In previous papers a stochastic model of the ageing process has been proposed. Some genetic parameters (redundance, repair) have been used to explain the observed differential predisposition to the process and family heredity. Because the process is basically due to effective random mutations, any individual of the population would be predisposed differently to ageing according to the structure of his/her genome. In the present paper, the previous model is generalized to take into account an additional genetic parameter, namely, the stability against random mutations, defined as the probability that a random mutation in a codon would produce no mutation in the corresponding protein. Estimation problems connected with the model are approached on the basis of twin data in maximum likelihood estimation as well as in bayesian framework. Some comparisons between the two methods are reported.

The population-based twin registries of Denmark, Finland, Norway and Sweden represent an extraordinary resource for scientific research. Although each register has its own history and composition, they share certain common qualities. All the Nordic countries have a long tradition of population registration, a high standard of living and health-related registers of high quality. The large size of the registers means that they are uniquely placed for representative studies of rare occurrences. Examples of studies that these registers make possible are illustrated with data from the Finnish Twin Cohort, which in its first phase consisted of over 17,000 like-sexed twin pairs born before 1958. It has been recently expanded to include multiple births between 1958 and 1986 (nearly 23,000 sets) and their first-degree relatives. During this period the DZ/MZ ratio decreased, while an increase in MZ twinning rates was observed between 1974 and 1986.

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.

We consider three alternative parametric models to describe genetic and environmental influences on smoking initiation and smoking persistence. Under the single liability dimension model, the same genetic and environmental influences which determine smoking initiation also influence smoking persistence. Under the independent liability dimensions model, independent initiation and persistence dimensions determine onset of smoking, and persistence in those who become smokers. The combined model also postulates separate initiation and persistence dimensions, but allows for the possibility that some smokers are so low on liability to smoke on the initiation dimension that they become ex-smokers for this reason. Reanalyses of London twin data published by Eaves and Eysenck support the single liability dimension model. We discuss the difficulty of reconciling this finding with the hypothesis that nicotine dependence is a major determinant of smoking persistence, but caution that sample sizes in the London twin study were small.

Many studies of embryogenesis and fate of twin pregnancies are invalidated because zygosity is not determined definitively, or is assumed on the basis of inadequate criteria. This paper briefly reviews methods of zygosity determination. It reports published results and a new series of twins in which zygosity was determined by DNA fingerprinting. Implications for methods of prenatal diagnosis of zygosity are discussed in the context of the occasional need for intervention in twin transfusion syndrome or in twins discordant for major malformations. Definitive zygosity and placental anatomy (number of chorions and amnions) is discussed as the firm substrate for studies of normal and abnormal twin development.

The use of DNA restriction fragment length polymorphisms (RFLP) to analyze variable number of tandem repeat (VNTR) sequences dispersed in the human genome, has become a powerful tool for the study of population genetics due to the very substantial polymorphism involved. Because the markers usually employed for twin zygosity determination (such as sex combination, placentation, HLA typing, blood group antigens, etc) may not be uniformly informative, we propose the use of synthetic olygonucleotides, representing VNTR “core” sequences, for the determination of zygosity in twins.

In order to examine the chronologic changes in genetic variance and heritability of anthropometric characteristics of Chinese infants in Taiwan, a total of 521 pairs of same-sexed twin neonates given birth in four major general teaching hospitals in Taipei City were studied. Based on the placental pattern and 12 red blood cell antigens, 428 MZ and 93 DZ twin pairs were identified and followed up to the age of one year. There was no significant genetic variance for all anthropometric characteristics adjusted for sex and gestational week before the age of six months. After adjusting for sex and gestational week, a significant genetic variance was observed at the age of six months, with heritability values of 0.51 (weight), 0.63 (head circumference), 0.77 (chest circumference), and 0.53 (arm circumference), as well as at one year, although with considerably lower heritability values. This implies that growth is dynamically determined by both genetic and environmental factors during infancy.

In order to examine the genetic variance and heritability of temperament among Chinese infants in Taiwan, a total of 62 pairs of same-sexed twin infants given birth in four major general teaching hospitals in Taipei City were studied. Based on placentation and 12 red blood cell antigens, 44 MZ and 18 DZ pairs were identified. Temperament was assessed at the age of six months by the Chinese edition of Carey's Temperament scale. Significant genetic variance was observed for activity level, approach or withdrawal, intensity of reaction, quality of mood, and threshold of responsiveness, with a heritability of 0.64, 0.56, 0.74, 0.39, and 0.45, respectively. There was no significant intrapair difference in temperamental characteristics between monochorionic and dichorionic MZ twins.

Data of 601 families with triplets and higher multiples have been collected. Since about nine years the number of higher-order births has been increasing enormously. The average pregnancy duration and the average birthweight of these mostly premature children have been declining from year to year. Despite the progress in neonatology, the death rate and the rate of handicapped children is very high. To prevent such disastrous outcomes, treatments for infertility should be performed only by physicians in centers with strong controls. Selective abortions are no regular solution to the problem of higher multiple gestation.

Using therapeutic intervention, the psychological and behavioural factors in the development of language retardation in two pairs of 4-year-old MZ twins have been examined. Although some factors are common to those found in singletons with language retardation, the factors peculiar to the twin situation are highlighted.

Out of 296 mothers of twins interviewed about difficulties encountered while rearing twins, 49% mentioned inadequate sleep, 18% financial strain, 39% emotional disturbance, 22% disturbance in the marital relationship, and 43% lack of time to take care of other children. With the growth of twins, the problems gradually decreased, except for emotional disturbance, the most difficult period being the first year. Only 15% reported leaving the babies in the care of relatives during the day time. Overall, 68.2% felt the support from the father, but 39% of them felt the support was useless; 49% felt support from the mother-in-law, but 43% of them regarded the mother-in-law to be of no help; 36% were supported by their own mothers, but 54% mentioned that they had received little help. One must conclude that social support from relatives was inadequate. Public policy should take this fact into account and provide public health nurses and social workers to help the mothers of twins, especially during the first year.

Over 80,000 multiple birth babies are born each year in the U.S. Their families must cope with a constellation of complex physical and psychosocial challenges, which jeopardizes their health and functioning. The demands of twin pregnancy and the parenting of twins, triplets, quadruplets, quintuplets or more puts these families at disproportionately high risk for infant mortality, birth defects, child abuse, substance abuse, financial problems and marital problems. Health and social service resources must be developed to alleviate the stresses associated with multiple birth and to empower parents to cope well. Guidelines for policies, parent education, professional training and service delivery developed for the California Department of Health Service, Maternal and Child Health Branch, are here offered for the consideration of policy makers and public health planners.