Blood glutathione and cysteine concentrations in twin children

Glutathione and cysteine are major antioxidants in blood that are associated with health and longevity. To ensure their measurement, careful attention to avoid auto-oxidation is necessary to stabilize the samples. Since no report of these compounds has been reported in children, our goal was to determine their levels of reduced and oxidized glutathione (GSH and GSSG) and cysteine (Cys and CSSC), To this end, 140 healthy children, ages 2 to 9 years from the Louisville Twin Study were studied. Blood samples were collected and analyzed for GSH, GSSG, Cys, and CSSC by our HPLC dual electrochemical method. The results showed that GSH and total GSH (GSH + GSSG) levels for monozygotic (MZ) twins were significantly higher (P < 0.001) than levels for dizygotic (DZ) twins. However, the opposite occurred for Cys and total Cys (Cys + CSSC) in that the levels were significantly higher for DZ twins than for MZ twins. (P < 0.005-0.013). In spite of this marked difference in zygosity, the within-pair correlations for twin pairs used for estimating heritability suggested that there was a major environmental influence for total GSH and total Cys. Finally. GSH levels were significantly lower for young (2-9 years) children than adults (P < 0.001).

Genetic and environmental influences on a measure of infant attachment security

A twin study of infant attachment security at age 24 months was conducted on archival data for a sample of 99 MZ pairs and 108 DZ pairs from the Louisville Twin Study. MZ concordance for attachment was 62.6%, which was significantly greater than the DZ concordance of 44.4%. Concordances were transformed into polychoric correlations, and LISREL was used to conduct a quantitative genetic analysis of the data. Results indicated that 25% of the variability in attachment was attributable to genetic factors, and the remaining 75% was attributable to non-shared environmental effects. No evidence was found for a contribution from shared environmental influences to attachment security. Possible concerns about the validity of twin methodology are addressed and various interpretations of the results are presented.

Preliminary results from a twin study of infant--caregiver attachment

Two pilot studies were conducted. First, the validity of a measure of infant-caregiver attachment for twins was assessed. Sixteen twin pairs from the Louisville Twin Study (LTS) were assessed in the Strange Situation (SS) at ages 19 or 25 months. Concordance between the LTS procedure and the SS procedure for assessing attachment was 78.1%, significantly greater than chance. Second, twin similarity for attachment as measured by the LTS procedure was assessed. Videotapes of 34 MZ pairs and 26 DZ pairs at ages 18 and 24 months were rated in terms of attachment behavior. MZ concordance for attachment was 67.6%, significantly greater than the DZ concordance rate of 38.5%. Results are discussed in the context of current debate in attachment theory.

Evidence for genetic influence on both cross-situation and situation-specific components of behavior

An assumption often made in the study of personality and in social psychology is that methods variance and situation-specific effects, as key components of measured behavioral variance, are environmental effects. The results of the present research refute that assumption. Nine measures-3 aspects of temperament measured in each of 3 ways-were obtained at age 24 months for twin sibships participating in the Louisville Twin Study. This report describes a new model that captures the unique information potentially available in such data, by combining multitrait-multimethod and twin-family analytic designs. The results indicated significant genetic influence on methods-situations components of variance along with genetic influence on traits. The findings support heuristics that include both situation-specific patterns of behavior and cross-situational consistencies.

Evidence for genetic influence on both cross-situation and situation-specific components of behavior

An assumption often made in the study of personality and in social psychology is that methods variance and situation-specific effects, as key components of measured behavioral variance, are environmental effects. The results of the present research refute that assumption. Nine measures-3 aspects of temperament measured in each of 3 ways-were obtained at age 24 months for twin sibships participating in the Louisville Twin Study. This report describes a new model that captures the unique information potentially available in such data, by combining multitrait-multimethod and twin-family analytic designs. The results indicated significant genetic influence on methods-situations components of variance along with genetic influence on traits. The findings support heuristics that include both situation-specific patterns of behavior and cross-situational consistencies.

Heredity and environment in phoneme articulation: hereditary and environmental contributions to articulation proficiency

Large twin samples and recent applications of multiple regression techniques to behavioral genetics methodology makes possible evaluation of genetic and environmental contributions to the articulation proficiency of individual phonemes. Factor analysis of the articulation scores from 256 MZ and DZ twins and 124 of their non-twin siblings (all children ranged from 2; 11 to 9; 8 years) were conducted to reduce a 50-item articulation test to a more manageable set of five articulation factors. The twins' factor scores were then analyzed using multiple regression procedures to determine the extent to which the individual factors resulted from genetic and/or environmental influences. The /r/ and /[symbol: see text], t[symbol: see text], d[symbol: see text]/ factors were found to have strong genetic components, while the /l, j, w/ factor was found to be strongly influenced by environmental sources of variation.

Quantitative genetic analysis of injury liability in infants and toddlers

A threshold model of latent liability was applied to infant and toddler twin data on total count of injuries sustained during the interval from birth to 36 months of age. A quantitative genetic analysis of estimated twin correlations in injury liability indicated strong genetic dominance effects, but no additive genetic variance was detected. Because interpretations involving overdominance have little research support, the results may be due to low order epistasis or other interaction effects. Boys had more injuries than girls, but this effect was found only for groups whose parents were prompted and questioned in detail about their children's injuries. Activity and impulsivity are two behavioral predictors of childhood injury, and the results are discussed in relation to animal research on infant and adult activity levels, and impulsivity in adult humans. Genetic epidemiological approaches to childhood injury should aid in targeting higher risk children for preventive intervention.

Insulin-like growth factor-I comparisons in healthy twin children

Insulin-like growth factor-I (IGF-I) levels in plasma were measured in healthy twin children. The within-pair correlation for 43 monozygotic pairs was r = 0.91 (P < or = 0.0001), an association significantly higher than that for same sex dizygotic pairs (r = 0.40; P < or = 0.06). The high correlation for monozygotic twins indicated a marked genetic influence on IGF-I levels. After correction for age and sex, the correlation between IGF-I level and height was r = 0.38 (P < or = 0.0001). These findings provide clear evidence that IGF-I levels correlate with height, a growth characteristic known to be genetically controlled.

Health consequences of behaviors: injury as a model

Injury is the third leading cause of death in the United States and the leading cause for children, adolescents, and young adults. Injury results from multiple factors and so may its prevention. The first and simplest approach toward preventing injuries has been to innovatively and aggressively apply a traditional public health model. Strategically, the goal has been to remove harmful agents of injury and to make the environment safer. Tactics such as public information, product regulation, legislative action, and the like have been credited with reductions in mortality and morbidity. To expand our understanding and our prevention strategies across multiple injuries, other scientific knowledge bases and intervention models from fields such as psychology and child development are being used to study childhood injury. These approaches show that in addition to environmental determinants, psychosocial factors involving both the care giver and the child are related to injury. The research programs described here illustrate the advantage of investigating psychosocial factors at both molar and molecular levels. General characteristics of mothers and children related to injury help define families at risk, as well as suggesting vehicles for intervention. Behavioral factors influencing risk perception highlight the etiology of increased risk in adolescence. Injury episodes, even slight, as well as "near injuries" and dangerous and risky behavior can be quantified and analyzed by retrospective ("postmortem") approaches yielding data on commonly occurring consequences (and the lack thereof) for minor injury. Finally, approaches that simulate dangerous situations can identify interaction patterns that result in childhood injury. Based on such research, we are coming to view injuries as the result of patterns of behaviors that develop and persist over time, and as such these patterns can be detected and, one hopes, altered before a serious medical event occurs. The role of the pediatrician after injury occurs is clear. With regard to prevention of injuries, pediatricians' roles are being defined by those individuals who have begun to investigate causes, educate families, and advocate for regulation and prevention. However, like the causes and methods for prevention, the disciplines involved in the study and prevention of injury are multiple. Such a multidisciplinary approach that considers multiple factors, theories, models, and interventions to prevent injury may be the approach that is as simple as possible.

Quantitative genetic analysis of longitudinal trends in height: preliminary results from the Louisville Twin Study

A preliminary series of quantitative genetic models was applied to a subset of longitudinal height data, spanning birth to maturity, gathered from twin families in the Louisville Twin Study. Descriptive Cholesky factor parameterization was found to give more satisfactory results than did a system of constraints based on a model of developmental transmission of a time-constant and time-specific factors. The results from application of two autosomal sex-limitation models are contrasted with those from a model specifying both autosomal and sex-chromosomal patterns of inheritance. The latter model was more conducive to parameter reduction. Although these models do not constitute conclusive tests of autosomal sex-limitation versus sex-linkage, the more parsimonious model is consistent with previous research suggesting a stature locus on the long arm of the Y chromosome. Heritability of height is estimated at about 90% or greater from 6 years of age on. Substantial and fairly constant longitudinal genetic correlations are found from 3 years of age on. Shared environmental effects unrelated to parental height were seen for birth length, corrected for gestational age, to height at 3 years of age, but these are not satisfactorily differentiated from possible twin effects in the present sample. The genetic consequences of assortative mating are emphasized since failure to take assortment into account can lead to overestimation of shared environmental effects and under-estimation of genetic effects. The results indicate that about 20% of within-gender variability for mature height can be attributed to the genetic consequences of assortment, even though the phenotypic marital correlation of 0.22 is quite modest. The importance of testing the assumption of multivariate normality underlying the application of the method of maximum-likelihood is also highlighted.

Children's behavioral inhibition over age and across situations: genetic similarity for a trait during change

Ratings of behaviors pertaining to inhibition were observed for 130 twins participating in a longitudinal study. Ratings were available for four ages (12, 18, 24, and 30 months) and from three sources at each age: direct observations obtained in a laboratory setting, direct observations obtained in conjunction with infant mental testing, and a temperament measure from a questionnaire completed by parents. For the individual twins, the age-to-age correlations were in the moderate range (.26 to .64). The situation-to-situation correlations were generally in the same range (.17 to .64). When the twins were recombined into twin pairs, within-pair (intraclass) correlations indicated that monozygotic (MZ) twins were more concordant than dizygotic (DZ) twins for each of the behaviors at each of the ages. Also, the MZ twins were more concordant for the direction and degree of behavioral change from age to age or from situation to situation. These data provide additional evidence for the biological influence on behavioral inhibition, a characteristic that has been studied in temperament and personality research. The results suggest that the trait of behavioral inhibition and a change in the trait are genetically conditioned. In addition, it is suggested that the concept of trait be expanded to include the person-centered biological regulation of change.

Assessment of infant mental development: toward a broader perspective

The limitations on predictions of intelligence provided by single-point neonatal and infant assessments continue to hamper efforts to certify the clinical outcome of children at risk. Although new methods of clinical assessment show promise of offsetting these limitations, each method alone has not markedly improved predictions. From a developmental perspective, a strategy for assessment should include a broad range of measures combined empirically to form a developmental risk registry. The strategy takes into account evolving changes, as well as levels of competencies of the infant, and considers these within the evolving environmental context of the family.

Developmental research of twin's temperament

In the Louisville Twin Study, pairs of 3- and 4-year-old twins were provided with standardized competitive or cooperative tasks in a laboratory setting. Some tasks required a pair of twins to share toys; other fostered a more competitive engagement between the twins. Behavioral ratings identified temperament and social components at both ages, and between the two ages, there were transformations in the links between the components. Parental ratings of the twins' temperament at the same ages were moderately correlated with the laboratory observations, but the pattern of the relations changed from one age to the next. The combined sets of measures were subjected to twin analyses for 43 pairs of twins. The results are discussed in terms of the similarity of MZ and DZ pairs for the dimensions of temperament and the transformations of temperament.

Multimethod assessment of temperament in twins: birth to six months

The predictive relationship between neonatal behavior and temperament at 6 months was assessed in 62 pairs of twins. The neonatal assessment evaluated irritability, resistance to soothing, reactivity, and activity level across various activities. Temperament at 6 months was appraised by a laboratory assessment, ratings on the Infant Behavior Record, and a questionnaire completed by the mother. Examination for temperament consistency from the neonatal period to 6 months indicated that neonates rated by an examiner as irritable and difficult to soothe were reported by the mother to be more negative, irritable, and slower to adapt at 6 months; neonates rated as more irritable and active were more attentive and responsive during mental testing at 6 months; there was no relationship between the neonatal measures and the laboratory ratings, possibly reflecting methodological problems influenced by maturation and vegetative processes at 6 months. Analyses for patterns of concordance within the various data sets for MZ and DZ twins indicated that there was no pattern of differential concordance favoring MZ twins for the temperament measures at birth or at 6 months. It was concluded that a modest predictive relationship between temperament variables had been demonstrated from birth to 6 months, but no specific genetic effects were detected at these early ages.

Twin similarity in the developmental transformations of infant temperament as measured in a multi-method, longitudinal study

In the Louisville Twin Study, laboratory observations of twin infants' temperament at 12, 18, and 24 months were linked with parental ratings from temperament questionnaires. Core dimensions of temperament were extracted by factor analysis applied to each set of measures at each age. The laboratory temperament dimension was recurrently represented by emotional tone, social orientation, attentiveness, and reaction to restraint. The questionnaire temperament dimension was recurrently represented by mood, approach/withdrawal and adaptability. The laboratory and questionnaire dimensions were found to be correlated at each age (convergent correlations: 0.38 to 0.52) and to be stable across ages (stability correlations 0.37 to 0.66). The temperament dimensions were used to demonstrate that temperament profiles were more concordant for identical than for fraternal twin pairs. The results demonstrate the genetic influences on the primary dimensions of temperament and the developmental transformations of temperament.

A longitudinal twin study of stability of components from Bayley's Infant Behavior Record

Factor scores from Bayley's Infant Behavior Record, obtained from about 300-400 infant twins at 6, 12, 18, and 24 months, were selected to represent 3 aspects of infant behavior: task orientation, test affect-extraversion, and activity. Age-to-age correlations for the scores yielded a simplex pattern of low to moderate order. Further age-to-age analyses by twin pairs generally showed that the identical twins, as compared with same-sex fraternal twins, generated more within-pair similarities for profiles of the factor scores across 2 overlapping age ranges: 6-18 months, and 12-24 months. Overall, the findings indicated that there is reordering of individual differences for aspects of infant behavior from one age to the next, but the sequence of reordering is somewhat structured in accord with genotypic similarity. The longitudinal findings are also discussed within the context of change and continuity of the individuality of infant temperament.

Behavioral contrasts in twinships: stability and patterns of differences in childhood

Semistructured interviews with 305 mothers of same-sex twins were conducted periodically over the course of the twins' first 6 years. During each of the interviews, the mothers were asked to contrast the twins on a variety of specific behaviors. From the body of reports, 5 ages and 16 behaviors were selected for analysis as most illustrative of the main developmental trends for intrapair differences and similarities. For each of the behaviors, intrapair contrasts were moderately stable across age spans as long as 18-36 months. A set of behaviors related to negative aspects of temperament and a set of behaviors related to sociability were obtained at 6 months and were replicated at later ages. Concordance rates for the identical twin pairs were uniformly higher than for the fraternal pairs, suggesting that genotypic similarity was associated with intrapair similarity for aspects of emotionality and sociability.

Bayley's infant behavior record: behavioral components and twin analyses

Factor analysis of 25 rating scales from Bayley's infant behavior record were performed for a sample of about 300-400 infant twins tested at 1 or more of the following ages: 3, 6, 9, 12, 18, and 24 months. The analysis provided 5 major factors and 2 minor factors that were considerably consistent at all ages. As defined by the scales with the strongest loadings, the major factors were denoted as task orientation, affect-extraversion, activity, auditory-visual awareness, and motor coordination, and the minor factors were denoted as kinds of mouthing behaviors. Further analyses by twin pairs generally provided higher concordance for identical pairs than for same-sex fraternal pairs, and the differences in concordance became more evident with an increase in age. Among the factors, task orientation showed the most persistent differences between identical and fraternal pairs. Profile analyses for all factors extracted at each age indicated that the profiles within identical pairs were more similar than profiles within fraternal pairs. Overall, the findings delineate some of the dimensions of temperament observed during testing and suggest that there are genetic influences on several behavioral characteristics and the organization of those characteristics during infancy.

Hereditary determinants of manual torque

Data from a longitudinal study of twin children and siblings, 155 girls and 134 boys (aged 4 to 9 yr.), on a torque test confirmed that during this age period manually produced circling patterns change from clockwise to counterclockwise orientation. A genetic influence is suggested.

Appraisal of parental bias in twin studies. Ascribed zygosity and IQ differences in twins

A criticism of twin studies has been that the difference between the behavioral similarities of identical and fraternal twins is largely created by parental influences based on their perception of the twins' zygosity. This issue is examined for differences in the IQ scores found within pairs classified by parents and bloodtyping. The systematic differences in IQ scores could be attributed to zygosity classified by bloodtyping rather than by parental belief. The available evidence indicates that the twin method is still appropriate for human behavior genetics.

Relations between twins' similarity of appearance and behavioral similarity: testing an assumption

Questionnaires rating twins' physical similarity and similarity of dress were obtained from the parents of 121 identical and 70 same-sex fraternal twin pairs. Within-pair difference scores on several behavioral measures (two intelligence tests, two perceptual tests, one reading test, one test of speech articulation, and one personality inventory) were correlated with the twins' scores for physical similarity and similarity of dress. The correlations revealed no systematic relation between the similarity of appearance and the similarity of behaviors for either the identical twin pairs or the same-sex fraternal twin pairs. The assumption that twins' behaviors are more alike because they are more similar in appearance does not seem warranted.

Retardation and twin concordance in infant mental development: a reassessment

A reappraisal is made of Nichols and Broman's conclusion that there is no evidence that genetic influences are important in infant mental development if severely retarded twins are excluded from the sample. The reappraisal touches on certain idiosyncrasies of their sample, the test scores reported, and their definition of severe retardation. Data are presented for a sample of White twins who have been followed longitudinally since birth. For Bayley Mental Scale scores at 9 months of age, the monozygotic within-pair correlation was 0.85 and the dizygotic within-pair correlation was 0.62. With 13 retarded twins excluded, the correlations became 0.81 and 0.64, respectively, with the monozygotic correlation still being significantly larger (p less than 0.01). The present results indicate that genetic factors play a role in infant mental development.

Twins: within-pair similarity on Bayley's Infant Behavior Record

Summary scores for behaviors rated on Bayley's Infant Behavior Record were obtained by summing sequential ratings for same-sex twins participating in a longitudinal study. The first year scores were based on visits made at 3, 6, 9, and 12 months, by 55 identical and 27 fraternal pairs; the second year scores were based on visits at 18, 24, and 30 months, by 47 identical and 27 fraternal pairs. During the first year, within-pair correlations for almost all behaviors were stronger for identical than for fraternal pairs; in the second year, within-pair correlations for identical twins were similar to those of fraternal twins except for the more cognitive behaviors. Composite scores were calculated for two behavioral clusters--one composed of behaviors related to Primary Cognition, and the other composed of behaviors related to Extraversion. Within-pair correlations were higher for identical than for fraternal twins in both the first and second years, indicating stronger and more consistent genetic influences on the more cognitive behaviors.

Persons, situations, and time: a genetic view of behavioral change in children

Twin children participating in a longitudinal study were observed at 9, 12, 18, 24, and 30 months of age in two settings: unstructured free play and relatively structured test taking. Behaviors relating to adaptability to the two settings were rated and the scores analyzed for evidence of continuity both across settings and across ages. Age-to-age correlations for the adaptability scores for each setting yielded a simplex pattern of low-moderate order. Correlations of individuals' scores across settings were also of low-moderate order. Within-pair correlations indicated that identical twins were more similar that same-sex fraternal twins in both settings, but correlations were consistently stronger in the playroom setting. Analysis of variance was performed to test for within-pair concordance in the direction and degree of change. Identical pairs remained significantly more similar in adaptability from one setting to another even when change in adaptability occurred. Similar results were found for change across ages; however, these differences were significant only in the playroom setting. Results suggest that situation variables contribute to the low stabilities frequently reported for personality dimensions but that the direction and degree of behavioral change are genetically conditioned.

Cognitive and personality factors associated with the double trapezium illusion

76 pairs of 9- to 11-yr.-old twins (41 female, 35 male) who were discordant on the magnitude of error for the double trapezium illusion were given the Children's Personality Questionnaire and the Wechsler Intelligence Scale for Children (WISC). The Wilcoxon matched-pair signed-ranks test was used to determine whether the twins with relatively smaller magnitudes of error than their co-twins also manifested systematic differences on the personality factor scores and the WISC subtest scores. Among the female pairs, twins with smaller magnitudes of the illusion were relatively more stable (Factor C), more shrewd (Factor N), obtained higher scores on Similarities and Digit Span, and had longer reaction times before responding to the figures. Among the male pairs, similar trends were found but the differences were not as pronounced. The results are discussed in terms of Piaget's theory concerning children's deployment of attention and Kagan's reflective-impulsive dimension in children's cognitive judgments.

Sex and genetic differences in hair color changes during early childhood

Hair color was assessed routinely from three months to six years for children participating in a longitudinal study of twins: 169 female twin pairs, 161 male pairs, and 60 opposite-sex pairs. Age trends, established by sampling only one number of every pair, showed marked changes in hair color for both sexes, but there was a consistent excess of light-haired males and dark-haired females. Within-pair concordance rates were calculated for same-sex pairs whose zygosity had been determined independently through bloodtyping. A high rate of concordance was found for MZ twins at every age in spite of the general change in hair color, indicating a strong genetic influence in the timing of color changes. The results are discussed in terms of accelerated maturation of females, and the need for genetic models of the inheritance of hair color which are age- and sex-specific.

Persons, situations, and time: A genetic view of behavioral change in children

Twin children participating in a longitudinal study were observed at 9, 12, 18, 24, and 30 months of age in two settings: unstructured free play and relatively structured test taking. Behaviors relating to adaptability to the two settings were rated and the scores analyzed for evidence of continuity both across settings and across ages. Age-to-age correlations for the adaptability scores for each setting yielded a simplex pattern of low-moderate order. Correlations of individuals' scores across settings were also of low-moderate order. Within-pair correlations indicated that identical twins were more similar that same-sex fraternal twins in both settings, but correlations were consistently stronger in the playroom setting. Analysis of variance was performed to test for within-pair concordance in the direction and degree of change. Identical pairs remained significantly more similar in adaptability from one setting to another even when change in adaptability occurred. Similar results were found for change across ages; however, these differences were significant only in the playroom setting. Results suggest that situation variables contribute to the low stabilities frequently reported for personality dimensions but that the direction and degree of behavioral change are genetically conditioned.

Hereditary components of response to the double trapezium illusion

The double trapezium illusion was studied in 68 pairs of same-sex twins ranging in age from 9 to 11 yr. Serological tests indicated that there were 39 identical and 29 fraternal twin pairs. Intrapair differences in the magnitude of the illusion were significantly less for identical pairs, indicating an hereditary component in the illusion.

Articulation proficiency in twins and singletons from families of twins

Two hundred sixty-three twins and 94 singletons from families of twins were administered the 1960 version of the Templin-Darley Screening Test of Articulation for three to eight year olds. Mean scores for the twins fell significantly below the standard test norms. This study provides additional evidence that position of birth in large families, as well as SES level, can affect development in articulation for twins and singletons alike. The within-pair similarity of articulation was significantly greater for male identical twins than for male fraternal twins. No such difference was found between the female identical and fraternal twins.

Personality factors associated with the Ponzo illusion: study using the co-twin method

29 pairs of 9 to 11-yr.-old MZ twins (11 male, 18 female) who were discordant on the magnitude of their susceptibility to the Ponzo illusion were given the Children's Personality Questionnaire (CPQ). The Wilcoxon matched-pair signed-ranks test was used to determine whether the twins with relatively larger magnitudes of the Ponzo illusion than their co-twins also manifested systematic differences on CPQ factor scores. Among males, twins with larger magnitudes of the illusion were relatively more calm and stable (Factor C), conscientious (Factor G), critical, restrained and evaluative (Factor J); among females the twins with larger magnitudes of the illusion were more calm and stable (Factor C), more shrewd (Factor N), and less apprehensive and anxious (Factor O) than their co-twins. Results are discussed in terms of Piaget's theory of children's deployment of attention and Kagan's reflective-impulsive dimension in children's cognitive judgments.

Activity, motor coordination and attention: individual differences in twins

Differences in temperament, attention span, and activity level as reported by mothers of 56 pairs of 12-mo.-old twins were related to tested differences in cognitive abilities at age 4 yr. The more active twins were found to show temper more, have shorter attention span in play, and walk earlier but to have poorer manual dexterity than their co-twins. The twins with longer attention showed less temper and walked later but had better manual dexterity than their co-twins. Twins who were less active or had longer attention span at 12 mo. had significantly higher WPPSI Performance IQs than their co-twins when tested at age 4 years. Verbal IQ differences were in the same direction but not significant.