The age of attainment for four motor developmental traits, such as turning over, sitting up without support, pulling up to a standing position and walking without support, was examined in 822 children, including 626 siblings from families with 2 to 6 children, 68 pairs of dizygotic twins and 30 pairs of monozygotic twins. Correlation analysis, carried out separately for each type of sibship, showed the highest pairwise correlations in monozygotic twins and the lowest correlation in non-twin siblings for all motor milestones. Variance component analysis was used to decompose the different independent components forming the variation of the studied trait, such as genetic effect, common twin environment, common sib environment and residual factors. The results revealed that the major proportion of the total variance after adjustment for gestation age for the attainment of each motor skill, except pulling up to standing position, is explained by the common twin environment (50.5 to 66.6%), whilst a moderate proportion is explained by additive genetic factors (22.2 to 33.5%). Gestational age was found to be an important predictor of appearance of all motor milestones, affecting delay of 4.5 to 8.6 days for the attainment of the motor abilities for each week of earlier gestation. The age of attainment of the standing position was affected only by shared sibs environment (33.3% of the total variance) and showed no influence of either genetic or common twin environment. Phenotypic between trait correlations were high and significant for all studied traits (range between 0.40 and 0.67, P < 0.01 in all instances). Genetic cross correlations, however, were not easily interpreted and did not show clear variance trends among the different groups of children.

The estimation of genetic effects from twin studies usually relies upon the equal environment assumption — that monozygous (MZ) and dizygous (DZ) twin pairs experience equal similarity of their environments from prenatal experiences through adulthood. However, the sharing of a chorion may make a subset of identical twins more similar, or in some cases, more different, than twins that do not share a chorion. Recent studies suggest monochorionic MZ twins resemble one another more than dichorionic MZ twins in cognitive abilities, personality, and risk for psychiatric disorder. To the extent that prenatal environment affects these characteristics, the traditional twin method will yield biased estimates of genetic and environmental influences. We develop models for quantifying this bias and estimating the influence of chorion type on estimates of heritability.

Measures of four dimensions of personality (Psychoticism, Extraversion, Neuroticism, and Lie scores) and six aspects of social attitudes (to sex, taxation, militarism, politics, religion and a general conservatism scale) were obtained by mailed questionnaire from 29 691 US subjects including adult twins (n = 14 761) their parents (n = 2360), their spouses (n = 4391), siblings (n = 3184) and adult children (n = 4800). After correction for the average effects of age, sex and source of sample, familial correlations were computed for 80 distinct biological and social relationships. The data allow for the estimation of the additive and non-additive effects of genes, assortative mating, vertical cultural inheritance and other non-parental effects of the shared environment on differences in personality and social attitudes. The interaction of genetic and environmental effects with sex may also be analyzed. Model-fitting analyses show that personality and social attitude measures differ markedly in major features of family resemblance. Additive and dominant genetic effects contribute to differences in both personality and attitudes, but the effects of the family environment, including vertical cultural transmission from parent to child, are much more marked for social attitudes than for personality. There is substantial assortative mating for social attitudes and almost none for personality. The causes of family resemblance depend significantly on sex for almost every variable studied. These findings clarify and extend the more tentative findings derived from previous twin, family and adoption studies.

The results presented in this paper are part of a current comparative study of genetic and environmental influences in three educational settings: Stockholm, Jerusalem and the Israeli kibbutz. We specifically wanted to investigate whether a more restrictive educational setting would decrease genetic influences. Here we report on comparisons of cognitive performance measures at several time points for twins/controls, boys/girls and within-pair similarity in MZ, DZ and controls. The tests used were the Raven Progressive Matrices, verbal, reading comprehension and arithmetic. The results show no differences between twins and controls, whilst gender differences seem to be smallest in the Stockholm sample and largest in Jerusalem. A pattern of genetic influences on cognitive performance was also clearly visible in Jerusalem. In Stockholm shared environmental influences at home and at school seemed even stronger than in the kibbutz. No consistent differences were found between tests or occasions.

Using vital statistics, yearly changes in the twinning and triplet rates by zygosity were investigated in the Czech Republic and the Slovak Republic during the period 1972–1995. Monozygotic (MZ) twinning rates in both countries had remained nearly constant (about 3 per 1000 total births) during that period. With a few exceptions, the dizygotic (DZ) twinning rates remained constant from 1972 to 1994, and increased in 1995 for both countries. MZ twinning rates for both countries were the lowest in Europe. As for triplet rates, overall rates increased significantly year by year in the Czech Republic, but not in the Slovak Republic. The triplet rate was significantly higher in 1995 than in the period 1972–1982 for both countries. The MZ triplet rate remained constant during that period in the Czech Republic. The trizygotic (TZ) triplet rates increased 3-fold for the Czech Republic and 4-fold for the Slovak Republic in 1972–1976 and 1992–1995. In the later period, the TZ rate was 1.5-fold higher in the Czech Republic than in the Slovak Republic. The quadruplet rate increased 2.3-fold from 2.9 per million births in 1982–1986 to 6.7 in 1992–1995 in the Czech Republic. The corresponding values were 2.7, 2.20 and 5.9-fold in the Slovak Republic. Both the Czech and the Slovak Republics were not affected by fertility drugs and assisted reproductive techniques until recently.

The genetic and environmental determinants of variation in blood cell size and number were investigated in 392 pairs of 12-year-old twins. The following blood cell indices were measured: haemoglobin, red blood cell count, haematocrit, mean corpuscular volume, platelet number, total white cell count, level of neutrophils, monocytes, eosinophils, total lymphocytes, CD3+ lymphocytes, CD4+ lymphocytes, CD8+ lymphocytes, CD19+ lymphocytes, CD56+ lymphocytes and CD4+ /CD8+ ratio. Genetic factors contributed significantly to all blood cell measures accounting for between 61 and 96% of variance. Heritability estimates did not differ significantly between males and females, although the sample size of the present study was not large enough to exclude the possibility of sex-limited gene expression. Common environmental factors were important in determining red blood cell count and haematocrit, but were not important in determining basal levels of any white blood cell type.

Measures of self-transcendence, physical health and psychological well-being were included in a self-report Health and Lifestyle questionnaire administered to Australian twins aged over 50 between 1993 and 1995. Self-transcendence appears to be higher among older Australian women than men, and was significantly associated with religious affiliation, marital status (in women) and age (in men). No strong correlations were observed between self-transcendence and any measure of psychological or physical health. Additive genetic effects were found to be important in influencing self-transcendence, with heritability estimates of 0.37 and 0.41 for men and women respectively, whilst shared environment effects were not found to be significant. Multivariate modelling of self-transcendence scores and self-reported church attendance behavior indicated substantially different etiologies for these variables, with implications for methods of investigation of religiosity and spirituality.

Previous behaviour genetic studies of aggression have yielded inconsistent results: reported heritabilities for different types of aggressive behaviour ranging from 0 to 0.98. In the present study, 247 adult twin pairs (183 MZ pairs; 64 same-sex DZ pairs) were administered seven self-report questionnaires which yielded 18 measures of aggression. Univariate genetic analyses showed moderate to high heritabilities for 14 of these 18 measures and for a general aggression factor and three correlated aggression factors extracted from the measures. Multivariate genetic analyses showed sizeable genetic correlations between the different dimensions of aggression. Thus, individual differences in many types of aggressive behaviour are attributable to some extent to genetic factors and there is considerable overlap between the genes that operate on different types of aggressive behaviour.

In our three-stage questionnaire study we investigated patterns of twin and familial aggregation of osteoarthritis (OA) for commonly affected joints. The baseline questionnaire study of the Finnish Twin Cohort was performed in 1975. In 1990, 4095 twin pairs of the same gender born 1930–1957 responded to a questionnaire and reported whether they had OA diagnosed by a physician. In 1996 both twins of 266 pairs of which at least one had reported OA in 1990 responded to a detailed questionnaire on joint-specific OA, including family history of OA. In male pairs shared (non-genetic) familial effects accounted for 37% of the total variance in liability to OA and unshared environmental effects for 63%. In female pairs additive gene effects explained 44% of the variance in liability to OA, and unshared environmental effects for 36%. Familial aggregation of finger and knee OA was clearly higher than that of hip OA. Twin-pair discordance for OA was, to some extent, associated with body-mass index, occupational loading and trauma. Our results indicate that genetic effects may be modulated by sex or by environmental factors distributed differently between men and women. Based on our joint-specific data finger and knee joints are the most optimal targets for studies of genetic factors predisposing to the development of OA.

There have been few secular analyses of the seasonal variation in human twinning and the results are conflicting. One reason for this is that the seasonal pattern of twinning varies in different populations and at different periods. Another reason is that the statistical methods used are different. The changing pattern of seasonal variation in twinning rates and total maternities in Denmark was traced for three periods (1855–69, 1870–94, and 1937–84). Two alternative methods of analysis are considered. The method of Walter and Elwood and a trigonometric regression model give closely similar results. The seasonal distribution of twin maternities for the periods in the 19th century showed highly significant departures. For both twin and general maternities, the main peaks can be seen from March to June and a local peak in September. During the spring– summer season the twinning rates were higher than the total birth rates, indicating a stronger seasonal variation for the twin maternities than for the general maternities. For 1937–84, there was a similar, but less accentuated, pattern. Studies of other populations are compared with the Danish results. The more accentuated seasonal variation of twinning in the past indicate that some factors in the past affected women during summer–autumn and around Christmas time, making them more fecund and particularly to be more prone to polyovulation and/or more able to complete a gestation with multiple embryos.

This report presents findings for the Intrinsic (IR) and Extrinsic (ER) religiousness scales from the Minnesota Study of Twins Reared Apart. The scales were shown to be internally consistent, sufficiently distinct from the scales of the California Psychological Inventory and the Multidimensional Personality Questionnaire and unrelated to a number of measures of response style to justify treating them as distinct traits. The I scales also showed considerable evidence of construct validity in its correlations with religious fundamentalism and authoritarianism as assessed by the MMPI and Altemeyer's Right-Wing Authoritarianism scale. Data on IR and ER from 35 pairs of monozygotic twins reared apart (MZA) and 37 pairs of dizygotic twins reared apart (DZA) were fitted to a biometric model and demonstrated significant heritability (0.43 and 0.39), with a model containing genetic plus environmental factors fitting significantly better than a model containing only an environmental component. Twin similarity could not be explained by placement on a self-reported measure of family Moral Religious Emphasis as measured by the Family Environment Scale.

Parental adjustment following the death of a premature singleton or multiple birth infant has hitherto been studied by mailed questionnaires or telephone survey. In the present study, using an in-depth personal interview, grief reactions and adjustment patterns of nine families who lost a singleton premature infant (‘Single Group’) were compared with those of nine families who lost one of a premature multiple birth cohort (‘Multiple Group’). The interview was conducted 1–4 years after the death of the infant and evaluated specific areas or ‘scales’ of life adjustment, including individual feelings, relationship between husband and wife, and functioning at home and at work. There was no significant difference between the paternal and maternal level of adjustment of the two groups in any of the studied scales. A positive correlation was found between maternal and paternal grief reaction of the same family in the scales of individual feelings (r = 0.65), relationships between husband and wife (r = 0.70), and functioning at home (r = 0.57). Comparing the father's scale with the mother's scale revealed a significant difference only in the area of ‘individual feelings’. The gestational age, maternal bonding during hospitalisation of the infant and the parental attendance at the event of death were significantly associated with the process of parental adjustment. The results of this study support previous reports of similar parental reactions following the demise of a premature singleton or multiple birth infant. Since societal environment may not recognise the need for consolation of these families, care, compassion, and sensitivity should be encouraged in dealing with these parents at the time of their infant' death, and for a long time thereafter.

Chronic fatigue syndrome (CFS) and the symptom of chronic fatigue are conditions of unknown etiology. The Centers for Disease Control and Prevention (CDC) define CFS as an illness characterized by ≥ 6 months of disabling fatigue associated with muscle pain, pharyngitis, and alterations in mood, sleep and neurocognition. We constructed a registry of twins with chronic fatigue to facilitate research on the impact of illness, the associated medical and psychosocial factors, and the heterogeneous proposed mechanisms for these conditions. We have recruited 204 twin pairs in which one or both members reported persistent fatigue through patient support group newsletters (60%), clinicians/researchers familiar with CFS (12%), notices placed on electronic bulletin boards for CFS (11%), twin organizations and researchers (6%), relatives and friends (3%) and other sources (8%). Complete data are available for 177 pairs (87%). Twins completed an extensive questionnaire booklet that included measures of physical and mental health, functional status, and psychosocial factors; a structured psychiatric interview was also conducted by telephone. Twins were classified using three increasingly more stringent diagnostic criteria for chronic fatigue: 1) ≥ 6 months of fatigue (115 discordant and 61 concordant pairs); 2) chronic fatigue with additional symptoms and application of the medial exclusions of the CDC CFS case definition as obtained by self-report (92 discordant and 41 concordant pairs) and; 3) chronic fatigue with additional symptoms unexplained by self-reported medical conditions and psychiatric diagnoses as determined by the structured interview (69 discordant pairs and 25 concordant pairs). Despite the limitations of a volunteer registry, the Chronic Fatigue Twin Registry promises to be an important resource for research on CFS and chronic fatigue.

Data on frequency of church attendance have been obtained from separate cohorts of twins and their families from the USA and Australia (29 063 and 20 714 individuals from 5670 and 5615 families, respectively). The United States sample displayed considerably higher frequency of attendance at church services. Sources of family resemblance for this trait also differed between the Australian and US data, but both indicated significant additive genetic and shared environment effects on church attendance, with minor contributions from twin environment, assortative mating and parent–offspring environmental transmission. Principal differences between the populations were in greater maternal environmental effects in the US sample, as opposed to paternal effects in the Australian sample, and smaller shared environment effects observed for both women and men in the US cohort.

The long-term effect of differences in placentation in MZ twins is a controversial subject. An effect has been clearly established for birth weight but data on psychological traits are still under debate. We studied 20 pairs of monochorionic MZ (MCMZ) and 24 pairs of dichorionic MZ (DCMZ) twins. A chorion effect was observed for Block Design (WISC-R) confirming a previous report: MC MZ co-twins were more similar that DC MZ co-twins. For anthropometrical measures, an expected effect in the opposite direction was found. No chorion effect was significant for the other variables. A follow up was undertaken 3 years later using cognitive, national academic evaluations, and personality variables. The sample included 16 pairs of MC MZ and 22 pairs of DC MZ twins. Again a chorion effect was observed on anthropometrical variables but results on the Block Design were not replicated. However, the MC MZ co-twins were more similar than the DC MZ co-twins for two other cognitive variables: Perceptual Organization Index from the WISC-R and Global Visualisation from a Belgian reasoning test. Among the personality variables only one was sensitive to a chorion effect. The discussion focuses on the need for larger samples to achieve adequate power in statistical comparisons.

Diagnostic advances have made it possible to use ultrasonograph to assess placentation and therefore zygosity in utero in the case of monochorionic-monozygotic twins. Foetal behaviour of 15 monozygotic and 15 unlike-sexed dizygotic twin pairs was studied serially with ultrasounds from 10 to 22 weeks gestational age. Each twin, regardless of its zygosity, showed individualised behavioural styles. One twin was found to be ‘dominant’ in the sense of being more active, but less reactive, possibly due to the fewer stimuli being generated by its co-twin. Monozygotic twins, as opposed to dizygotic twins, showed greater similarities in activity and reactivity levels, but were never behaviourally identical and decreased in likeness with increasing age. Our data suggest that so-called identical twins are very similar, but not behaviourally identical, from very early in pregnancy. The unequally shared intrauterine environment contributes to putting each monozygotic twin on a progressively distinct behavioural path.

Case reports of thoracopagus/xiphopagus conjoined twins by Godofredus Christell (1751) and Francesco Luigi Fanzago (1803) have been recovered and can be added to the history of conjoined twins.