Birth dates of 1,536 twin-pairs in 1971-1984 were collected from the members of an association of twins' mothers. The seasonal variation of twinning changed every 2-4 years. Years when twinning rate was higher in the summer-fall season (1971-72, 1976-77, 1982-84) and those when a peak of the rate was not observed (1973-75, 1978-81) appeared alternately. In years with a summer-fall peak, the elevation of twinning in the summer-fall season was detected consistently in both like- and unlike-sexed and in both MZ and DZ twin groups. The twinning seasonality in these years, however, was not evident in twin births of mothers who were born in May-July. These results suggest the possibility that seasonal factors which influence the twinning rate be not multiple-ovulation-inducing but probably abortion-inducing factors and most likely seasonally epidemic microbes.

Record linkage of the Finnish Twin Cohort Study with the Hospital Discharge Registry and with the Registry of Rights to Free Medication kept by the Social Insurance Institution gave following numbers of twin pairs with an ophthalmic disease (one or both members of the pair had the disease) in each disease category:98 with glaucoma simplex (5 concordant pairs), 38 with capsular glaucoma (no concordant pairs), 58 with iritis (no concordant pairs) and 149 with strabismus (2 concordant pairs).

The number of concordant pairs in each disease category was small, except for glaucoma simplex in which concordant pairs could be broken down by zygosity. The ratio of observed to expected (based on association by chance) was 6.96 for MZ and 1.74 for DZ pairs. This result suggests that genetic factors play some role in the variability of prevalence of simple glaucoma. Main etiologic factors are stili to be found in the environment. Data on occurrence of other diseases of ophtalmologic importance is presented.

While language, articulation and reading problems have been well documented in young twins, it is not clear how extensive such problems can be or how early in childhood they become evident. At the age of 30 months, twin boys in the La Trobe Twin Study were 8 months behind matched singletons and twin girls on expressive language and 6 months behind on verbal comprehension. They were also 5 months behind on symbolic play and this delay was closely related to language. “Secret” language characterised most of the twin boys but not the girls and the relation of this to articulation delays is discussed. To examine if exposure to other children helps the twin boys, 38-53 month old twins and singletons were matched on the Columbia Mental Maturity Scale at the time of starting preschool. The twin boys had more articulation problems and all twins scored much lower on a Sociability questionnaire completed by the teacher. After 8 months at preschool, all children had advanced in Sociability, but the twins remained just as far behind with poor Sociability relating to poor articulation. The role of intervention programs is discussed.

Twins and singletons were matched for several confounding factors. The monthly distribution of twin births and that of singleton births were compared with a uniform allocation of births over the year. Opposite seasonal variations emerged that were confirmed by a case/control comparison. Twins occurred more often in winter and early spring while singletons proved to be relatively few; singleton peaked in the fall season when the risk of a twin birth was low. These trends held across maternal age at birth and the time period of birth. Results suggest that the conception of a twin pair is higest in spring and early summer and lowest in winter. The role of sunlight in the twinning liability is discussed along with the role of sexual intercourse. That twin-prone mothers are usually more fecund in spring and early summer is a distinct possibility.

This is a follow-up study aiming to describe personality development of twins and possible differences at different ages. The investigation was conducted at the Mother Clinic in the Karolinska Hospital in Stockholm. This report describes the language development of 35 twin pairs at 9 months of age. The development values show that no greater differences exist for girls between twins and singletons. The language result is however lower. The boys, on the other hand, have considerably lower results both on total development scale and on the language scale.

In order to reassess the relative importance of genetic and environmental factors in the development of myopia in Chinese schoolchildren, 90 pairs of MZ and 36 pairs of like-sex DZ twins were enrolled for detailed ophthalmological examination about their ocular refractions. Corneal curvatures and axial lengths were also measured. With equivalent settings of the range for concordance, corneal curvatures showed higher concordance rate (84%) than ocular refractions (65%) and axial lenghts (59%) in MZ twins. F-test on the intrapair variances between MZ and DZ twins revealed a significant hereditary role in determining the ocular refraction and its optical components. The degree of genetic determination was expressed by heritability indices, which were derived from intraclass correlation coefficients. The diversity of sample ages and refractions, while making the obtained data difficult to further explore the gene-environment interaction, led to the observation of more intrapair differences with age and myopic progression in MZ twins.

Twins are regularly reported to invent languages of their own, unintelligible to others. These languages are known as autonomous languages, cryptophasia or idioglossia. Despite current belief, this is not a rare phenomenon. Autonomous languages exist in about 40% of all twins, but often disappear soon. In this study, nine autonomous languages are compared: the circumstances in which they emerge, how these languages relate to the parents' language (the model language) and how they are structured. The prototypical situation is one in which two or more close siblings (not necessarily twins) grow up closely together during the language acquisition period. If an adult model language is frequently absent, the children use each other as a model and acquire the language imperfectly. The language may stabilise at that level. If a model is completely absent, the children probably do not create a language. In all cases known, the language consists of onomatopoeic expressions, some invented words, but for the greatest part of words from the adult language adopted to the constrained phonological possibilities of young children. These words being hardly recognizable, the language may turn out to be completely unintelligible to speakers of the model languages, but they resemble each other in that they lack morphology and that word order is based on pragmatic principles such as saliency and the semantic scope of words. Neither the structure of the languages nor its emergence can be explained by other than situational factors.

Weinberg's difference method, applied to twin birth statistics, usually shows a dependence of the MZ rate on maternal age, like a thin shadow of the DZ rate. Some of this MZ variation could be explained away by James' finding of more same-sex (SS) than opposite-sex (OS) DZ twins, the excess being mistakenly classified as MZ by Weinberg's assumption of equal numbers. By several methods one can extract a Constant value for the MZ rate and a Constant or nearly Constant value for the DZ SS/OS ratio, but these “constants” are actually arbitrary and they vary between populations.

Information about menstrual cycle variables was obtained by questionnaire using 462 female twin pairs. The twins were either members of the Institute of Psychiatry Volunteer Twin Register, or of the Birmingham Population-based Register. The two samples were analysed separately using univariate and multivariate methods so that an independent replication was obtained. Maximum likelihood estimation was used to fit simple models of genetic and environmental variation to these data. The results suggest that age of menarche, menstrual cycle regularity and premenstrual symptom reporting may be heritable, whereas menstrual cycle length is not. The results should be interpreted with caution as not all variables were replicated in the smaller sample, and the method of retrospective menstrual cycle data collection has been questioned.

To study genetic contribution to complex body size traits, the intrafamilial correlation and regression analyses along with twin study method have been used. The data are based on a sample of 45 MZ and 101 DZ twin pairs, their 125 singleton siblings, 104 fathers and 103 mothers in 146 Punjabi families living in Chandigarh, India. Twin study gives no evidence of inequality of means and variances between zygosities. Within-pair genetic variance ratios, correlations, regressions of offspring on midparent and single parent are all significant at 0.1%, thus indicating strong genetic component. Heritability estimates are higher for longitudinal body traits than the breadth dimensions. The resemblance of the children with the parents of either sex is not equal. Higher maternal influence is indicated for a number of body traits. The results on familial correlations do not support the hypothesis of sex-linked inheritance for any of the traits considered in this investigation. These results have been compared with those from other such studies.

Recent changes in the estimated incidence of monozygotic twinning in 15 European populations are described. The overall trend was an increase in the monozygotic twinning rate (MZTR) since the 1960s, particularly in those countries in which the use of oral contraceptives (OC) was widespread. A slower increase or even a decrease in the MZTR was observed in countries with low use of OC. Some countries, eg, Sweden, demonstrated an unexpectedly sharp increase since the 1960s. In Poland and the Federal Republic of Germany the MZTR was already strongly increasing as early as in the 1950s, clearly before the introduction of the pill. The influence of several other factors on the MZTR is discussed, such as toxic and teratogenic agents, pelvic infection diseases caused by the use of intrauterine devices, the increased use of ovulation inducers and neuroleptics as well as changes in the registration of perinatal deaths.

The birth of twins is often accompanied by complications which may affect both twins equally (eg, prematurity) or which may establish differences between the twins (eg, one coming home from hospital first). Parents' ratings of behaviour may reflect and even perpetuate the influence of such variables and it is of interest to see if people less familiar with the twins' history can observe any long-term effects. In the La Trobe Twin Study teachers routinely complete the Bristol Social Adjustment Guide, an assessment of social maladjustment in children aged 5-16 years. Extreme prematurity and problems at delivery were generally associated with the Underreaction syndrome - the child who is unforthcoming, withdrawn and depressed. Withdrawal, depression and maladaptive behaviour to classroom peers were much more common among those twins discharged from hospital after the cotwin. A major mediating factor was that the parents admitted to greatly preferring the one to come home first. These raise questions both about the practice of bringing twins home separately (which occurred in 21% of the cases) and about intervening variables in studies of personality and temperament.

A sample of 122 twin pairs and 166 singletons from the urban Punjabi population of Chandigarh was studied. The observed frequency of double occipital hair whorls was higher in twins than in singletons (P < 0.01). The possible association between twinning and double occipital hair whorls is discussed.

Factors predicting dominance and submissiveness between twins were analyzed in a longitudinal study of 234 twin pairs. Boys were found to be physically dominating, while the girls more often were psychically dominating and in the role of the spokesman. Low birth weight, to be second born, or low Apgar score tended to predict submissiveness, but not significantly. The same was true concerning perinatal asphyxia and hypoglycemia, which also showed some potential cumulative effect when occurring simultaneously. Those having developed faster or with higher intelligence were more often the leaders of the twin pair. Probably good neonatal care in modern hospitals tends to prevent perinatal damage, and therefore the previous findings of second born or small birth weight twins being most submissive was not fully established.

Data based on total births from a geographically defined population with zygosity determined from blood samples and placentation and with data on the use of oral contraceptives routinely collected in early pregnancy showed no association between oral contraceptive use prior to pregnancy in either MZ or DZ twinning. Three mutually exclusive control groups of singletons were used to take account of age, parity and secular trends.

Using high-resolution electrocardiography (HRE), we have previously examined the morphological aspects of the atriogram in a set of DZ triplets comparing the MZ and the DZ pairs [3]. We now repeated the recordings on the triplets, three years after the first studies, in order to evaluate the individual morphological reproducibility of the atriogram and the cardiovascular response to effort and to stress by means of provocative tests [1-3].

We studied a set of female DZ triplets aged 15 years, born normally at the 39th week of gestation, with two chorions and three amnions. We compared the responses to the individual provocative tests, and repeated the analysis of the atriograms after three years. We therefore used the HRE, the Bruce protocol ergometric test, and the sympathetic stimulation tests: handgrip test, cold pressure test, arithmetic mental test (Table).

In a follow-up study of 234 twin pairs, now aged 12-20 yrs, the intertwin relationships were evaluated by the parents and the twins themselves. The dominance-submissiveness aspect was inquired from three separate points of view, physical dominance, psychic dominance, and role of the spokesman. These three different aspects of dominance reflected on the twins' well being and mental health in somewhat different ways. The submissiveness in one area was often compensated by equality or dominance in another area, with only about 10% of adolescents being submissive or dominant in all three areas. According to the parents' reports, the most submissive twins suffered most often from psychosomatic symptoms, and the most dominant ones from nervous symptoms. The twins who themselves reported to be most submissive had most often somatic complaints and scored highest in the Kovacs' Depression Inventory.

To test the assumption that monochorionic twins are monozygotic, cord bloods from 12 sets of monochorionic twins were tested for a total of 16 red cell antigens. Discrepancies were noted among the minor blood groups in 3 of the 12 sets. The result of this pilot study strengthens the contention of others who have previously questioned the general acceptance that monochorionicity and monozygosity are synonymous.