Phenotypic variation in human population may contain contributions from a number of different sex-associated genetic influences. These influences include maternal effects, the effects of sex-linked loci, and the effects of sex-limited autosomally linked loci. The families produced by MZ and DZ twins provide statistics which permit the detection and estimation of these effects. In particular, they provide statistics derived from various types of age-matched half-sibs and cousins in addition to those derived from the more usually studied full-sib or parent-offspring relationships. Specific models for genetic maternal effects, sex-linkage and sex-limitation are used to explore the use of extended twin design for the detection of and the discrimination between various sex-associated effects. The sample sizes required to detect maternal effects and sex-linkage were considered for some simple cases and it is concluded that comparison derived from the progeny of twins will often provide better tests for these effects than those derived from parent-offspring comparison.

A concordance study of 6 infectious diseases of childhood has been carried out in a sample of 656 twin pairs classified by sex and zygosity. A new approach is proposed to estimate the respective influence of heredity and of common environment. The estimates thus obtained range from 86% hereditary component in the case of measles to 100% environmental component in the case of scarlet fever.

Problems of ascertainment bedevil the investigation of the etiology of congenital anomalies in singletons and in multiple births by epidemiological methods. It is shown that the definition of the population of affected births is ambiguous and that the problem of tracing etiology is complicated by the systematic way in which anomalies may be missed at birth. The available methods of dealing with problems of ascertainment are reviewed. Methods of adjusting for possible bias of ascertainment of affected births, by fitting statistical models to data from several sources, have been employed in some previous studies. In these methods, it is assumed that there are no errors of diagnosis or of recording. However, it is shown that there may be discrepancies in recorded diagnoses between sources, rendering this assumption untenable. In these methods, it is also assumed that the model which is the best fit to the data on the ascertained cases in also the best model for the cases which were not ascertained. This assumption is tested indirectly in a retrospective analysis of data from Aberdeen and Belfast collected concurrently through routine recording systems. It is demonstrated that there was a social process in ascertainment which renders the methods of adjusting for bias of ascertainment at best very complicated and at worst inapplicable.

Seventy-nine pairs of same-sex twins were examined at the age of three years at a municipal clinic and their mothers were interviewed to assess the twins' current and past behaviours. The zygosity was determined after the interview by fingerprints and/or bloodtyping in the majority of cases. The following significant differences in concordance between monozygotic and dizygotic twins were found: 1) fear of strangers during observation at the clinic; 2) marked fear of strangers in the first year of life; 3) whether or not the child was startled by sudden noises during infancy; 4) whether or not the child was able to sleep alone at three years without a parent sitting nearby; 5) susceptibility to motion sickness; 6) nocturnal enuresis; 7) short attention span or restlessness during the test.

The rates of MZ and DZ twin births and of triplet births slightly decreased in Japan in 1955-1967 and 1974, whereas that of trizygotic triplets tripled. The rates of triplet and quadruplet births were comparable to those of Caucasians. Although the sample size is small, Japan seems to be characterized by higher rates of triplets and quadruplets of polyembryonal origin and lower rates of those of polyovulational origin as compared to Caucasian populations. Stillbirth rates in both MZ and DZ twins gradually decreased in 1960-1967 and 1974. The rate was higher in MZ than DZ twins, and higher in males than females. The secular trends of stillbirth rates in like-sexed triplets was similar to that in twins, whereas the rate in unlike-sexed triplets remained almost constant for the entire period. The rate was higher in like-sexed than unlike-sexed triplets.

A case of monozygotic male twins discordant for skeletal and cardiac defect is reported. One twin had the hemifacial microsomia type of the oculo-auriculo-vertebral dysplasia. The cotwin had no asymmetry of the face and normal ears, but preaxial polydactyly and ventricular and auricular septal defects. The cotwins were concordant for craniostenosis with a ridge metopic suture. Karyotypes were normal.

A total of 178 placentas of twins born in Tokyo in 1961-1982 were examined. Only just over one third were dichorionic, 110 being monochorionic and DZ twins were estimated to account for 24.7% of the pairs. The lower frequency of DZ twinning in Japan is attributed to lower gonadotropin values in Japanese women as compared to European, American, or African women. High fetal morbidity was found in monochorionic twins and this is attributed to a frequent occurrence of vascular anastomoses.

Three case reports are presented concerning 1) Albino MZ female twins, 2) Raynaud's syndrome in MZ female twins, and 3) Pseudohermaphroditism in MZ male twins.

Some health related psychosocial correlates of the Eysenck neuroticism scale were examined in a questionnaire study of 1501 monozygotic (MZ) and 3455 dizygotic (DZ) male twin pairs representing the adult male twin population in Finland. In analyses of the individuals, 34% of the variance in neuroticism was associated to: psychological variables (stress of daily activities, life satisfaction, quality of sleep, and extroversion – the explanatory rate of this variable set was 30%), psychotropic drugs (5%), alcohol use (4%), and smoking (2%). Neuroticism was also associated to social, life change, and medical variables. In pairwise analyses, the heritability estimate (h2) was 0.54 for pairs living together and 0.39 for pairs living apart. It seems that heritability estimates are confounded by the closer intrapair relationship between members of MZ than DZ pairs. In pairwise analyses, 23% of the intrapair difference of neuroticism in MZ pairs was associated to intrapair differences in the aforementioned variables. The following explanatory rates were found: psychological variables, 21%; psychotropic drugs, 2%; alcohol use, 2%; and smoking, 1%. Neuroticism of pairs discordant for background variables showed similar intrapair differences as between individuals in the following variables: service vs farming work, use of alcohol, use of antacids, hypertension, heavy physical work, quality of sleep, changes of workplace for negative reasons, smoking, and use of tranquillizers. It appears that in Finland environmental factors explain at least 61% of the variability in neuroticism, and that factors determining neuroticism are also associated to health related behavior such as smoking, use of alcohol and psychotropic drugs.

Old-age twinning rates were evaluated over a period of 250 years (1611-1860), from 80,256 parish records of baptism in a church office in Görlitz, East Germany. Two periods with high and two periods with low twinning rates were observed. Twinning rates were relatively stable in spring and fall, but varied greatly in winter and summer.

Hospitalization rates of monozygotic (MZ) and dizygotic (DZ) twin pairs in Finland were compared for schizophrenia, neuroses, and alcoholism. Record-linkage of hospital records and death certificates for the years 1972-1979 was carried out for persons in the Finnish Twin Cohort (16,649 like-sexed twin pairs). The ratio of the number of observed vs that of expected concordant pairs and the ratio of concordance rates between MZ and DZ pairs were greater among males than females, and greater among young (40 years old or less) than among older pairs. The highest difference was found in schizophrenia and the lowest in neuroses. Pairwise concordance rates for schizophrenia (11.0% for MZ and 1.8% for DZ) seem to indicate great environmental influence (high proportion of discordant pairs) with apparent genetic liability (6.1-fold ratio in concordance between MZ and DZ pairs). In neurotic disorders, the difference of pairwise concordance rates between MZ and DZ pairs (6.8% vs 4.0%) was quite low, not strongly supporting a genetic hypothesis. Of the MZ pairs concordant for psychiatric hospitalization, 47% had lived together for their whole life time; of those discordant, 16% lived together. The corresponding figures for DZ pairs were 18% and 15%. The effect of intrapair relationships in disease-concordant pairs should be taken into account when evaluating the effect of genetic and environmental factors in psychiatric disorders.

Results on genetic variability in some of the morphometric characters on head and face, body girths and skin folds, based on 45 MZ and 67 like-sex DZ twin pairs, are presented. The data were subjected to a method which eliminated possible biases in the estimated genetic variances that could result from heterogeneity of total variances between zygosities of the 17 head and face measurements, heterogeneity was observed for only bizygomatic diameter. Head breadth means differed between MZ and DZ twins, indicating bias in the trait's genetic variance analysis. The results indicated a significant genetic component in these morphometric traits. For 11 girth and skinfold measurements, the t'-test based on hierarchical structure of twin data, also failed to reveal any appreciable difference between the mean values of MZ and DZ twins. Heterogeneity of total variance between zygosities was observed for three skinfold measurements, ie, biceps, triceps and suprailiac. The girth measurements, however, did not reveal any heterogeneity of total variances between the zygosities. The estimates of genetic variance revealed stronger genetic component for the girths than for the skinfolds.

A study of 74 twin pairs with alcoholic probands from the Maudsley Hospital is reported. Pairwise concordance for alcoholism as categorised by the SADS-L Research Diagnostic Criteria is similar in MZ twins (29%, 8/22) and DZ twins (33%, 13/39). Concordance for all diagnoses other than alcoholism is however significantly greater in MZ twins (48%, 13/27) than DZ twins (21%, 8/39, P < 0.001). Concordance in MZ and DZ twins for all diagnoses other than alcoholism, inlcuding depression, did not coincide with concordance for alcoholism. Data on criminality revealed that 21% (32/148) of the twin sample had non-alcohol, non-traffic offences on record at the UK Home Office. Of the 32 with criminal records, 28 were alcoholic probands and pairwise concordance for criminality was found in only 1 MZ and 2 DZ pairs.

Records of two hospitals and of an association of mothers of twins were analysed for the years 1924-1980, finding a total of 656,109 and 1185 twin births, respectively. Twinning rates were higher in summer or fall, in general, but these seasonal variations occasionally changed within a short period of years, as in 1973-75 in the Association group. The twinning rate was also affected by the month of mother's birth. It was low among the mothers born around May-July in maternities up to 1960, and these mothers did not show seasonal variation of twin births. The unlike-sexed twinning rate of the mothers born in May-July was also lower than that of the other mothers, and constantly low even at higher maternal ages. A possible explanation for these phenomena is discussed.

The immediate global impressions of zygosity (based upon physical similarity) were recorded by the investigator at initial meetings with 105 pairs of twins. The accuracy of these ratings, as well as classifications provided by two objective procedures (physical resemblance questionnaire; dermatoglyphic analyses) and two subjective procedures (parental impressions; physician's impressions), were evaluated by comparison with results from bloodtyping for 53 pairs. The judgments of the investigator furnished the most accurate indication of zygosity (94-96% accuracy). Laboratory tests were repeated for five pairs when the results proved incompatible with the investigator's ratings. In all five cases, the investigator's judgments were confirmed, indicating that a laboratory error had occurred. It appears that the opinion of a skilled observer of twins can provide a convenient and highly effective alternative to bloodtyping.

Serum gonadotropin, estradiol, prolactin and alpha-1-antitrypsin levels were measured in the first four days of the menstrual cycle in 14 women who were cycling normally. FSH, and to a lesser extent LH levels, were significantly higher in a group of 8 women who had at least one set of DZ twins (6 of whom had 2 sets) than in a control group of 6 women with no DZ twins. Estradiol levels also tended to be higher in mothers of twins but there were no significant differences in prolactin or alpha-1-antitrypsin concentrations. There were no differences between the means of the two groups in age, height, weight or number of pregnancies.

Revelations about twins are abundant, but the information often is inaccessible. Therefore, The Twins Foundation, a multifaceted, international, nonprofit agency has been formed. Through development of a Research Library, Museum and Hall of Fame, it will (1) provide archival and research support to the growing number of disciplines involved in twin studies; (2) provide a central repository and clearing house of information about twins for the general public; (3) enlist twin achievers to act as role models for adolescent twins; and (4) collect information about the contributions twins have made to their societies in order to dispel still prevalent negative stereotypes about multiples. (Note: The term “twin” is used generically and implies all multiples).

Mothers of twins, as compared with mothers of singletons, are taller, and for a given height are heavier; they have slightly earlier menarche and menopause. Our recent studies have shown that mothers of DZ twins are at greater risk of cancer of the pancreas (but not for cancers of other sites), as well as of death due to diabetes, other endocrine diseases, and allergies. These findings are consistent with the clinical and epidemiological literature. We have also found that singleton siblings of twins are slightly heavier at birth than children born into sibships consisting of singletons only. The need for further research on the characteristics and long-term health of mothers of twins is indicated.