A cotwin control study and a twin study of reflection-impulsivity in children.
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Child↗
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Results for “Twin Study”
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The validity of the masseteric post-stimulus electromyographic complex: a twin study.
The twin model was used to assess the validity of an electromyographically recorded, masseter muscle reflex by measuring the sensitivity and specificity. Results were satisfying, implying that in future studies this reflex could be used to calculate heritability estimates between monozygotic and dizygotic twins.
Adolescent↗
Northwestern University Twin Study X: Outcome of twin gestations complicated by gestational diabetes mellitus.
Existing data concerning the effect of gestational diabetes on perinatal outcome in twin pregnancies is scant. We hypothesized that altered carbohydrate metabolism would worsen perinatal outcome in twin gestation in a manner similar to singleton gestation. Thirteen twin pregnancies complicated by gestational diabetes mellitus were matched by gestational age at delivery to 13 twin pregnancies unaffected by gestational diabetes. Comparing infants of diabetic mothers to infants of control mothers, there was a trend of greater likelihood of respiratory distress syndrome, hyperbilirubinemia, and prolonged neonatal intensive care nursery admissions. Our experience suggests that altered carbohydrate metabolism in multiple gestations increases the potential for neonatal morbidity.
Adult↗
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.
Blood Grouping and Crossmatching↗
Genetic studies in outpatients: plasma cholesterol in family and twin studies.
Quantitative genetic studies have much potential in partitioning the causes of variation of quantitative traits such as risk factors for atherosclerosis. Only if specific causes of variation are identified can specific therapy be developed to modify risks. There have been extensive family studies of plasma cholesterol which reveal that the level of plasma cholesterol is correlated in family members. However, except for the relatively rare familial hypercholesterolemia the evidence is not convincing that correlations of relatives are due to genetic rather than environmental factors. Early twin studies were interpreted as supporting the hypothesis that levels of plasma cholesterol were strongly influenced by genetic factors. However a recent large study of twins cast doubt upon this hypothesis by finding no significant genetic variance of plasma cholesterol after correcting for differences in total variance of monozygotic and dizygotic twins.
Arteriosclerosis↗
Growing up and growing apart: a developmental meta-analysis of twin studies.
Developmental change in twin similarity was examined with age contrasts in a meta-analysis of twin studies from 1967 through 1985. Intraclass rs were coded from 103 papers that included data for monozygotic or dizygotic twins, or for both, on personality or intelligence variables. Analyses indicated that there was a general tendency for some intraclass rs to decrease with age. In other words, as twins grow up, they grow apart. There were also developmental differences associated with components of variance for heritability, the shared environment, and the nonshared environment. Mechanisms through which the nonshared environment may operate are discussed.
Adolescent↗
Selection bias in disease-related twin studies. Data on 11,154 adult Finnish twin pairs from a nationwide panel.
The effect of migration on pairwise concordance for disease was assessed in 11,154 twin pairs of the Finnish Twin Cohort Study by comparing the pairs living in the same province to the pairs which members were living in different provinces of Finland. The cumulative incidence of psychosis and hypertension for the years 1972-1985 were analyzed. The cumulative concordance of psychosis for those MZ twin pairs living in the same province were higher than for those MZ pairs living further apart. Similar findings were found among DZ pairs for psychosis. The cumulative concordance of hypertension was only slightly higher among those MZ and DZ pairs living in the same province compared with pairs living in different provinces. These results indicate an overestimation of concordance of psychosis caused by selective migration. This bias in twin studies is likely to influence heritability estimates in a sample of limited geographical area.
Adult↗
The study of nevi in British twins: study design and description of the data set.
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Adult↗
Uses and limitations of twin studies.
Series of twin pairs selected because one (or both) is ill are prone to biassed ascertainment, and great care has to be taken to avoid this. Such bias is absent if the primary source is a twin registry established at birth. In general, series of twin pairs have no advantage over studies on sibs in assessing the size of genetic contribution to disease. However, individual monozygotic twin pairs who are discordant for single gene or multifactorial conditions offer unique opportunities for investigating postzygotic mutations, for searching for factors that may precipitate disease or influence its course, and for assessing the effect of prophylactic measures.
Humans↗
Twin studies in familial Mediterranean fever.
Familial Mediterranean fever (FMF) is a genetic disease characterized by recurrent short episodes of fever, accompanied by peritonitis, pleuritis, or arthritis. The disease is almost completely ethnically restricted to patients of Mediterranean descent--Sephardic Jews, Armenians, Anatolian Turks, and Arabs. Although many family studies have been performed, no twin study has been reported as yet. We studied 21 di- and monozygotic twin sets, identified among the 1,943 FMF patients in our registry. Full concordance was observed in all the 10 monozygotic twin sets. In the 11 dizygotic twins, concordance for FMF disease was found in only 3 pairs. Variability in the clinical manifestations and degree of severity have been noted within twins. These findings provide definitive evidence for the genetic cause of FMF. They also support the single gene autosomal recessive model, and provide support for the contention that the lower observed than expected incidence found in FMF is due to genetically affected but clinically undiagnosed patients.
Adolescent↗
The Northwestern University Multihospital Twin Study. II. Mortality of first versus second twins.
The results of a multihospital study involving a total of 588 twin pairs born in Chicago in 1970-1975 are reported, with special respect to differences in mortality between first and second twins by time as well as by cause of death. Mortality was higher in second than in first twins and most commonly occurred after delivery and was the result of immaturity and of respiratory distress syndrome.
Asphyxia Neonatorum↗
Influence of placentation on high density lipoproteins in adult males: the NHLBI twin study.
Dizygotic (DZ) World War II veteran twins who participated in the National Heart Lung and Blood Institute (NHLBI) Twin Study have been reported to have greater variance than monozygotic (MZ) twins for plasma high-density lipoprotein cholesterol (HDL-C), cholesterol in the low-density fraction of HDL (HDL2-C) and apolipoprotein A-I, a major protein component of HDL. It was hypothesized that a possible source of this difference in zygosity variance could be prenatal environmental influences related to placental type. Dermatoglyphics were used to provide a retrospective index of placental type in a subset of the NHLBI MZ twins aged 59-70. The MZ twins classified as dichorionic were found to have significantly greater within-pair variability than the monochorionic MZ twins for HDL-C, HDL2-C and Apo A-I. These findings indicate that intrauterine environmental influences on HDL are manifest later in life.
Aged↗
Twin studies of psychopathology: why do the concordance rates vary?
Many current discussions of hereditary factors in psychopathology focus on twin studies as the primary source of evidence supporting the importance of genetic determinants. In summarizing the results of these studies, authors often derive estimates of concordance rates by collapsing across studies and presenting mean or median rates. This practice implicitly assumes that the concordance rates yielded by different studies represent equally reliable estimates of the population mean. The present study evaluates the validity of this assumption. Reports of twin studies of schizophrenia and affective disorder were reviewed. A meta-analysis was conducted to examine the influence of methodological factors on concordance rates. Analyses indicated that both sample-selection and zygosity-determination procedure are systematically associated with concordance rates. For schizophrenia, MZ concordance rates are significantly lower when samples are selected from a twin register as opposed to a psychiatric facility. Lower MZ concordance rates are also yielded by studies that employ laboratory procedures to determine zygosity. Implications of the findings for future research are discussed.
Affective Disorders, Psychotic↗
Twin studies in metabolic diseases.
The traditional role of twin studies has been to assess the relative role of genetic factors as a first step in defining the genetic architecture of complex traits. This has been based on the realization that monozygotic pairs (MZ) share all their genes, while dizygotic pairs (DZ) share 50% of their genes on average. Thus, greater similarity of MZ pairs compared to DZ pairs has been taken as prima facie evidence of the role of genetic factors. This is true provided the environmental similarity of MZ pairs is not greater than for DZ pairs for effects relevant to the trait in question. This first step in genetic studies was carried out long ago in many research areas, but not in others. More detailed knowledge of the genetic architecture of traits is then obtained by other means. In this paper, we give a brief overview of some results for metabolic diseases (ischaemic heart disease, hypertension, subarachnoid haemorrhage, NIDDM and IDDM) using the classical twin approach in a large, unselected population-based twin cohort. We also outline approaches to using twins that we believe will continue to be useful, particularly for the study of environmental effects.
Cohort Studies↗
Resemblance in appearance and the equal environments assumption in twin studies of personality traits.
The equal environments hypothesis of twin methodology was examined for the variable of similarity of appearance as it affects the personality ratings of young twins. There were two separate samples, the first with 95 pairs of same-sex twins and the second with 111 pairs. The average age of the twins in both samples was 3-1/2 years. Mothers rated their twins on four personality traits and on confusability of appearance. Not surprisingly, identical twins were markedly more similar in appearance than fraternal twins. The effect of this inequality on the personality ratings of the two types of twins was examined by correlating ratings of similarity of appearance with the absolute difference on the four personality traits for each pair of twins. None of the correlations was significant for the identical twins, suggesting that greater resemblance in appearance in identical twins does not make them more similar in personality. Indeed, the data suggested a contrast effect in which identical twins who were easily mistaken in appearance tended to be rated as less similar in personality. Thus, although similarity of appearance may create unequal environments for the two types of twins, it does not appear to bias twin studies in the direction of inflated heritabilities, at least for rating studies of the personality of young twins.
Child, Preschool↗
[A twin study on the heredity of electroencephalogram].
A human twin study was undertaken to research the heredity of the EEG. The accuracy of the twins zygosity diagnosis was more than 98 per cent. Thirteen pairs of monozygotic twin had an intraclass correlation of 0.43 in alpha frequency and 0.65 in alpha sequence time per minute, and twelve pairs of dizygotic twins had an intraclass correlation of 0.20 and 0.34. Their hereditability was estimated by three methods as 29-58 per cent and 47-66 per cent, respectively.
Adult↗
Concordance of ischemic heart disease in the NHLBI twin study after 14-18 years of follow-up.
Morbidity and mortality were assessed in the NHLBI twin study at the end of 1987. Deaths were greater in DZ twins (58/520, 11.2%) than MZ twins (38/508, 7.5%). Ischemic heart disease concordances were 2.3 times higher in MZ pairs and 2.8 times higher in DZ pairs than expected based on the prevalence of ischemic heart disease in the cohort. Family history scores for heart disease, calculated 14-18 years earlier at entry to the study, were significantly higher in DZ pairs where one or both members later developed ischemic heart disease and in corcordant MZ pairs than in twin-pairs without any subsequent heart disease. Concordance rates were not significantly different between MZ and DZ pairs. The results agree with previous suggestions that selection at enlistment into the armed services over 40 years ago, as well as later volunteering for the NHLBI twin study, resulted in a decline in the number of concordant MZ pairs.
Cause of Death↗
Effects of selective return on estimates of heritability for body mass index in the National Heart, Lung, and Blood Institute Twin Study.
In the National Heart, Lung, and Blood Institute Twin Study, body mass index (BMI) was studied at military induction and at three subsequent examinations spanning five decades in a cohort of white, male World War II veterans. At military induction (1940s) and again at the first clinical examination of this study (1969-1973), there was close agreement of three commonly used estimates of heritability (range 0.72 to 0.80), and no evidence of a difference in total variance of BMI between the zygosities. However, at the last two examinations (1980s), the total variance in dizygotic (DZ) twins was significantly greater than that of monozygotic (MZ) twins (P less than 0.01) and these same heritability estimates varied widely. The among-pair estimate of heritability fell to unrealistic negative values, the within-pair estimate rose to values of 1.0 or greater, and the intraclass correlation coefficient estimate was slightly lower than in the entire cohort at baseline. The cause of the unequal zygosity total variance appears to have been nonparticipation at later examinations of MZ twins with extreme values of BMI, with no evidence of a similar selection process influencing DZ twins. This selection process biased the three estimates of heritability, making it difficult to determine which estimate is the most appropriate. Despite these biases, it remains clear that genetic factors contribute substantially to BMI in this population.
Adult↗