Discrete distributions for use in twin studies

We describe several new discrete distributions motivated by the study of longevity in twins. All individuals both living and dead at a given age are randomly paired. The univariate distribution models the number of these pairs where both individuals are alive. If there is a positive association to longevity in twins, then we would expect to see an excessive number of twin pairs both alive at older ages relative to the number of living individuals. We obtain Poisson and normal approximations to the exact distribution. Multivariate distributions are developed to allow for simultaneous and conditional inference at different ages. Odds-ratio parameter models provide a measure of the association of longevity within twin pairs. These models indicate an excessive number of identical twin pairs both alive after age 60 in a cohort of twins born between 1870 and 1880 in Denmark. Monozygotic twins are contrasted with dizygotic twins to separate the genetic and environmental contributions to the similarity in longevity among twins.     

Substantial genetic influence on cognitive abilities in twins 80 or more years old

General and specific cognitive abilities were studied in intact Swedish same-sex twin pairs 80 or more years old for whom neither twin had major cognitive, sensory, or motor impairment. Resemblance for 110 identical twin pairs significantly exceeded resemblance for 130 fraternal same-sex twin pairs for all abilities. Maximum-likelihood model-fitting estimates of heritability were 62 percent for general cognitive ability, 55 percent for verbal ability, 32 percent for spatial ability, 62 percent for speed of processing, and 52 percent for memory. There was also evidence for the significant influence of idiosyncratic experience as the environmental component that most determines individual differences in cognitive abilities late in life.     

A new cardiology patient simulator

The apolipoprotein E (APOE) epsilon4 allele is a risk factor in Alzheimer disease (AD), but not in vascular dementia (VaD). We have investigated whether the epsilon4 allele is more common in twin pairs concordant for AD, compared with those discordant for AD, and whether the epsilon4 allele is more common in AD twins than in VaD twins. In addition, we have investigated the relationship of the epsilon4 allele and the age at onset in AD and VaD. APOE genotype was analysed in 29 senile demented twin pairs. The epsilon4 allele was associated with AD and not with VaD. However, there was no difference in the frequency of the APOE epsilon4 allele in concordant (33.3%) and discordant (31.3%) AD dizygotic twin pairs. Age at onset in AD was significantly lower in epsilon4 homozygotes than in individuals with one or no copies of epsilon4 (62.4 vs. 73.5, p<0.01). In concordant AD twin pairs, the epsilon4 allele frequency was somewhat higher in the twins with earlier onset (41.7% vs. 25%), but the difference was not statistically significant. In the VaD group the age at onset was not significantly different between individuals with or without epsilon4 in their genotypes.     

Specific cognitive abilities in 5- to 12-year-old twins

Eleven tests of specific cognitive abilities were administered to 108 pairs of young twins (average age of 7.6 years). Internal consistencies are high for all measures except Raven's Coloured Progressive Matrices and Delayed Picture Memory. Two-month, test-retest reliabilities are also reported. The twin sample is representative in terms of both means and variances when compared to normative data from standardization samples, and twin correlations for height and weight are similar to those obtained in six other twin studies. Because all measures were highly correlated with age (average correlation with age was 0.64), scores were aged adjusted. Previous twin studies of specific cognitive abilities in adolescents and adults found genetic variance for nearly all tests. In contrast, our study of young twins yielded significant genetic influence for only 1 of the 11 measures, PIAT Reading Recognition, and suggested the possibility of genetic influence on 2 others (vocabulary and WISC-R mazes). Environmental influences seem to dominate, particularly for nonverbal measures, as children begin their education. In accord with other studies, we found that between-family environmental factors have an important influence on the development of nearly all of the measures of specific cognitive abilities. However, we found that our tests of perceptual speed and memory were substantially influenced by within-family environmental factors independent of error.     

Developmental genetic studies of adult personality.

Investigated genetic determinants of personality development and vulnerability to environmental influences during young adulthood, using a longitudinal twin study design. 133 college-age twin pairs completed an abbreviated MMPI twice over a period of 4.5 yrs at ages 20 and 25 yrs. Of the 6 MMPI scales investigated, significant genetic variance was found for 4 at both time periods (Wiggins's Social Maladjustment, Schizophrenia, Welsh A, and D?? scales). Two scales (Psychopathic Deviate and Wiggins's Religiosity) showed significant genetic variance at age 20 but not at age 25. There were no significant genetic influences on directional developmental change over time, and instead, idiosyncratic environmental influences appeared most important for the developmental profiles. In contrast, significant genetic influences were present for generalized vulnerability, or instability, for the Sc scale. (61 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Sources of cumulative continuity in personality: A longitudinal multiple-rater twin study.

This study analyzed the etiology of rank-order stability and change in personality over a time period of 13 years in order to explain cumulative continuity with age. NEO five-factor inventory self- and peer report data from 696 monozygotic and 387 dizygotic twin pairs reared together were analyzed using a combination of multiple-rater twin, latent state-trait, and autoregressive simplex models. Correcting for measurement error, this model disentangled genetic and environmental effects on long- and short-term convergent valid stability, on occasional influences, and on self- and peer report-specific stability. Genetic factors represented the main sources that contributed to phenotypic long-term stability of personality in young and middle adulthood, whereas change was predominantly attributable to environmental factors. Phenotypic continuity increased as a function of cumulative environmental effects, which became manifest in stable trait variance and decreasing occasion-specific effects with age. This study's findings suggest a complex interplay between genetic and environmental factors resulting in the typical patterns of continuity in personality across young and middle adulthood. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Developmental stability of the relative influence of genes and environment on specific cognitive abilities during childhood.

Describes hierarchical multiple regression (HMR) model for analysis of cross-sectional data to examine developmental changes in the relative influence of genes and environment. The HMR model is advocated because it provides a test of significance for age changes without the loss of information and statistical power that results when continuous variables, such as age, are assigned to categories. This model was applied to 9 tests of specific cognitive abilities that were administered to 54 identical and 33 same-sex fraternal twin pairs whose ages ranged from 5 to 11 yrs. Results indicate that familial influences as well as the relative mixture of genetic and environmental influences on specific cognitive abilities remain stable during this fast-moving developmental era. (25 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The intrauterine ponderal index in relation to birth weight discordance in twin gestations

OBJECTIVE: To establish the relationship between the fetal ponderal index and birth weight discordance in twins. METHOD: The fetal ponderal index (estimated fetal weight divided by femur length3) was calculated in 86 pairs of twins delivered within 2 weeks of the last sonography and analyzed in relation to birth weight discordance. RESULTS: A weak but significant correlation between fetal ponderal index and birth weight (r = 0.26, P < 0.0007) but no correlation with gestational age (r = 0.035, P = 0.65) were found. Members of concordant pairs (< 15% birth weight difference) had a significantly higher fetal ponderal index compared with members of mildly (15-25%) discordant pairs (P < 0.02), but not as compared with members of severely discordant (> 25%) pairs. CONCLUSION: The characteristics of the fetal ponderal index in twins are similar to those in singletons. Fetal size seems to be diminished in severe but not in mild discordants. However, in its present form, the fetal ponderal index is a poor predictor of discordant growth and therefore should be employed cautiously in twin gestations.     

Auditory lateralization in monozygotic twins

In an attempt to investigate whether auditory lateralization has a heritable component, 20 monozygotic (MZ) twin pairs were examined with four different dichotic listening tests known to produce reliable right-ear advantages (REAs) in right-handers. Ten twin pairs were concordantly right-handed (MZ-RR), and ten twin pairs were discordant for handedness (MZ-RL). Intraclass correlations for MZ twin pairs were weak or nonexistent for ear advantage, but relatively strong for overall correct scores and mean reaction times, measures unrelated to laterality scores. These results support the hypothesis that auditory lateralization, as measured with dichotic tests, is nongenetic in origin. A comparison of MZ twins and right-handed siblings (n = 20) showed that right-handed siblings exhibited strong REAs, whereas left-handed siblings (n = 20) and MZ twins showed weak or absent REAs, indicating that twins may be atypically lateralized with respect to auditory lateralization.     

Effects of chorion type on variation in cord blood cholesterol of monozygotic twins

Cholesterol levels were measured in the cord blood of 30 pairs of monochorionic and 22 pairs of dichorionic monozygotic (MZ) twins. Cholesterol levels were found to be significantly higher in female twins when data was combined over chorion type. The type of chorionic development had no significant effect on variation among twin pairs. Chorion type did, however, have a significant effect on the within-pair variation. The variation within dichorionic pairs was more than five times that within monochorionic pairs (P less than .01). This result suggests that the variation in placentation has a significant effect on within-pair variation in serum cholesterol of newborn MZ twins.     

Parkinson disease in twins: an etiologic study

CONTEXT: The cause of Parkinson disease (PD) is unknown. Genetic linkages have been identified in families with PD, but whether most PD is inherited has not been determined. OBJECTIVE: To assess genetic inheritance of PD by studying monozygotic (MZ) and dizygotic (DZ) twin pairs. DESIGN: Twin study comparing concordance rates of PD in MZ and DZ twin pairs. SETTING AND PARTICIPANTS: A total of 19842 white male twins enrolled in the National Academy of Sciences/National Research Council World War II Veteran Twins Registry were screened for PD and standard diagnostic criteria for PD were applied. Zygosity was determined by polymerase chain reaction or questionnaire. MAIN OUTCOME MEASURE: Parkinson disease concordance in twin pairs, stratified by zygosity and age at diagnosis. RESULTS: Of 268 twins with suspected parkinsonism and 250 presumed unaffected twin brothers, 193 twins with PD were identified (concordance-adjusted prevalence, 8.67/1000). In 71 MZ and 90 DZ pairs with complete diagnoses, pairwise concordance was similar (0.129 overall, 0.155 MZ, 0.111 DZ; relative risk, 1.39; 95% confidence interval, 0.63-3.1). In 16 pairs with diagnosis at or before age 50 years in at least 1 twin, MZ concordance was 1.0 (4 pairs), and DZ was 0.167 (relative risk, 6.0; 95% confidence interval, 1.69-21.26). CONCLUSIONS: The similarity in concordance overall indicates that genetic factors do not play a major role in causing typical PD. No genetic component is evident when the disease begins after age 50 years. However, genetic factors appear to be important when disease begins at or before age 50 years.     

Minimal genetic influences on plasma fibrinogen level in adult males in the NHLBI twin study

Plasma fibrinogen was determined in 189 twins participating at the Indiana center during the third examination of the NHLBI twin study with a mean age of 63 years. Moderate heritability estimates were obtained from 44 complete MZ pairs and 39 complete DZ pairs. After adjustment of fibrinogen levels for age and other confounding variables related to cardiovascular disease risk, the maximum likelihood heritability estimate was only 30% (p = 0.03). Plasma fibrinogen was most strongly associated with smoking and the presence of diabetes. Omitting all subjects with diabetes or cardiovascular disease further reduced the heritability estimates slightly, and most path models including genetic parameters provided no significant improvement in fit over a model determined solely by random environmental effects. Our results are consistent with the environment rather than genetic influences having a greater influence on the level of plasma fibrinogen.     

Review of the evidence that pH is a determinant of nicotine dosage from oral use of smokeless tobacco

The effect of calcium supplementation on bone mineral density (BMD) was evaluated in female twin pairs aged 10-17 years with a mean age of 14 years. Forty-two twin pairs (22 monozygotic, 20 dizygotic; (including one monozygotic pair from a set of triplets) completed at least 6 months of the intervention: 37 pairs to 12 months and 28 pairs to 18 months. BMD was measured by dual-energy X-ray absorptiometry (DXA). In a double-blind manner, one twin in each pair was randomly assigned to receive daily a 1000 mg effervescent calcium tablet (Sandocal 1000), and the other a placebo tablet similar in taste and appearance to the calcium supplement but containing no calcium. Compliance (at least 80% tablets consumed), as measured by tablet count, was 85% in the placebo group and 83% in the calcium group over the 18 months of the study, on average increasing dietary calcium to over 1600 mg/day. There was no within-pair difference in the change in height or weight. When the effect of calcium supplementation on BMD was compared with placebo at approximately 6, 12 and 18 months, it was found that there was a 0.015 +/- 0.007 g/ cm2 greater increase in BMD (1.62 +/- 0.84%) at the spine in those on calcium after 18 months. At the end of the first 6 months there was a significant within-pair difference of 1.53 +/- 0.56% at the spine and 1.27 +/- 0.50% at the hip. However, there were no significant differences in the changes in BMD after the initial effect over the first 6 months. Therefore, we found an increase in BMD at the spine with calcium supplementation in females with a mean age of 14 years. The greatest effect was seen in the first 6 months; thereafter the difference was maintained, but there was no accelerated increase in BMD associated with calcium supplementation. The continuance of the intervention until the attainment of peak bone mass and follow-up after cessation of calcium supplementation will be important in clarifying the optimal timing for increased dietary calcium and the sustained, long-term effects of this intervention.     

Linguistic environment of 1- to 2-year-old twins.

Investigated differences in the language learning environments of singletons and twins, with special reference to pragmatic factors that might be expected to differ in dyadic and triadic interactive situations. Six twin pairs and 12 singleton children (all firstborn) were observed in natural interactions with their mothers, once at 15 mo of age and again at 21 mo of age. Findings reveal that twins were lower than singletons on all measures of language development and that their language learning environments were significantly different. Although twin mothers spoke and interacted with their children as much as singleton mothers when twins were analyzed together, when analyzed as individuals, twin children received less speech directed specifically to them, participated in fewer and shorter episodes of joint attentional focus, and had fewer and shorter conversations with their mothers. Twin mothers were also more directive in their interactional styles. Correlational analyses indicated that variation of these language learning environment factors for the sample as a whole, and variation for some of these factors within the twin group itself, were related to early language growth. It is proposed that both the quantitative and qualitative differences observed in the language learning environments of singletons and twins derive from the nature of the triadic situation and that these differences have important effects on the child's early language development. (23 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Selective preference in visual fixation away from negative images in old age? An eye-tracking study.

Recent studies have suggested that older individuals selectively forget negative information. However, findings on a positivity effect in the attention of older adults have been more mixed. In the current study, eye tracking was used to record visual fixation in nearly real-time to investigate whether older individuals show a positivity effect in their visual attention to emotional information. Young and old individuals (N = 64) viewed pairs of synthetic faces that included the same face in a nonemotional expression and in 1 of 4 emotional expressions (happiness, sadness, anger, or fear). Gaze patterns were recorded as individuals viewed the face pairs. Older adults showed an attentional preference toward happy faces and away from angry ones; the only preference shown by young adults was toward afraid faces. The age groups were not different in overall cognitive functioning, suggesting that these attentional differences are specific and motivated rather than due to general cognitive change with age. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The tocolytic effect of catecholamines in the gravid rat uterus

OBJECTIVE: The aim of the study was to determine the most cost-effective delivery management of vertex and nonvertex twin pair gestations. STUDY DESIGN: Two hundred sixty-six consecutive twin gestations were followed up prospectively in a special antepartum Twins Clinic directed by the Maternal-Fetal Medicine Division. Maternal demographic, obstetric, and neonatal data were compiled prospectively. Information regarding 84 vertex and nonvertex twin pair gestations was extracted for review. Comparison groups included 41 twin pairs managed by spontaneous vaginal delivery and breech extraction (group A), 19 twin pairs managed by spontaneous vaginal delivery and external cephalic version (group B), and 24 twin pairs managed by primary cesarean delivery (group C). In-hospital financial data were retrieved and adjusted for inflation to 1996 constant dollars (data from a single institution allow comparison of charges). The Kruskal-Wallis chi2 test, the Fisher exact test, and analysis of variance were used for statistical analysis. RESULTS: The 3 groups did not differ with respect to maternal demographics, medical complications, gestational age, birth weight, fetal sex, or intrauterine growth restriction. Maternal and neonatal hospital charges were both significantly lower (P = .01 and P = .0001, respectively) in the breech extraction group A ($5890 +/- $2304 and $3526 +/- $5017, respectively) than in either group B ($8638 +/- $4175 and $11,754 +/- $15,457, respectively) or group C ($7,608 +/- $3,256 and $36,994 +/- $54,318, respectively). Although maternal morbidity was similar among the 3 groups, length of stay was shorter for mothers in group A. The infants delivered by spontaneous vaginal delivery and breech extraction (group A) had significantly lower rates of pulmonary disease (7% vs 24% vs 31% for groups A, B, and C, respectively; P = .002) and neonatal infectious disease (1% vs 0% vs 16%; P = .0005). These infants required the use of a ventilator less frequently (5% vs 12% vs 24%; P = .01), were more likely to be admitted to the normal newborn nursery (71% vs 51% vs 50%; P = .0001), and had a significantly shorter length of hospitalization (4.8 vs 12.4 vs 17.8 days; P = .0001). CONCLUSIONS: There is no medical consensus regarding delivery management for vertex and nonvertex presenting twin pairs. When hospital charges are examined with clinical data, however, breech extraction of the nonvertex second twin is the most cost-effective delivery management strategy.     

Environmental differences in twin pairs discordant for Alzheimer's disease

The aim of this study was to examine the contribution of environmental factors to the pathogenesis of Alzheimer's disease by comparing environmental differences in twin pairs discordant for Alzheimer's disease. Seventy four twin pairs discordant for Alzheimer's disease were found by linking the Finnish twin cohort and the Hospital Discharge Register from years 1972-91. In 50 pairs (25 monozygotic and 25 dizygotic pairs), both co-twins had responded to a questionnaire survey in 1975. Exposure differences were compared between these pairs. A reduced risk of Alzheimer's disease was significantly associated with a higher level of schooling (relative risk 0.3; 95% confidence interval 0.1-0.9, p=0.029). In addition, a reduced risk was suggestively associated with ambidextrousness or left handedness (p=0.083) and an increased risk with marriage (p=0.052), widowhood (p=0.074), and a history of cholelithiasis (p=0.071). In conclusion, a reduced risk of Alzheimer's disease was associated with a higher level of schooling.     

Differential parent-child relationships and adolescent externalizing symptoms: Cross-lagged analyses within a monozygotic twin differences design.

Research has indicated that differential parental treatment is linked to differences in externalizing symptomology (EXT) across siblings, even those siblings who are genetically identical. However, the direction of causation and longitudinal significance of this relationship remains unclear. Thus, in the present study, the authors examined 486 monozygotic twin pairs, assessed at ages 11, 14, and 17 years, within a cross-lagged twin differences design. Results revealed that differential parent-child conflict at age 11 years uniquely contributed to differential sibling EXT 3 years later but only in the most discordant twin pairs. In the full, unselected sample, this relationship was not significant. These results suggest that markedly different parent-child conflict has an environmentally mediated impact on child behavior through mid-adolescence, findings that yield insights into environmental influences on behavior. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Normal personality characteristics in identical twins discordant for schizophrenia.

The authors examined normal personality characteristics of monozygotic (MZ) twins discordant for schizophrenia. Twenty pairs of discordant twins were analyzed; 11 pairs of MZ twins concordant for schizophrenia served as a comparison group. Personality was assessed using the Multidimensional Personality Questionnaire (MPQ; A. Tellegen, 1995). Among discordant twin pairs, twins with schizophrenia were more deviant than normal co-twins on all but 4 of the MPQ's scales. Analysis of MZ twin correlations among the discordant twin pairs revealed substantial erosion of personality similarity as compared to normal individuals, although a cluster of scales related to Constraint/Psychoticism showed significant correlations. The results highlight striking personality divergence related to nonshared environmental influences. It is not possible to determine the point at which the twins diverged, although previous findings from this sample suggest that the observed personality differences may reflect effects of schizophrenia on normal personality. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Genetic variance in processing speed drives variation in aging of spatial and memory abilities.

Previous analyses have identified a genetic contribution to the correlation between declines with age in processing speed and higher cognitive abilities. The goal of the current analysis was to apply the biometric dual change score model to consider the possibility of temporal dynamics underlying the genetic covariance between aging trajectories for processing speed and cognitive abilities. Longitudinal twin data from the Swedish Adoption/Twin Study of Aging, including up to 5 measurement occasions covering a 16-year period, were available from 806 participants ranging in age from 50 to 88 years at the 1st measurement wave. Factors were generated to tap 4 cognitive domains: verbal ability, spatial ability, memory, and processing speed. Model-fitting indicated that genetic variance for processing speed was a leading indicator of variation in age changes for spatial and memory ability, providing additional support for processing speed theories of cognitive aging. (PsycINFO Database Record (c) 2010 APA, all rights reserved)