male twin pairs who were examined at approximate ages of 48, 57, and 63 years. Because the sample was selected from military veterans, height and weight data were also available from their induction physical examinations when they were approximately 20 years of age. From the total NHLBI Twin Study cohort, 124 monozygotic and 119 dizygotic male twin pairs had complete data available for both members of the pair at induction and three examinations spanning 43 years of adult life. Using these data, the contributions of genetics and shared and non-shared environmental factors to BMI over the 43 year period were estimated by model fitting procedures. Model fitting included both a factor decomposition of these effects as well as a developmental path model. Results from the decomposition procedure indicate significant genetic effects at each examination cycle. Fitting a developmental path model, two independent genetic contributions to the variability of BMI were found: one at, or prior to, the induction examination about age 20, and a second between ages 20 and 48. Significant non-shared environmental contributions at each examination were also indicated, but shared environmental effects were not significant. We conclude that cumulative genetic effects explain most of the tracking in obesity over time; non-shared environmental effects, although significant at each age, are relatively short-lived and make only a minor contribution to tracking.

Adult↗
PubMed1991

Bivariate genetic analysis of left ventricular mass and weight in pubertal twins (the Medical College of Virginia twin study).

Left ventricular (LV) hypertrophy in adults is a recognized risk factor for the subsequent development of cardiovascular morbidity. To make informed preventive health decisions it is important to understand the interaction of genes and environment on LV mass. In both children and adults, weight is a strong correlate of LV mass. We hypothesized that genetic influences common to both of these variables could in part explain the strong relation between weight and LV mass in children. In a population of 341 twins (11 years old), these questions were asked: (1) How much of the total variance of LV mass is under genetic control? (2) After accounting for weight and weight adjusted for sexual maturity, how much of the remaining variance is genetic? (3) Of the total genetic variance, what proportion is specific for LV mass and what proportion is common to both weight and LV mass? (4) How much of the correlation between these 2 variables is explained by genes common to both LV mass and weight? Univariate genetic analyses documented that genes operating at different magnitudes in boys (63%) and girls (71%) explain a significant proportion of the variance of LV mass. After removing the effect of weight and sexual maturity by regression methods, genes remain an important influence. Bivariate genetic analyses confirmed that genes common to LV mass and weight significantly influence the covariation of these variables and that greater than 90% of the correlation of LV mass and weight is due to common genes.

Body Mass Index↗
PubMed1991

Genetic analysis of cerebellar foliation patterns in mice (Mus musculus).

Four inbred strains of mice, DBA/2J, C57BL/10J, BALB/cJ, and SJL/J, were mated in a diallel cross. The cerebella of the F1 generation were examined for the presence (Type I) or absence (Type II) of an intraculminate fissure between vermian lobule IV and vermian lobule V (the ventral and dorsal lobules of the culmen). One strain (DBA/2J) consistently expressed the Type I pattern. Another strain (SJL/J) expressed predominantly the Type II pattern. The other two strains (C57BL/10J and BALB/cJ) and many of the hybrids exhibited variability in their expression of the foliation patterns. The results were analyzed using biometrical genetic procedures and showed significant additive and dominance genetic effects and a maternal effect. Correlations of these cerebellar anatomical variants with the development of behavior are discussed.

Animals↗
PubMed1991

Genetics of Type A behavior in two European countries: evidence for sibling interaction.

Young male twins in The Netherlands and England completed the Jenkins Activity Survey (Dutch and English versions, respectively), a measure of Type A behavior. Separate model fitting analysis revealed a similar pattern of variance estimates and associated goodness of fit across the two countries. The data were then analyzed concurrently, with a scalar parameter included to account for differences in variance due to the disparity of the measurement scales. A model including additive genetic and individual environmental effects gave a good explanation to the data. The heritability estimate was 0.28. Models of social interaction and dominance explained the data even better, the former being preferred. The twins' parents were included in the analysis to examine population variation for Type A behavior intergenerationally. There was evidence for individual environmental experiences having a greater influence on Type A behavior in the older generation.

Adolescent↗
PubMed1991

The structure of schizotypy: a pilot multitrait twin study.

This report of a pilot study examines 29 pairs of twins from a population-based registry on whom four domains of schizotypy have been measured: personal interview using the Structured Interview for Schizotypy, self-report questionnaire formed from eight published self-report scales, attentional battery of eight individual tests, and root mean square error on smooth pursuit eye tracking. Analyzing the twins as individuals revealed two independent dimensions of clinically rated schizotypy (positive symptom schizotypy and negative symptom schizotypy) and two independent dimensions of self-rated schizotypy (positive trait schizotypy and trait anhedonia). Positive symptom schizotypy was highly correlated with positive trait schizotypy, but not with attentional dysfunction or eye-tracking error. By contrast, negative symptom schizotypy was significantly related to trait anhedonia, attentional dysfunction, and eye-tracking error. Correlations in monozygotic and dizygotic twins suggested that genetic factors were important in all four domains of schizotypy. Except for eye-tracking error, the results are more consistent with a dimensional than a "disease" model of schizotypy. Replication of these results with a larger group of subjects is needed.

Adult↗
PubMed1991

A twin study approach towards understanding genetic contributions to body size and metabolic rate.

The genetic and environmental determinants of a brief assessment of metabolic rate at rest and under psychological stress were studied in 40 pairs of monozygotic and 40 pairs of dizygotic young adult male twins. Height, weight and age were employed as covariates. Univariate analyses showed a high heritability for height and weight and moderate heritability for metabolic rate. Classical twin analyses and multivariate genetic modeling indicated that genetic influences on resting metabolic rate were entirely explained by body weight: there was no independent genetic contribution to resting metabolic rate. Metabolic rate under psychological stress, on the other hand, showed a significant genetic effect. The exponent (3/4) in the power function relating body weight to resting metabolic rate was the same as that found in a wide variety of animal species, a value that has been proposed as defining a body weight set point. We speculate that an adult body weight set point is genetically transmitted. Independent genetic effects on resting metabolic rate would be observed only when the normal equilibrium between body weight and metabolic rate is unbalanced during development, aging or disease. The study illustrates the use of multivariate genetic analyses of twin data which may be readily applied to widely used metabolic rate assessments.

Adolescent↗
PubMed1990

Personality and reproductive fitness.

The relationship between reproductive success (number of biological children) and personality was explored in 1101 postmenopausal females from the Australian twin registry. The quadratic response surface relating fitness to extraversion (E) and neuroticism (N) showed a saddle point at intermediate levels of E and N. Selection was shown to be stabilizing, i.e., having an intermediate optimum, along the axis low E, low N-high E, high N and more mildly disruptive, having greater fitness in the extremes, along the axis low N, high E-high N, low E. Neither dimension of personality considered by itself showed a significant linear or quadratic relationship to reproductive success. Sections through the fitness surface, however, show selection tends to favor high neuroticism levels in introverts and low neuroticism levels in extroverts.

Family Characteristics↗
PubMed1990

Lipoprotein and oxygen transport alterations in passive smoking preadolescent children. The MCV Twin Study.

We investigated the cardiovascular effects of lifelong passive cigarette smoke exposure in preadolescent children and examined the following questions: 1) Is systemic oxygen transport altered? 2) Are coronary heart disease risk factors adversely affected? We recruited 216 families from the MCV Twin Study; 105 had at least one smoking parent. Serum thiocyanate and cotinine levels were used as measures of smoke exposure in the children and thiocyanate was proportional to the number of parental cigarettes smoked each day (p = 0.0001). Paternal smoking had no effect on these measures. Whole blood 2,3-diphosphoglycerate was higher in smoke-exposed than unexposed children (p less than 0.01) and was related to the thiocyanate level (p less than 0.02). High density lipoprotein (HDL) cholesterol was lower in passive smoking children (p less than 0.05); the HDL2 subfraction was reduced in passive smoking boys, while the HDL3 subfraction was reduced in passive smoking girls. Significant adverse alterations in systemic oxygen transport and lipoprotein profiles are already present in preadolescent children exposed to long-term passive cigarette smoke, primarily from maternal smoke. Children with long-term exposure to passive smoke may be at elevated risk for the development of premature coronary heart disease.

2,3-Diphosphoglycerate↗
PubMed1990

Genetic analysis of anthropometric measures in 11-year-old twins: the Medical College of Virginia Twin Study.

We have conducted a cross-sectional analysis of the genetic and environmental contributions to the variance of anthropometric measurements in children during early adolescence. Univariate path analysis was used to estimate the relative contributions of genes, individual environment, and family environment to measures of childhood obesity in 259 11-y-old Caucasian twin pairs. Triceps, subcapular, and suprailiac skinfold thicknesses, as well as waist circumferences, ht, and wt were measured in a standardized protocol. In this sample, a parsimonious model that included only additive genetic effects and environmental factors unique to the individual provided an adequate explanation for the variation in ht, wt, quetelet index, and subscapular and triceps skinfolds. In this largely preadolescent population, different magnitudes of genetic effects were seen in males and females for waist circumference, biiliac diameter, and suprailiac skinfold.

Anthropometry↗
PubMed1989

Univariate genetic analysis of blood pressure in children (the Medical College of Virginia Twin Study).

The relative contributions of genetic, individual environmental and shared environmental effects on resting blood pressure (BP) and heart rate (HR) were studied in prepubescent twins. The study population consisted of 251 caucasian 11-year-old twin pairs. Correlations were higher for all variables in monozygotic twins compared to dizygotic twins; this is consistent with a significant genetic effect. Path analysis revealed that the model of additive genetic and individual environmental effects fit systolic BP, diastolic BP and HR. In boys and girls, sex-specific genetic effects controlled systolic BP. The magnitudes of the sex-specific genetic effects on systolic BP were similar in both boys and girls and accounted for 66% of the variance. In boys, for diastolic BP, genetic effects accounted for 64% of the variance while in girls they accounted for 51%. These results provide no evidence for different genetic effects on HR in boys or girls. No shared environmental effects were detected. The large sample size and design, using different-sex dizygotic twins of the same age, establish that genes play an important role in the influence of resting BP and HR and that there are sex-specific genetic contributions in early pubertal children.

Blood Pressure↗
PubMed1989

Testing structural equation models for twin data using LISREL.

Simple genetic models can be fitted to twin data using software packages such as LISREL (Jöreskog and Sörbom, 1986a). After discussion of data preparation and routine checks on possible violation of assumptions of the twin method, we illustrate univariate, bivariate, and multivariate genetic models which can be tested in cross-sectional twin data using LISREL. These include models for cohort or cohabitation effects, genotype x sex interaction, and certain types of genotype x environment interaction and genotype-environment correlation.

Computer Simulation↗
PubMed1989

Fitting genetic models with LISREL: hypothesis testing.

A brief introduction to the mathematical theory involved in model fitting is provided. The properties of maximum-likelihood estimates are described, and their advantages in fitting structural models are given. Identification of models is considered. Standard errors of parameter estimates are compared with the use of likelihood-ratio (L-R) statistics. For structural modeling, L-R tests are invariant to parameter transformation and give robust tests of significance. Some guidelines for fitting models to data collected from twins are given, with discussion of the relative merits of parsimony and data description.

Computer Simulation↗
PubMed1989

Bias in correlations from selected samples of relatives: the effects of soft selection.

Martin and Wilson (1982) describe two forms of sampling bias in twin studies. One is "hard selection," where individuals above a threshold participate, and those below do not. The second is "soft selection," where the probability of including a pair of relatives varies over the range of the character. We present an alternative model of soft selection which has strikingly different consequences for the resemblance between relatives. In general, the softer the threshold, the more the correlation resembles that in the underlying population. Results are presented where the probability of selection equals the cumulative distribution function of a normal distribution with 10% of the variance of the selected variable. In these circumstances, soft selection usually leads to less severely attenuated correlations than truncate selection.

Genetics, Behavioral↗
PubMed1989

Of biases and more in the study of twins reared together: a reply to Grayson.

Grayson (see the preceding paper) discusses some circumstances in which estimates of genetic and environmental parameters derived from the study of twins reared together may be biased and documents in those circumstances what the magnitude of the biases may be. As Grayson suggests, the points he makes have been made previously by various authors and issues such as the power to detect dominance have been analyzed at some length. This paper draws attention to some other sources of variation which Grayson does not consider but which have been considered by other writers and which might have somewhat different consequences for the estimation of shared environmental effects. The classical twin study has never been an end in itself, but it is the nucleus of a systematic genetic approach to the study of human behavior.

Genetic Variation↗