The risk of preterm delivery ( < 37 weeks of gestation) is approximately nine times higher in women with multifetal pregnancies than in women with singleton pregnancies. However, it is possible that the risk will vary according to gestational week. To assess the risk of premature birth within 1 week by gestational age among multifetal pregnancies and compare the estimated risk with that of singleton pregnancies, we analyzed 6 036 475 infants born in singleton pregnancies and 90 887 infants born in multifetal pregnancies in Japan ( ≥ 22 weeks) over the 5-year period 1989–1993. An estimate of the risk of birth within 1 week at gestational week n was obtained by dividing the number of infants delivered at gestational week n by the number of infants delivered at or beyond gestational week n. The risk at 22 weeks was 0.9 per 1000 fetuses for singleton pregnancies and 5.0 per 1000 for multifetal pregnancies. The risk remained relatively stable until 27 weeks of gestation, then sharply increased toward 36 weeks of gestation in both singleton and multifetal pregnancies. The odds ratio for birth within 1 week for fetuses of multifetal pregnancies compared with fetuses of singleton pregnancies was 5.9 (95% CI, 5.4–6.5) at 22 weeks of gestation, increasing gradually with increasing gestational age until 33 weeks of gestation (13.7; 95% CI, 13.1–14.2) but declining thereafter to 8.8 (95% CI, 8.6–8.9) at 36 weeks of gestation. Results of data analysis for each year of the 5-year period did not differ substantially. Twin Research (2000) 3, 2–6.

Twin pregnancy is considered to be a risk factor for congenital dysplasia of the hip. From 1987 until 1996, the hips of 4476 (2260 male, 2216 female) newborn babies were examined by ultrasound according to Graf's technique and classification in our hospital. In this study, we compare the results of twins and singletons for this risk factor. Of the newborns, 97 (2.2%) were twins (40 male, 57 female); 39 pairs of twins (10 male/male, 19 female/female, 10 male/female) and 19 individual twins (6 male, 13 female) were investigated. Hips of typeIa, Ib and IIa (α ≥ 55°) are not pathologic; hips of typeIIa (α < 55°) need an early control examination; and hips of typeIIc, D, IIIa, IIIb and IV require therapy. TypesIa, Ib, and IIa (α ≥ 55°) were found in 4207 (94.0%) of all newborns, in 4112 (93.9%) of the singletons, and in 95 (97.9%) of the twins. Early control examination and/or therapy (indicated for typesIIa (α < 55°), IIc, D, IIIa, IIIb, and IV) were necessary in 269 (6.0%) of all cases, in 267 (6.1%) of singletons and two (2.1%) of twins. Twins with additional factors such as breech position birth, hip dysplasia in the family or premature birth did not show the types of hip IIa (α < 55°), IIc, D, IIIa, IIIb, IV. We did find these hips in two (3.5%) of the female twins, but not at all in the male twins. Statistically, twins with or without other risk factors that are known before birth did not show significantly more of type hip IIa (α < 55°), IIc, D, IIIa, IIIb, IV (P > 0.05). Twin Research (2000) 3, 7–11.

The incidence of dizygotic (Dz) twinning can be used as an index of natural human fertility. A retrospective study was done at The Royal Women's Hospital, Australia, to see whether the dizygotic to monozygotic (Mz) twinning ratio from one hospital can accurately reflect the national incidence of Dz twinning. The yearly twinning incidence from 1947–1997 was expressed as a Dz:Mz ratio, standardised for maternal age and plotted against previously published national statistics. The proportion of mothers born in Asia (of both singleton and multiples) between 1983–1997 was analysed to see whether different racial mixes might influence twinning trends. There were 5275 twins born of known sex and maternal age between 1947–1997. The agestandardised Dz:Mz ratio increased non-significantly from 1.39 in 1947 to 2.29 in 1953 (P = 0.08), underwent a significant decline to 0.73 in 1977, then remained stable until 1997 (P > 0.05). The same trends were also apparent when the data was pooled into 2-year groups with the increase from 1947/48–1953/54 becoming highly significant (P < 0.009). These trends observed in the hospital-based data were in close agreement with those found in the national statistics, with the exception of a rise in 1977–1982 only reflected in the Australia-wide data. In 1993, 2.6% of mothers were born in Asian countries; by 1997, this had risen to 10.6%. We found that the Dz:Mz ratio from one hospital closely reflects national twinning trends. Prospective studies must account for race, and would need around 200–300 twin pairs per year to minimise fluctuations of the ratio. Twin Research (2000) 3, 12–16.

The objective of this study was to estimate how much of an individual's appreciation of humour is influenced by genetic factors, the shared environment or the individual's unique environment. A population-based classical twin study of 127 pairs of female twins (71 monozygous (MZ) and 56 dizygous (DZ) pairs) aged 20–75 from the St Thomas' UK Adult Twin Registry elicited responses to five ‘Far Side’ Larson cartoons on a scale of 0–10. Within both MZ and DZ twin pairs, the tetrachoric correlations of responses to all five cartoons were significantly greater than zero. Furthermore, the correlations for MZ and DZ twins were of similar magnitude and in some cases the DZ correlation was greater than that of the MZ twins. This pattern of correlations suggests that shared environment rather then genetic effects contributes to cartoon appreciation. Multivariate model-fitting confirmed that these data were best explained by a model that allowed for the contribution of the shared environment and random environmental factors, but not genetic effects. However, there did not appear to be a general humour factor underlying responses to all five cartoons and no effect of age was seen. The shared environment, rather than genetic factors, explains the familial aggregation of humour appreciation as assessed by the specific ‘off the wall’ cognitive type of cartoons used in this study. Twin Research (2000) 3, 17–22.

The aims of the present study were to evaluate the contribution of the genetic and environmental factors to the risk of teenage childbearing, and to study whether life style, socio-economic conditions, and personality traits could explain possible familial effects. We linked two population-based registers: the Swedish Twin Register and the Swedish Medical Birth Register. The study covers female twin pairs born between 1953 and 1958, having their first infant before the age of 30 years (n = 1885). In order to separate familial effects from other environmental influences, and genetic effects from shared environmental effects, only complete twin pairs with known zygosity were included, in all 260 monozygotic and 370 dizygotic twin pairs. We used quantitative genetic analyses to evaluate the importance of genetic and environmental effects for liability to teenage childbearing. Logistic regression analyses were used to estimate the effects of life style, socio-economic situation, and personality on the probability of teenage childbearing, and to study whether psychosocial factors could explain possible familial effects. Fifty-nine percent (0–76%) of the variance in being a teenage mother was attributable to heritable factors; 0% (0–49%) was due to shared environmental factors; and 41% (23–67%) was explained by non-shared environmental factors. Thus, the data were consistent with the hypothesis that the familial aggregation of teenage childbearing is completely explained by genetic factors, although the alternative hypothesis that familial aggregation is entirely explained by shared environmental factors cannot be ruled out. Significant effects of smoking habits, housing conditions, and educational level were found in relation to liability to teenage childbearing. However, the familial effects on risk of teenage childbearing were not mediated through similarities in life style and socio-economic factors. When studying risk factors for teenage childbearing, it is recommended to include life style and socio-economic variables as well as information about family history of teenage childbearing. Twin Research (2000) 3, 23–27.

There have been long questions about the relationship of schizophrenia to other mental disorders. Lifetime DSM-III-R diagnoses of mood and anxiety disorders in twins with clinically diagnosed schizophrenia (n = 24) and their non-affected co-twins (n = 24) were compared with twins from pairs without schizophrenia (n = 3327) using a sample from the Vietnam Era Twin Registry. Schizophrenic probands had significantly elevated rates of all included disorders (bipolar disorder, major depression, dysthymia, generalized anxiety disorder, panic disorder, and PTSD) compared with controls (P < 0.01). The odd ratios comparing co-twins of schizophrenic probands with controls was greater than three for every disorder, but did not attain statistical significance. A similar pattern was observed when analyses were restricted to only monozygotic twins (n = 12). Consistent with other studies, schizophrenics appeared to have higher rates of a range of mental disorders. Our results suggest that schizophrenia per se represents a risk factor for other psychiatric disorders, but the absence of significantly elevated risk among non-schizophrenic co-twins suggested that family environmental and/or genetic factors that contribute to risk of schizophrenia do not increase the risk of mood and anxiety disorders to the same extent that the risk of these other disorders is increased by the presence of schizophrenia. Twin Research (2000) 3, 28–32.

To identify large numbers of twins affected by chronic disease as potential subjects for studies of environmental and genetic chronic disease determinants, we advertised for affected twins over the period 1980–91 in newspapers across North America. Responses were received from 17 245 twin pairs in which cases of cancer or other chronic disease had occurred. To assess the representativeness of affected twins identified by advertising, we evaluated the pattern of reporting, compared the cases identified to the number of cases estimated to be prevalent among all North American twins, compared the cases to population-based singleton case series, compared the healthy co-twins to population-based samples of healthy persons, assessed the impact on ascertainment of opinions about disease causation, compared the pattern of prospective to retrospective ascertainment of disease in the originally unaffected co-twins of cases, and compared the results of the prospective ascertainment of disease in co-twins to comparable published estimates. Youth, gender, zygosity, education, and disease concordance were found to be overall determinants of ascertainment. Disease-discordant DZ twins appeared to be modestly underascertained. While somewhat better educated, both concordant and discordant pairs were judged to be reasonably representative of affected non-Hispanic white North American twin pairs of comparable status, ie of comparable age, sex, race, and zygosity. If interpreted with caution, the concordance patterns of such twins can be used to generate genetic hypotheses, but should not be the basis of definitive heritability analyses. We conclude that advertising offers a method of identifying pairs of twins that can serve as subjects for studies designed to identify disease determinants. Twin Research (2000) 3, 33–42.

Central body fat distribution has been shown to be related to hyperinsulinemia, insulin resistance, hypertriglyceridemia, and atherosclerosis to a greater degree than general obesity. There are known to be both genetic and environmental effects on all components of this clustering. Whether these genetic effects are due to one set of genes in common to the components or whether genetic influences on insulin resistance and/or general/abdominal fatness ‘turn on’ other genes that affect other components of the syndrome is not clear. We analyzed data from the Swedish Adoption/Twin Study of Aging (60% female; monozygotic = 116, dizygotic = 202; average age 65 years) to determine whether there were genetic and/or environmental factors shared among general body fat distribution, abdominal body fat distribution, fasting insulin levels and cardiovascular disease. We found additive genetic effects in males to be significantly different from those in females with genetic effects accounting for variance in waist–hip ratio (males = 28%; females = 49%), body mass index (males = 58%; females = 73%), fasting insulin levels (FI) (males = 27%; females = 49%), and cardiovascular disease (CVD) (males = 18%; females = 37%). There were also shared genetic and environmental effects among all the variables except CVD, but a majority of the genetic variance for these measures was trait specific. Twin Research (2000) 3, 43–50.

The traditional frailty models used in genetic analysis of bivariate survival data assume that individual frailty (and longevity) is influenced by thousands of genes, and that the contribution of each separate gene is small. This assumption, however, does not have a solid biological basis. It may just happen that one or a small number of genes makes a major contribution to determining the human life span. To answer the questions about the nature of the genetic influence on life span using survival data, models are needed that specify the influence of major genes on individual frailty and longevity. The goal of this paper is to test the nature of genetic influences on individual frailty and longevity using survival data on Danish twins. We use a new bivariate survival model with one major gene influencing life span to analyse survival data on MZ (monozygotic) and DZ (dizygotic) twins. The analysis shows that two radically different classes of model provide an equally good fit to the data. However, the asymptotic behaviour of some conditional statistics is different in models from different classes. Because of the limited sample size of bivariate survival data we cannot draw reliable conclusions about the nature of genetic effects on life span. Additional information about tails of bivariate distribution or risk factors may help to solve this problem. Twin Research (2000) 3, 51–57.