This article reviews the treatment options of the twin–twin transfusion syndrome (TTTS). No single therapy is associated with a uniformly improved outcome for the involved twins and success is primarily related to gestational age and severity at diagnosis. Treatment options for severe cases include digitalization, ligation of the umbilical cord, serial amniocenteses, septostomy, laser occlusion of placental vessels, and selective feticide. These modalities are associated with significant risks of complications, and variable results of fetal morbidity and mortality. Therefore, they should be considered when risks of withholding treatment clearly outweigh those associated with intervention.

Twin–twin transfusion syndrome is the most common complication of monochorionic pregnancies affecting between 5 and 15% of such pregnancies and accounts for 15–77% of perinatal mortality in twins. The management of twin–twin transfusion syndrome is complex and various treatment modalities have been tried. This review critically evaluates the different management options available for the syndrome and recommends some guidelines for future interventional studies.

The major morphological types of conjoined twins had all been described by 1600, often in publications that included details of the time and place of birth, morphology, outcome, behaviour, and, on occasion, autopsy findings. These descriptions differ from modern reports in emphasizing the twins’ supposed “meaning” rather than their cause. Understanding the symbolic aspects of these accounts is important for their interpretation. This article summarizes the records of 53 cases of conjoined twins born in 16th-century Europe and examines the cultural background of these accounts and the use of the pediatric autopsy as a means of investigating the pathology of twins. Studies of conjoined twins were central to the debate over whether the possession of separate hearts was a prerequisite for individuality.

This population-based study determined the impact of co-twin gender on twin intrauterine growth in addition to their infant gender, maternal height, maternal age and parity on intrauterine growth rate of singletons and twins. All singletons and twins born in Western Australia during the period of 1980 to 1995 were considered for the study. The multiple linear regression models showed that 76% of the variance in the mean birthweight was explained by the selected variables for twins and 51% for singletons. Twins grew more slowly than singletons from 26 weeks gestation. Among twins, opposite-sex twin pairs grew consistently faster than like-sex twins. Primiparous twin pairs grew more slowly than subsequently born twins. These regression equations can be used to assess the appropriateness of intrauterine growth in twin pairs of various gender combinations.

The purpose of this study was to examine the impact of the sex makeup of pairs of twins on language acquisition. Past research indicated that this variable plays a role in speech problems of twin children. The questions raised were whether being a boy or a girl and having a boy or girl co-twin affected linguistic performance. A language test was given to 30 pairs of boy-girl twins, 16 pairs of boy twins, and 16 pairs of girl twins whose average age was 4 years 8 months. Their test scores confirmed our hypotheses. The poorest performance was obtained by the boy twin pairs and the best performance, by either the girl twin pairs or the different-sex pairs. The results were interpreted in the light of findings on language learning differences between girls and boys, and also in terms of Vygotsky’s zone of proximal development.

In order to determine high school entrance level in the Netherlands, nowadays, much value is attached to the results of a national test of educational achievement (CITO), administered around age 12. Surprisingly, up until now, no attention has been paid to the etiology of individual differences in the results of this national test of educational achievement. No attempt has been made to address the question about the nature of a possible association between the results of the CITO and cognitive abilities, as measured by psychometric IQ. The aim of this study is to explore to what extent psychometric IQ and scholastic achievement, as assessed by the CITO high school entrance test, are correlated. In addition, it was investigated whether this expected correlation was due to a common genetic background, shared or nonshared environmental influences common to CITO and intelligence or a combination of these influences. To this end multivariate behavior genetic analyses with CITO and IQ at ages 5, 7, 10 and 12 years have been conducted. The correlations were .41, .50, .60, and .63 between CITO and IQ assessed at age 5, 7, 10, and 12 respectively. The results of the analyses pointed to genetic effects as the main source of variance in CITO and an important source of covariance between CITO and IQ. Additive genetic effects accounted for 60% of the individual differences found in CITO scores in a large sample of Dutch 12-year-olds. This high heritability indicated that the CITO might be a valuable instrument to assess individual differences in cognitive abilities in children but might not be the right instrument to put the effect of education to the test.

Gene–environment interaction is likely to be a common and important source of variation for complex behavioral traits. Often conceptualized as the genetic control of sensitivity to the environment, it can be incorporated in variance components twin analyses by partitioning genetic effects into a mean part, which is independent of the environment, and a part that is a linear function of the environment. The model allows for one or more environmental moderator variables (that possibly interact with each other) that may i) be continuous or binary ii) differ between twins within a pair iii) interact with residual environmental as well as genetic effects iv) have nonlinear moderating properties v) show scalar (different magnitudes) or qualitative (different genes) interactions vi) be correlated with genetic effects acting upon the trait, to allow for a test of gene–environment interaction in the presence of gene–envi-ronment correlation. Aspects and applications of a class of models are explored by simulation, in the context of both individual differences twin analysis and, in a companion paper (Purcell & Sham, 2002) sibpair quantitative trait locus linkage analysis. As well as elucidating environmental pathways, consideration of gene–environment interaction in quantitative and molecular studies will potentially direct and enhance gene-mapping efforts.

Gene–environment interaction (G × E) is likely to be a common and important source of variation for complex behavioral traits. Gene–environment interaction, or genetic control of sensitivity to the environment, can be incorporated into variance components twin and sib-pair analyses by partitioning genetic effects into a mean part, which is independent of the environment, and a part that is a linear function of the environment. An approach described in a companion paper (Purcell, 2002) is applied to sib-pair variance components linkage analysis in two ways: allowing for quantitative trait locus by environment interaction and utilizing information on any residual interactions detected prior to analysis. As well as elucidating environmental pathways, consideration of G × E in quantitative and molecular studies will potentially direct and enhance gene-mapping efforts.

Twin researchers from Russia have announced new opportunities for research collaboration at the Psychological Institute of the Russian Academy of Education, in Moscow. Members of the laboratory are also establishing Moscow's first Psychological Twins counseling room to facilitate practical activities, such as parental advisement and teacher training. Reference is also made to the work of twin researcher, Bronson Price, who spent several years at the Medico-Genetical Research Institute in Moscow. His papers are available at the library of the American Philosophical Society, in Philadelphia. Brief reviews of twin studies involving facial asymmetry and parental favoritism, social relationships and disputed memories are also presented. The article concludes with a look at several twin-related human interest items.