The heritability of smoking behaviour in pregnancy, and the birth weights of offspring of smoking-discordant twins

Questionnaire information on smoking habits in pregnancy was collated in 341 monozygotic (MZ) and 321 dizygotic (DZ) female twin pair cases from a population-based Norwegian Twin Panel. In a multifactorial model, the intra-pair correlation in smoking was 0.797 (+/- 0.042) in monozygotic (MZ) and 0.443 (+/- 0.075) in dizygotic (DZ) twin pairs, indicating a substantial genetic influence on liability to smoke in pregnancy. The questionnaire information was linked with birth records in the Medical Birth Registry of Norway, and birth weights of offspring of 62 MZ and 100 DZ smoking-discordant twin pairs were studied. Offspring of smoking MZ twins weighed 127 g less than birth order matched offspring of the non-smoking co-twins. This finding is additional evidence that smoking is a direct cause of reduced birth weight in offspring.     

Twins, smoking and mortality: a 12-year prospective study of smoking-discordant twin pairs

Despite the increasing scientific evidence for a causal role of tobacco smoking in lung cancer and coronary heart disease, critics, several decades ago, put forward an alternative hypothesis. The constitutional hypothesis has stated that there are genetic or other common factors, which predispose both to smoking and disease, but that the two are not causally related. A critical test of this hypothesis is the study of disease in monozygotic (MZ) twin pairs in which one smokes and the other never has. Earlier twin studies found only small differences in the mortality of smoking and nonsmoking twins of discordant pairs. In the Finnish Twin Cohort, a population-based panel of adult like-sexed twin pairs, a questionnaire study carried in 1975 permitted identification of twin pairs discordant for cigarette smoking. The nonsmoking cotwins had never been regular smokers. The smoking twins were divided into 1278 current smokers [CS; 143 MZ and 598 dizygotic (DZ) males and 171 MZ and 585 DZ females] and 1210 former smokers (FS; 129 MZ and 408 DZ males and 113 MZ and 341 DZ females). Exposure to tobacco was much higher among males; over 25% of men smoked 20 or more cigarettes daily compared to less than 10% of women. Follow-up of mortality yielded data on time and cause of death. Analyzing on first deaths from concordant pairs, there were 13 deaths in the smokers of male CS MZ pairs and 1 death in the nonsmoking cotwins (relative risk = 13.0, P less than 0.01). Excess mortality was also found for male CS DZ smokers (RR = 2.43, P less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)     

双生子A型人格与高血压及血生化指标研究

目的 了解双生子A型人格与高血压及血液生化指标的关系。方法 利用遗传流行病学方法对青岛市89对24岁以上双生子(同卵55对，异卵34对)进行调查。并进行A型人格测试，以比较同卵与异卵双生于A型人格得分的相关程度、A型人格及血压的一致性。推测遗传与环境因素对A型人格的影响，A型人格与高血压的关系，并探讨血液生化指标与A型人格的关系。结果 经KAPPA一致性检验，同卵(MZ)双生子之间A型人格存在着显的一致性(P＜0．001)，而异卵(DZ)双生子之间的一致性无显性差异(P＝0．802)。同时，MZ双生子之间A型人格和血压也存在显的一致性(P＜0．001)，而DZ双生子之间A型人格和血压无明显一致性(P＝0．102)。有A型人格的双生子血压的收缩压明显高于非A型人格的双生子(P＜0．05)。许多生化指标与A型人格因素相关，但是所计算出的相关系数大都小于0．30，属于弱相关。结论 MZ双生子A型人格及高血压之间存在着显的一致性，而这种一致性在DZ双生子表现不明显。A型人格是高血压的危险因素之一。A型人格与所研究血液基本生化指标之间相关较弱。     

Lifestyle factors in monozygotic and dizygotic twins

In examining genetic influences on biological variables using twins, it may be important to examine the distribution between and within twin pairs of demographic and lifestyle factors that may themselves affect the biological variable being studied. We explored the distribution of demographic and lifestyle factors that may affect blood lipid levels or ischaemic heart disease (IHD) risk among a sample of 106 monozygotic (MZ) and 94 like-sex dizygotic (DZ) twin pairs. In our sample, MZ twins were statistically significantly different from DZ twins only in marital status, cigarette smoking habits, and the ratio of polyunsaturated to saturated fat (P:S ratio) in their dietary intake. The latter variable was among many dietary variables examined (using 4-day weighed food diaries), and the size of the difference in intake was small. When comparisons were made of the similarities within twin pairs, we found members of MZ twin pairs to be statistically significantly closer than DZ twins in educational achievement, occupation, cigarette smoking, and exercise habits, and the number of days a week on which alcohol was consumed. These last three variables were consistently closer among twins with closer contact than among those with a smaller degree of current shared environment. For 12 of the 13 nutrients examined, the within-pair correlations were higher for MZ than for DZ twins, although our test for significant genetic variance showed statistical significance only for intake of complex carbohydrates. We conclude that MZ twins share demographic and lifestyle factors that might influence the risk of IHD and blood lipid levels to a greater degree than do DZ twins, although it is difficult to say if these similarities in lifestyle result from genetic influences or not. Nevertheless, ascribing differences between correlations in MZ and DZ twin pairs for lipid levels as being purely "genetic"--as implicit in conventional measures of heritability--is likely to overestimate the influence of genetic factors.     

Socioeconomic position and twins' health: a life-course analysis of 1266 pairs of middle-aged Danish twins

BACKGROUND: The association between socioeconomic circumstances and health in adulthood could come about through processes that may be divided into factors experienced early in life and those experienced in later adulthood. In order to disentangle the influences on health of the early genetic, prenatal and rearing environmental factors from environmental factor later in life, we compared the health status among male and female twin pairs who lived together during childhood and were discordant or concordant on adult socioeconomic position. METHODS: A cross-sectional survey among a random sample of middle-aged Danish twins was conducted in 1998-99. The study population included 1266 like-sex twin pairs [52.5% monozygotic (MZ) and 47.6% dizygotic (DZ)]. Data were obtained on childhood and adult social class and on height, BMI, grip strength, depression symptoms, self-rated health, cognitive function, physical activity, smoking, alcohol and food intake. RESULTS: The expected associations between the individual twins' adult social class and health measures were observed. Among DZ male twins discordant on adult social class, the higher social class twin was on average significantly taller and had higher cognitive test scores. Among DZ female twins discordant on adult social class, the higher social class female twin was more physically active and had a higher cognitive test score. There were no significant health disparities or behavioural differences between the members of MZ twin pairs discordant on adult social class. For most health outcomes, the variability within twin pairs was related to zygosity (higher for DZ than for MZ) but not to social class. CONCLUSION: This study suggests that the relationship between adult social class and health outcomes in Denmark is due mainly to selection effects rather than a causal effect of social class exposures on health and behaviour.     

双生子艾森克人格与血液生化指标的相关性研究

[目的 ]了解双生子艾森克人格特点及遗传因素对艾森克人格的影响 ,探讨血液生化指标与艾森克人格的相关性。 [方法 ] 2 0 0 1年 12月对青岛市 89对 2 4岁以上的双生子 (同卵 5 5对 ,异卵 3 4对 )进行艾森克人格测试 ,检测 3 7项血液生化指标 ,并进行相关分析。 [结果 ]同卵双胞胎之间在N(情绪稳定性 )因子的相关系数高于异卵双胞胎 ,N因子的遗传度为 0 45。人格因素中只有N因子与几项生化指标有相关关系 (r <0 3 0 )。 [结论 ]艾森克人格因素中的N因子受遗传作用的倾向较大 ,血液生化指标水平并不决定人的个性心理特征。     

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.     

Heritability estimate for refractive errors--a population-based sample of adult twins

The population-based Finnish Twin Cohort study was used to establish a heritability estimate for refractive errors especially for myopia. The twin cohort was derived from adult same-sexed twins in Finland. The total number of twins with both members alive in 1984 was 23,570. Of these, 3,676 twin pairs were monozygotic, and 8,109 pairs dizygotic. The sample for the present study was linked from the Finnish Police Force data base in 1984, where information of a person's possession of a driver's license and the obligation to wear glasses for far correction when driving a motor vehicle is recorded. Correlations in liability were estimated according to a multifactorial method of Smith. Falconer's heritability was 0.62 among males and 0.98 among females in the age group 28-29 years. When compared to previous twin studies of myopia, the proband concordance rates were higher for both MZ and DZ twin pairs.     

The epidemiology of anencephaly and spina bifida in Izmir, Turkey, in the light of recent aetiological theories

The data from this study suggest that, in western Turkey, potato consumption and water hardness do not play a significant role in the aetiology of anencephaly and spina bifida (ASB). Several of the predictions of the fetus-fetus interaction theory are not supported. Other predictions could not be tested because of lack of information on the dizygotic (DZ) twinning rate. Twin data from this study, together with previous material containing explicit information on zygosity, suggest that true concordance of a particular neural tube anomaly occurs only in monozygotic (MZ) twins. It also seems that even ASB concordance occurs in DZ twins only at a rate comparable with recurrence in siblings. This confirms the earlier refutation (Field and Kerr, 1974) of the supposition of Nance (1971) that MZ twin pairs are strikingly discordant for ASB compared with DZ pairs. However, the evidence seems to go further than Field's assertion that MZ and DZ pairs are affected about equally. Concordance in DZ twins is in comparison with sibling data, but MZ pairs show a significantly higher rate of concordance in both categories. Finally, the incidence of ASB in Izmir compared with the areas of ethnic origin of the Turks, and the high representation of families from Balkan areas where the incidence more closely resembles that of Izmir, suggest that the genetic factor is important in this area.     

A genetic and environmental analysis of a twin family study of alcohol use,anxiety, and depression

Alcohol consumption, anxiety, and depression were measured by questionnaire in 572 twin families ascertained from the Institute of Psychiatry (London) normal twin register, each family consisting of an adult twin pair, their parents, and siblings-a total of 1,742 individuals. A multivariate normal model for pedigree analysis was applied to each variable, with power transformations fitted to maximise the fit with distributional assumptions. The effect of shared twin environment was estimated by considering the measured cohabitation history of twin pairs. For log-transformed alcohol consumption, amongst current drinkers this effect was the same for MZ and DZ pairs but depended on the cohabitation status of pairs. For both anxiety and depression the effect was clearly not the same for MZ and DZ pairs. Therefore the basic assumption of the classical twin method appears to be invalid for all three traits. Estimates of heritability derived from these analyses were compared with those obtained (1) by applying the classical twin method to twin data only, and (2) by a pedigree analysis ignoring the effect of shared twin environment. For all variables there were considerable differences between estimates based on the three models. This study illustrates that data from twins and their relatives which includes information on cohabitation history might distinguish shared genes and shared environment as causes of familial aggregation. In these behavioral traits the effect of shared twin environment may depend on zygosity and play a major role in explaining familial aggregation in twin family data.     

The association between body height and coronary heart disease among Finnish twins and singletons

OBJECTIVE: An inverse association between body height and the incidence of coronary heart disease (CHD) has been observed. However, the mechanisms behind this association are still largely unknown. We will examine the role of genetic and familial factors behind the association in a large twin data set. DESIGN AND SETTING: The data were derived from the Finnish Twin cohort including 2438 singletons, 4073 monozygotic (MZ) twins, and 9202 dizygotic (DZ) twins aged 25-69 years at baseline in 1976. Incident CHD cases were derived from hospital discharge data and cause of death data between 1977 and 1995. Cox regression analysis and conditional logistic regression analysis were used. RESULTS: In population-level analyses no differences in the general risk of CHD between zygosity groups were found. The association between body height and CHD was similar between sexes and zygosity groups. When men and women in all zygosity groups were studied together an increased risk of CHD was found only among the shortest quartile (hazard ratio [HR] = 1.34, 95% CI: 1.14-1.57). Among the twin pairs discordant for CHD a suggestive increased risk for the shorter twin was seen among DZ twins (odds ratio [OR] = 1.19, 95% CI: 0.95-1.48) when men and women were studied together. CONCLUSION: An inverse association between body height and CHD was broadly similar between sexes and twin zygosity groups and was associated with short stature. Among discordant twin pairs we found a weak association among DZ twins but not MZ twins. This may suggest the role of genetic liability behind the association between body height and CHD.     

Heritability of body size and muscle strength in young adulthood: a study of one million Swedish men

Moderate heritability for skeletal muscle strength has been reported in twin studies, but genetic co-variation between muscle strength at different parts of body and body size is not well known. Further, representativeness of twin cohorts needs to be critically evaluated. Height, weight, elbow flexion, hand grip and knee extension strength were measured in young adulthood in 1,139,963 Swedish men born between 1951 and 1976. We identified 154,970 full-brother pairs and 1582 monozygotic (MZ) and 1864 same-sex dizygotic (DZ) complete twin pairs. The data were analyzed using quantitative genetic modeling for twin and family data. Twins compared to singletons and MZ twins compared to DZ twins were shorter, lighter and had lower muscle strength. In singletons, there was more variation in weight and the strength measures compared to twins with known zygosity but not when compared to twins with unknown zygosity. Full-sib correlations for these traits were lower than DZ correlations. Additive genetic factors explained 81% of variation in height, 59% in body mass index and 50-60% in the strength measures. Additive genetic correlations varied from 0.13 between height and elbow flexion strength to 0.78 between elbow flexion and hand grip strength. Our results suggest that extra variation may exist in general populations not found in twin samples, probably because of selection due to non-participation. This may have inflated heritability estimates in previous twin studies. Nonetheless, we showed that genetic factors affect muscle strength and part of these genes are common to different strength indicators and body size.     

Male factor infertility: a twin study

There is a considerable body of literature on the causes of female infertility, but far less is known about male factor infertility. We conducted a classical twin study to estimate the genetic influence on 12-month male factor infertility. The study used the Vietnam Era Twin (VET) Registry, which includes male twin pairs born between 1939 and 1957, and who served in the US military between 1965 and 1975. In 1987, a health survey was mailed to all twins and obtained a 74% response rate. The current analyses comprised 1795 complete pairs in which both twins were married only once. Proband-wise concordance rates, tetrachoric correlations, and a bivariate probit model were used to calculate estimates of familial clustering and heritability for male factor infertility. The proband concordance rate for male factor infertility was 38% [95% CI 32.8, 42.4] in monozygotic (MZ) pairs and 33% [95% CI 28.0, 38.6] in dizygotic (DZ) pairs. The tetrachoric correlations for male infertility were 0.15 in MZ and 0.04 in DZ pairs. This pattern provides evidence of familial clustering, although genetic influence was not evident (P = 0.21). The current study identified that 12-month male factor infertility clustered within families. However, results suggest that factors unique to individual twins may play a more prominent role in male infertility than additive genetic effects or the common environment.     

The effect of non-response on estimates of health care utilisation: linking health surveys and registers

BACKGROUND: Non-response in health surveys may lead to bias in estimates of health care utilisation. The magnitude, direction and composition of the bias are usually not well known. When data from health surveys are merged with data from registers at the individual level, analyses can reveal non-response bias. Our aim was to estimate the composition, direction and magnitude of non-response bias in the estimation of health care costs in two types of health interview surveys. METHODS: The surveys were (1) a national personal interview survey of 22 484 Danes (2) a telephone interview survey of 5000 Danes living in Funen County. Data were linked with register information on health care utilisation in hospitals and primary care. Health care utilisation was estimated for respondents and non-respondents, and the difference was explained by a decomposition method of bias components. RESULTS: The surveys produced the same pattern of non-response, but with slight differences in non-response bias. Response rates for the interview and telephone surveys were 75 and 69%, respectively. Refusal was the most frequent reason for non-response (22 and 20% of those sampled, respectively), whereas illness, non-contact, and other reasons were less frequent. Respondents used 3-6% less health care than non-respondents at the aggregate level, but the opposite was true for some specific types of health care. Non-response due to illness was the main contributor to non-response bias. CONCLUSIONS: Different types of non-response have different bias effects. However, the magnitude of the bias encourages the continued use of interview health surveys.     

Social capital and health. A study of adult twins in the U.S

BACKGROUND: A growing number of studies have suggested a link between social capital and health. However, the association may reflect confounding by factors, such as personality or early childhood environment, that are unmeasured prior common causes of both social capital and health outcomes. The purpose of this study was to investigate the impact of social capital on physical and mental health among adult twins in the U.S. METHODS: A cross-sectional national survey of twins within the National Survey of Midlife Development in the U.S. (MIDUS), 1995--1996 was analyzed in 2007. The study population included 944 twin pairs (37.2% monozygotic [MZ] and 62.8% dizygotic [DZ]). Data were obtained on individual-level social capital variables (social trust, sense of belonging, volunteer activity, and community participation); health outcomes (perceived physical and mental health, depressive symptoms and major depression); and individual covariates (age, gender, race, education, working status, and marital status). A fixed-effects model was used to examine health status among twin pairs who were discordant on levels of social capital. RESULTS: In the individual data analysis, social trust, sense of belonging, and community participation were each significantly associated with health outcomes. In the fixed-effects model, physical health remained significantly positively associated with social trust among MZ and DZ twins. However, major depression was not associated with social capital. CONCLUSIONS: The present study is the first to find the independent positive effect of social trust on self-rated physical health using fixed-effects models of twin data. The results suggest that the association between social capital and physical health status is not explained by unobserved confounds, such as personality or early childhood environment.     

Selective international migration by social position, health behaviour and personality

BACKGROUND: Immigrants is an important minority in many countries, but little is known how they are self-selected. We analysed differences in psycho-social and health behavioural factors between international migrants and non-migrants prior to migration in a large cohort of Finnish twins. METHODS: A questionnaire was sent to Finnish twins in 1975 (response rate 89%, N = 26555 twin individuals). Follow-up data on migration and mortality were derived from population registries in Finland and Sweden up to 31 March 2002. In 1998, another questionnaire was sent to Finnish twins migrated to Sweden and their co-twins (response rate 71%, N = 1534 twin individuals). The data were analysed using Cox and conditional logistic regression models. RESULTS: Life dissatisfaction, higher alcohol use and smoking at baseline predicted future migration. In men additionally, unemployment, neuroticism and extroversion increased the probability to migrate. Similar associations were found for alcohol use in men and smoking in men and women within twin pairs discordant for migration. Twins also reported retrospectively that prior to migration the migrated twin had been less satisfied with his/her educational institution or job and was generally less satisfied with life, used more alcohol (men) and smoked more (women) than the co-twin stayed in Finland. CONCLUSION: Migrants are self-selected by health behavioural and personality factors, which may compromise their health. The special requirements of migrants should be recognized in health care.     

Univariate genetic analyses of epilepsy and seizures in a population-based twin study: The Virginia twin registry

The purpose of this study was to examine the roles of genetic and environmental factors in the etiology of epilepsy and seizures in twins ascertained from the Virginia Twin Registry. Health history information on twins was collected by questionnaire. Concordance rates were calculated and used to estimate degree of concordance for seizure types in monozygotic (MZ) and dizygotic (DZ) twin pairs. Univariate twin analyses were performed for each epilepsy and seizure type to determine models which best explained observed variation. Health history information concerning epilepsy and febrile seizure occurrences was provided by members of 8,655 twin pairs; 6,684 of these supplied additional information reporting absence, complex partial, tonic-clonic, and unspecified seizures. Models including additive genetic and unique environmental factors best explained febrile seizures, epilepsy, complex partial seizures, and unspecified seizures. For complex partial seizures, however, the contributions of genetic and environmental effects did not vary across gender. These results show that, under univariate analysis methods, genetic factors played an important role in the expression of seizures in epilepsy, febrile seizures, unspecified seizures, and complex partial seizures. Additional support for these findings was provided by the concordance results for all categories except male twins reporting complex partial seizure occurrence. However, environmental influences still remained an important factor in seizure expression in these specific categories. Genet. Epidemiol. 15:33–49,1998. © 1998 Wiley-Liss, Inc.     

Characteristics of monozygotic twins discordant for bulimia nervosa

OBJECTIVE: We studied monozygotic (MZ) twins who were discordant for bulimia nervosa in order to identify environmental factors that could have contributed to the expression of bulimia nervosa in only one member of these genetically identical twin pairs. METHOD: Female twin pairs from the Virginia Twin Registry were identified. One twin met DSM-III-R criteria for bulimia nervosa (excluding the frequency and duration criterion) and the co-twin either reported some symptoms of bulimia nervosa but never met full diagnostic criteria (n = 20 pairs) or reported no symptoms of bulimia nervosa (n = 10). We compared the twins on demographic, developmental, personality, and psychiatric dimensions using self- and maternal report. RESULTS: Affected twins were more likely to report lifetime generalized anxiety disorder and were described by their mothers as more anxious and fearful as children. They also had significantly lower mastery, optimism, and self-esteem, and significantly greater obsessive and compulsive symptoms than their unaffected co-twins. Affected twins recalled greater discord in their families but viewed their parents as more warm toward them than toward their unaffected co-twin. DISCUSSION: Given that MZ twins are genetically identical and share a common family background, differences between them must be attributable to unique environmental factors. Although it is difficult to disentangle predisposing risk factors from sequelae of psychiatric illness in a retrospective co-twin control design, the affected twins were perceived to have been more anxious as children by their mothers-possibly reflecting a predisposing trait. More extensive interviews are required to determine the nature of environmental events that contribute to the expression of the observed personality, behavioral, anxious, and eating-related traits in the affected twins.     

Concordance rates and modifiable risk factors for lower urinary tract symptoms in twins

INTRODUCTION: Lower urinary tract symptoms, which are common in older men, are thought to be determined genetically and by modifiable environmental risk factors. We examined the contribution of these 2 etiologic components in a cohort of U.S. twins. METHODS: In 1998, a questionnaire that assessed lower urinary tract symptoms, weight, height, alcohol consumption, cigarette smoking, and physical activity was sent out to members of the National Academy of Science-National Research Council Twins Registry. We analyzed 1,723 complete twin pairs with information on lower urinary tract symptoms and zygosity and who did not have a previous diagnosis of prostate cancer. We calculated concordance rates of categories of the International Prostate Symptom Score in monozygotic (MZ) and dizygotic (DZ) twins. Generalized estimating equations were used to calculate the odds ratio of having high-moderate/severe lower urinary tract symptoms. RESULTS: Concordance rates were higher in MZ than in DZ twins with concordance rate ratios of 2.2 and 6.9 depending on the specificity of definition of symptoms. Genetic factors contributed 72% to the risk of high-moderate/severe lower urinary tract symptoms. Taking into account correlated individuals, we observed high odds of lower urinary tract symptoms in obese men compared with lean men (odds ratio = 1.91; 95% confidence interval = 1.16-3.15 comparing first versus fourth quartile). Cigarette smoking was not associated with lower urinary tract symptoms, but alcohol consumption was positively associated. Men who were more physically active tended to have lower odds of lower urinary tract symptoms compared with less active men (0.62; 0.36-1.08). CONCLUSION: The findings indicate a strong genetic component of lower urinary tract symptoms, but also support previous studies that modifiable environmental risk factors are associated with this condition.