Depression scores (DSSI) were available for 1232 MZ and 751 DZ female twin pairs who completed a mailed questionnaire. Pairs were divided into those concordant for being in a marriage-like state, concordant for having no partners, and those discordant. The pattern of twin correlations differed according to marital status. Our results suggest that having a marriage-like relationship acts as a protective factor in reducing the impact of inherited liability to symptoms of depression in the general population.

Birth weight is in large extent influenced by gestational age. In addition genetic and environmental factors determine intrauterine growth and birth weight. The contributions of these factors may be influenced by maternal smoking during pregnancy. We examined birth weight and maternal smoking in a sample of 2930 twin pairs from the Netherlands Twin Register using structural equation modelling. Gestational age accounted for 27–44% of the variance in birth weight. A lower variability of birth weight and a lower association of birth weight with gestational age was found in twins whose mothers smoked during pregnancy. The variance not associated with gestational age was independent of maternal smoking during pregnancy. A systematic smaller part of the variability in birth weight was associated with variability in gestational age in second born twins compared to first born twins. The heritability of interindividual differences in birth weight was modest (10% for twins with non-smoking mothers and 11% for twins with smoking mothers). Common environmental influences other than gestational age accounted for a slightly larger part of the variance not associated with gestational age (17–20%).

New large-sample data show that non-additive genetic effects, probably epistatic interactions between loci, and sex-limited gene expression are significant features of the genetic architecture of human personality as measured by questionnaire scales of extraversion and neuroticism. Three large data sets – new data on large samples (n = 20 554) of US twins, their spouses, parents, siblings and children, correlations for Australian twins (n = 7 532), and previously published twin data from Finland (n = 14 288) – are subjected to an integrated analysis to test alternative hypotheses about the genetic causes of family resemblance in personality. When allowance is made for differences in reliability of the scales, the combined data are consistent with the same model for variation. There are significant amounts of genetic non-additivity for both dimensions of personality. The evidence favours additive × additive epistatic interactions rather than dominance. In the case of neuroticism, there is especially strong evidence of sex differences in genetic architecture favouring a greater relative contribution of non-additive genetic effects in males. The data confirm previous claims to find no major contribution of the shared environment of twins and siblings to these dimensions of personality. Correlations between spouses are zero, and the correlations for very large samples of siblings and non-identical twins do not differ significantly.

A fear of interlocking twins is one factor that has led to a high Cesarean section (C/S) rate in breech (A)/vertex (B) (Bra/Vtxb) twins. We sought to estimate the frequency of occurrence of twin entanglement, and of interlocking Bra/Vtxb twins in vaginal deliveries. 541 twins and 48 195 deliveries were retrospectively studied for the period 1987–1995. The incidence of Bra/Vtxb deliveries was noted, and the number of deliveries marked by interlocking and collision of fetuses. The mode of delivery, reason for C/S, and sources of perinatal mortality were noted. Only 43 deliveries were Bra/Vtxb (7.9% of all twins) and of these only 14 (32.6%) were delivered vaginally. One case of interlocking (2.3% of all Bra/Vtxb pairs) and five cases of collision of twins (ie competition for entry into the pelvic inlet with obstruction) were noted. All cases mentioned were delivered by C/S. No perinatal mortality resulted from these cases. In recent years the trend has been for greater use of C/S and ultrasound in managing twin deliveries. In this study 67.4% of Bra/Vtxb twins were delivered by C/S. Although there are fewer vaginal deliveries of these twins and the rate for interlocking (2.3%; 95% CI: 0.06–12.3%) for the whole group has remained about the same, we found the rate in those twins allowed vaginal delivery was 6.7% (95% CI: 0.2–31.9%). The presentation at greatest risk for entanglement was found to be Bra/Vtxb.

In this study, expectant parents of twins were questioned about their feelings and need for information and counselling on the parenting of twins. Data were obtained from self-rating questionnaires and an interview with the expectant mother. From the results it is argued that expectant parents of twins need specific information and counselling about the medical, emotional and parenting issues of having twins. Guidelines for organising this counselling are proposed.

At least two twin registries in the United States have been or are being assembled using the similarity of Social Security Numbers in computerized records to help identify possible twin pairs. While the success of such enterprises depends directly on a high probability of twinness given Social Security Numbers, there are theoretical and practical reasons to study the probability of Social Security Number similarity given twinness. For example, the number of twin pairs with similar Social Security Numbers obviously determines the maximum number of twin pairs that can be discovered by similarity algorithms. To study this issue, we examined the similarity of known Social Security Numbers in twin pairs from the Virginia Twin Registry by age, sex, race, and zygosity of the pair. We found that similarity between the Social Security Numbers of twin pairs varies markedly by age, and MZ twin pairs have significantly more similar Social Security Numbers than DZ pairs at all ages. Among older twins, there are also significant differences by sex and race. For younger twins, algorithms that identify putative twin pairs on the basis of the similarity of their Social Security Numbers hold the promise of being able to identify a large proportion of all true twin pairs. Such algorithms will be substantially less successful, however, in identifying a large proportion of older twin pairs.

A threefold increase for testicular carcinoma has been reported in male dizygotic twins. In this comment we suggest the hypothesis that over-exposure to endogenously hypersecreted Follicle Stimulating Hormone (FSH) may underlie the pathogenesis. This is supported by several findings. 1) FSH hypersecretion in mothers of dizygotic twins is most likely an autosomal trait implicating the possibility of male offspring with the same hormone characteristic. 2) In testicular carcinoma higher levels of cyclin D2 are found. This is an FSH dependent stimulatory regulator of mitosis. 3) There is a marked similarity between geographical distribution in occurence of dizygotic twinning and testicular carcinoma. 4) Men undergoing surgery for testicular carcinoma have higher FSH concentrations and males with Down syndrome have higher FSH levels and are more at risk to develop testicular carcinoma. We suggest to study FSH secretion in males of familial dizygotic twins and furthermore the risk of developing testicular carcinoma in males with elevated FSH. These men with one testicle and/or with dysfunctioning Sertoli/Leydig cells.