Respiratory distress syndrome occurred in 77 of 1,130 liveborn twins (1: 15) and appears to be more frequent than in singletons. This increase cannot be entirely accounted for by the higher prematurity rate of twins over singletons. In 46 twin pairs with at least one affected, there was a significantly higher concordance rate among MZ than DZ pairs, suggesting that genetic factors are of some etiologic importance in this disease.

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.

Skin reflectances were obtained at 685 mμ wave length from the medial upper arm and the forehead regions among 105 Tibetan males and 105 Tibetan females, in order to study sex differences in human skin pigmentation. The forehead pigmentation of males was always found to be darker than that of females, and the difference to be significant during middle and late adolescence The medial upper arm pigmentation of girls was found to be darker than that of boys, during early adolescence; but during middle adolescence the two sexes exhibited just similar pigmentation and during late adolescence females emerged to be significantly lighter than males. The present study, thus, suggests that one of the factors causing sex differences in human skin pigmentation may be the physiological changes occurring during adolescence, which may be genetically determined.

Behavioral development during childhood is examined in relation to recent concepts and data from evolutionary theory and developmental genetics. The epigenetic framework of Waddington is proposed as a powerful tool for analyzing the progressions in behavior, particularly for recognizing that development involves coordinated pathways of change over time. Many of these pathways appear to depend upon the activity of timed gene-action systems that switch off and on according to a predetermined plan. Behavioral development thus gives expression to the dynamics of preprogrammed change; and in this perspective, behavioral discontinuities may be as strongly rooted in the epigenetic ground plan as the continuities are. The present paper aims to pull together some common themes from different areas that bear on the central issues of behavioral development—the neural foundations, the time course followed, the interplay of maturation and experience, the extent of preorganization furnished by the genetic program, and the adaptive significance of such behaviors in an evolutionary perspective. The final section touches on some hypotheses drawn from developmental neurobiology and developmental genetics that may enrich the analyses of human behavioral development.

This report examines the impact of two major kinds of unequal ascertainment on the estimation of true probandwise concordance (Cpbt ) in twin studies: 1) concordance-dependent – where the ascertainment rate differs in affected members of concordant vs discordant pairs, and 2) non-independent – where ascertainment rates differ in affected members of concordant pairs where the cotwin has vs has not been ascertained. Concordance-dependent ascertainment is easily modeled algebraically; non-independent ascertainment is more complex and we here propose a model based on survival analysis. Overall, concordance-dependent ascertainment produces greater bias in estimates of probandwise concordance than does non-independent ascertainment. The bias introduced by concordance-dependent ascertainment is greatest when Cpbt is low and/or when the ascertainment rate for twins in concordant pairs is low. The bias introduced by non-independent ascertainment is greatest when Cpbt is high and/or when the ascertainment probability for an affected twin in a concordant pair where the cotwin has already been ascertained approaches unity. The impact of concordance-dependent and non-independent ascertainment on estimates of heritability and common environment is examined. Correction terms to estimate Cpbt in the presence of concordance-dependent and/or non-independent ascertainment are presented.

This study compares standardized measures of childhood behavior problems in a community-based twin sample with those for normative samples from the general population. Maternal parent ratings on the Child Behavior Checklist (CBCL) for 1824 twins were compared with the CBCL normative sample. The results indicated that twins showed small but consistently higher levels of problem behaviors. These elevations were significant for older children on both internalizing and externalizing behaviors; for younger children the elevations were significant for externalizing but not internalizing behaviors.

Delivery records of public hospitals and birth certificates of household registration offices were examined to study the epidemiological characteristics of twinning rate from 1955 to 1984 in Taiwan. The MZ twinning rate was consistently higer than the DZ rate during the study period. The DZ rate declined steadily from 2.7 per 1000 in 1955 to 1.3 per 1000 in 1975, and then gradually increased to 3.6 per 1000 in 1984. The MZ rate peaked periodically in 1956, 1966 and 1976, and gradually increased from 3.3 per 1000 in 1978 to 5.9 per 1000 in 1986. Both MZ and DZ rates were higher in urban than in rural areas and they were also higher in northern Taiwan than elsewhere in the island. While both MZ and DZ rates increased with maternal age and parity, the maternal age difference and the parity difference were more striking in DZ than in MZ rates. The international comparison also showed a greater racial difference in maternal age-specific DZ than MZ twinning rates; and the older the maternal age, the greater the international discrepancy in DZ rates.

The 20 reported cases of MZ female twins discordant for X-linked diseases are reviewed. In such twins the X-inactivation pattern is opposite skewing (abnormal allele inactivated in most cells of the normal twin, and normal allele inactivated in most cells of the affected twin) or skewing in one twin and random in the cotwin. The diseases involved map in two specific regions: Xq27-28 and Xp21. The only exceptions are Fabry's disease and Aicardi's syndrome, which map in Xq22 and Xp22 respectively. No concordant MZ female carrier twins, either normal or affected, have been described. Three main hypotheses have been proposed to explain such characteristics [2, 5, 14], but none is completely satisfactory. The constant discordance for X-linked diseases in MZ female twins has important consequences for genetic counselling.

An up-to-date review of literature on the distribution of haemoglobinopathies in the Indian subcontinent has been presented. Haemoglobins S, D, E, J, K, L, M, Q, β-thalassaemia major, β-thalassaemia minor, α-thalassaemia minor, haemoglobin H, haemoglobin Lepore and persistence of foetal haemoglobin have been reported from India, β-thalassaemia in combination with haemoglobins S, E, D, J, K, Q has also been reported. Haemoglobin S is found to be prevalent among tribal populations in various regions of India, while haemoglobin D is prevalent among Sikhs, Pathans and Gujratis. Haemoglobin E is present in high frequency among populations of Assam, Bengal and Veddahs of Ceylon. β-thalassaemia major is widespread in various populations. There is paucity of data regarding distribution of β-thalassaemia minor and α-thalassaemias in this region.

In 1988, questionnaires were received from 1,400 twin pairs (17% MZM, 23% MZF, 17% DZM, 19% DZF, 24% DZO) aged 11 to 18, registered with the Australian NHMRC Twin Registry. Twins reported independently on themselves and on the perceived behaviour of their parents, siblings and friends. For smoking and for drinking in the previous month, the prevalence was modelled as a logistic function of age, sex, perceived smoking or drinking behaviour of family and friends, and the Junior Eysenck Personality Questionnaire (JEPQ) scales. Strenghts of association were: family behaviour, odds ratio (OR) ≤2; Extraversion and Psychoticism, interquartile OR ≈ 1.6; behaviour of friend, OR ≈ 3 to 6. Twin associations were represented by odds ratios. For smoking they were 16 in MZ and 7 in DZ same-sex pairs, and 3 in DZO pairs. Although the former is consistent with genetic factors determining adolescent smoking behaviour, the reduced association in DZO pairs and strong association with smoking by friends argue to the contrary. For drinking, twin odds ratios were 11 in MZM, MZF and DZF pairs, and 4 in DZM and DZO pairs, consistent with genetic factors determining alcohol use in male but not female, adolescents. Twin odds ratios were not influenced by adjustment for the JEPQ scales; this does not support the hypothesis that genetic factors which determine personality also determine smoking or drinking behaviour during adolescence.

Data with regard to the proportion of congenital malformations found at birth are presented from a survey of 657 pairs of twins of known zygosity and placentation delivered in Aberdeen and Northeast Scotland between 1968 and 1979. There was an excess of malformed individuals from monozygotic (MZ) pairs, but this did not reach statistical significance. The possible effect of monochorionic placentation in the causation of malformations in MZ twins in general is discussed. It seems that this type of placentation may be of less causative importance than has been previously suggested.

Two cases of holoacardius are presented. Both had a normal 46,XX female karyotype, identical to that of their respective cotwin. Data are presented supporting the concept that the placental vascular anastomoses are the primary agents in the formation of an acardiac. Cytogenetics, epidemiology and other theories of pathogenesis are discussed.

Twins, more than any other home reared infants, experience intimate and usually continuous interaction with an age-mate from early infancy. The effects of this situation are examined and its implications for both the emotional and cognitive development are discussed in the light of evidence from two studies in which play sessions were video-recorded and subsequently analysed into behavioural categories. The first was a longitudinal study of a single pair of twins, while the second was a short-term study of interaction and cooperation in infants in the 9 to 20 months range. Five pairs of twins, 5 pairs of familiar peers, and the children's mothers were observed playing a structured cooperative game. The hypothesis is advanced that twins enjoy more emotional support but less intellectual stimulation than singletons and it is suggested that social enrichment of the environment of twins could foster social and cognitive development.

A simple path model applicable to twins and their parents and involving both cultural and genetic transmission in the presence of phenotypic assortative mating was extended to cover the bivariate case. The model allows for cross assortative mating as well as cross cultural transmission. It was applied to two correlated measures derived from a fear survey questionnaire given to 1000 subjects. In allowing for the impact of more than one variable, the model allows for a much more realistic picture of cultural transmission than provided by the univariate model. The logic of the model and an application are outlined. (The authors are indebted to Professor R.J. Rose for providing the illustrative data.)

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.

The concordance of physiological and pathological times in human identical twin pairs induced the authors to postulate the existence of a hereditary biological time.

Having formulated the hypothesis that the information of each gene has a given period of existence and that, therefore, every gene has its own inherited temporal dimension, the authors report on five different experimental studies intended to verify their hypothesis.

In the first study (cf II. 1) a twin research on bone age and dental age is performed. The chronological study of the appearance of ossification nuclei in carpal bones and of mineralization of the gems of permanent dentition, in 20 MZ and 20 DZ human twin pairs, indicates that these wellknown “biological timetables” exhibit about 70% of genotypical control.

In order to verify whether biological time is a function of the genotype as a whole, or a property of each individual gene, the authors carried out an experimental study on the mean lifespan in different strains of Drosophila melanogaster whose genotypes were fully known (cf II.2). Their results indicate that the specific information of certain genes controls the insect's lifespan; it may also be inferred that the differential persistence of its specific information is an attribute of each individual gene. This chronological dimension of the gene is called chronon, which the authors also define as “the period during which the original information of the gene remains unchanged” — whether it is used for transcription or duplication, or it remains at the potential stage.

The determination of alkaline phosphatase activity in the same strains of D. melanogaster (cf II.3) affords an estimate of the amount of genie information (intensity of the individual trait) and the variation thereof during the gene's chronon. The authors observe that the amount of information decreases gradually during the gene's chronon, suggesting that this be due to the gradual exhaustion of a given specific energy. The decrease in the amount of information in the longitudinal study of chronon leads the authors to identify a further fundamental parametric unit of the gene which they call ergon.

Ergon is defined as “the degree of stability of a gene”.

In the fourth study (cf II.4) the twin test is applied to the chromosome association index in subcultures of lymphocytes from MZ and DZ twins at age 6 and age 60. This study affords a parallel estimate of chronon (i.e., duration of information) and ergon (i.e., stability of information).

Chronon and ergon are found to be interrelated; they may be considered as variables in a dimensional equation of the gene. Thus, the existence of the Ergon/Chronon (E/C) system is postulated.

Nine parameters of development and of senescence (first smile, first word, first steps, first pubic hair, menarche, first white hair, first loss of a permanent tooth, first use of reading glasses, onset of menopause) are studied in an experimental population of 666 twin pairs of either zygosity, leading the authors to formulate several conclusions concerning the characteristics of the E/C system (cf II.5).

The interpretation of their experimental findings leads the authors to consider the ergon (energy of stability) of a gene as the total result of the stabilities of all the nucleotides making up the DNA sequence of that gene. Since it is well known that the stability of adenine-thymine (AT) bonds exceeds the stability of guanine-cytosine (GC) bonds, and that different combinations of codons (differing in at least one nucleotide) may provide the same information, it is clear that identical polypeptide chains may be produced under the control of genetically different ergons resulting in genetically different chronons.

The authors summarize these concepts in the following two aphorisms: “one gene, one stability” (ergon) and “one gene, one time” (chronon).

Biological time, development, senescence, homeostasis and disease are interpreted by the authors in the light of the E/C system.

From 1971 to 1981, 16752 women were delivered at the University Women's Clinic in Heidelberg; 228 of these were twin pregnancies (1.36%). The relationships between sex, gestational age, birthweight, mortality, and fetal outcome were investigated. For the reason of grading, so-called χ2 curves are used. These curves show the strength and the course of relationship between two parameters. Mortality of the twin (8.3%) depended on birthweight and on sex of the infant, but not on the mode of delivery. All twins were ‘small-for-date’ (25th weight percentile of single births). Starting at the 35th week of gestation, the increase in weight decelerated additionally. So, immaturity as well as twin-related factors led to the high mortality rate for twins. Fetal outcome was evaluated in respect to the birthweight and to the delivery interval.

In the years 1966-1976, 50 pairs of Warsaw twins (27 MZ and 23 DZ) were subjected to longitudinal investigations. The twins were aged 9-11 years at the beginning of the observations, and 19-22 at their conclusion. The observations consisted of comprehensive somatic measurements and of the evaluation of biological maturity. The rate of growth of the twins, defined by the magnitude of yearly increments of body height, proved to be more concordant in MZ than DZ pairs. The appearance of the adolescent spurt occurred in the same year in 81 % of MZ and ca. 43% of DZ pairs. A very slight relationship was found between the rate of growth and final body height. The attaining of subsequent stages of sexual maturity in MZ twins was concordant in 88%, and in DZ twins in 30-39%, according to the respective stage. The body height curves were represented be means of three numerical parameters.

A new approach is proposed in twin research based on the study of twins who, though reared together, have subsequently lived apart for a period of at least five years. With respect to the more powerful study of twins reared apart, the twins living apart test has the advantages of being more realistic and affording easier access to sufficiently large samples. In this pilot study, the test has been applied to a sample of 92 monozygotic pairs now aged 35–45; 15 pairs were still living together and 77 had lived apart for over five years. As a first approach, comparisons have been made, in the cotwins of the two subsamples, with respect to the following traits: height, weight, presbyopia, presbyacusia, alcohol consumption, tobacco consumption, blood pressure (systolic and diastolic), dental caries, and hours of sleep.

The results of the small number of existing investigations of criminality among twins have mostly been considered as contributions to the discussion of the relative importance of heredity and environment. I attempt to look at the problem in a slightly different context, but find it useful to give a brief summary of the older investigations. The results of previous studies utilizing the twin method to throw light on the problem of crime appear from Tab. I.

I am sure you will remember Lange's conclusion from 1929: “ As far as crime is concerned, MZ twins on the whole react in a definitely similar manner, DZ twins behave quite differently. In accordance with the significance of the twin method we must conclude that innate tendencies play a preponderant part among the causes of crime”.

There is one fundamental assumption behind the conclusions about the heredity of criminal behaviour, which is not fulfilled. The environment of each of the two twins in a pair must be equally similar or equally different, irrespective of zygosity. However, the milieus of the cotwins in a DZ pair are in fact more different than in a MZ one.

The Danish study is carried out in cooperation with the Danish Twin Register (Harvald and Hauge, 1965). The material consists of data about nearly 6000 pairs of twins, i. e., all twins born on the Danish islands in 1881-1910, where both twins have survived to the age of at least 15 years. The source material may be considered complete. About 900 pairs have been entered into the Central Police Register and/or into the local penal registers.