Population-based studies in twins have been of insufficient size to explore the relationship between risk of cerebral palsy and intrauterine growth. Earlier studies in singletons have suggested an optimum size at birth for minimum cerebral palsy risk between the 75th and 90th percentiles of weight for gestational age. We aggregated data from nine European cerebral palsy registers for 1976 to 1990. Using sex-specific fetal growth standards for twins, a z score of weight-for-gestation was derived for each of the 373 twin cases. The rates of cerebral palsy in each z-score band were compared to the rate in the a priori reference band of 0.67 to less than 1.28 (equivalent to the 75th to less than 90th percentiles). In twins born at 32 weeks' gestation or more (92% of all twins), cerebral palsy rates were higher for both light and heavy-forgestation babies compared to an optimum (i.e., minimum risk) in the reference band. However, the rate ratio for heavy babies (90th percentile or greater) did not reach conventional (95% confidence intervals [CI]) statistical significance (rate ratios = 1.76; 90% CI 1.02–3.03). For twins born at less than 32 weeks, the significantly higher risk for cerebral palsy was observed consistently in all z-score bands less than average compared to the reference band. This multi-center study demonstrates that for twins born at 32 weeks' gestation or more, an increased risk of cerebral palsy is associated with deviations from optimal intrauterine growth at about 1 standard deviation above mean weight, as was earlier reported for singletons. For twins born at less than 32 weeks' gestation, this pattern is only demonstrable for babies weighing below the optimum weight-for-gestation.

Since its start as a database of ‘possible twins’, the Italian Twin Register has developed remarkably in terms of twin approach and recruitment, data-management tools, the cohorts enrolled, and the breadth of information gathered, making the Italian Twin Register a valuable resource for genetic epidemiological research. The Italian Twin Register is a random population of twins at both the national level and within targeted geographical areas or birth cohorts. Further, the Register is linked with disease records and has recently implemented a web-based method for volunteer twin recruitment specifically designed to promote the Register and to disseminate information on genetic epidemiology. To date, approximately 9000 twins have joined the Italian Twin Register, the majority of whom (approximately 70%) represent young adults aged 20 at time of enrolment. Although the total number of twins recruited to date is far below the expected figure initially predicted, the newly established standardized procedures guarantee an increase of around 2000 twins each year. Following the collaboration between the Italian Twin Register and the main Italian nonprofit association for blood donors, twin DNA sampling and storage has recently accelerated contributing to the large amount of phenotypic data collected. The Italian Twin Register is currently involved in both population and clinical based studies on various complex pheno-types and diseases, some conducted within large European consortia.

Arecent large-scale study of Finnish adolescent twins (Pulkkinen et al., 2003) reported that individuals from opposite-sex twin pairs were more socially adaptive than individuals from same-sex pairs or singletons. This finding raised questions about the social learning effects of being an opposite-sex twin. The current article predicted on the basis of this finding, and evidence from singleton populations, that having an opposite-sex twin would yield social advantage. It sought to examine the social competencies of opposite-sex twins and compare them with same-sex twins and singletons. The study focused on the preschool years (age 3 to 6), a period in which the majority of children encounter their first large group, nonfamilial social experiences. The study obtained reports from parents and teachers of children aged 3 to 6 years: 72 children (36 pairs) who were dizygotic opposite-sex twins (DZOS), 50 children (25 pairs) who were dizygotic same-sex twins (DZSS), and 85 singletons of the same age and sex as the twins, who had at least one sibling. Reports were made using standardized measures of social competencies, behavior problems, language development and friendships. The main effects found were of differences in social competency between twins and singletons. Twins had lower social competency scores. No differences between same-sex and opposite-sex twins were found. The findings did not support the hypothesis of social advantage for opposite-sex twins in early childhood.

More than 100 pairs of adult twins, reared apart and growing old separately, have participated in the Japanese Study of Adult Twins Reared Apart and Growing Old Separately since it began in 1974. The subjects are 161 pairs of adult twins born between 1910 and 1945 in Japan. The main focus of this study is to investigate the influences of environmental factors and life history on life satisfaction and psychological well-being in later adulthood. A mail survey has been conducted on these twins each year since 1974. To date, the Wechsler Adult Intelligent Scale, Maudsley Personality Inventory, Newgarten Life Satisfaction Index, comprehensive medical examinations and personal interviews have been conducted for 12 twin pairs residing in various areas of Japan.

The nature and frequency of quadruplet births are reviewed, followed by a close-up look at the 6-year-old monozygotic Mathias quadruplets. This essay is followed by reviews of new twin research on color–number association, male–female twins' finger-length ratios, and links between twinning and dietary practices. The article finishes by focusing on unusual twin athletes in football, basketball and golf, and on twin co-authors of a new book on the 1972 silver medal victory by the United States hockey team.

Research on twins has tended to focus around the question of heredity/environment. As part of this, attention has been paid to how twins' family environments impact upon them. By examining twinship as a social identity, this article considers the social experience of twinship as it is framed by parent–child relations and also actively shaped by twins themselves. Sameness constitutes one central defining component of ‘twin’ identity within Western societies. In preparing their twins' bodies for public presentation, parents play a key role in communicating twin identity on their children's behalf. However, children also construct and convey their own identities through presenting their bodies in certain ways. Drawing on findings from a small-scale qualitative study of twinship, this article examines how twin identity is created, modified and reproduced by parents and child twins. It highlights the active role that twins take in constructing their own identities and in shaping the ‘twin situation’.

Do women experience orgasm because this trait was shaped by natural selection to augment female fitness? Or are women merely the lucky recipients of developmental patterns favored by selection to produce orgasm in males? A recent and widely publicized book by Elisabeth Lloyd (2005a) contends that there is insufficient evidence to validate any of the adaptive explanations yet proposed for female orgasm. We agree. But our reading of the data differs from Lloyd's. In this essay, we outline why, unlike Caton (2006), whose review of Lloyd's book appeared previously in this journal, we are not persuaded by Lloyd's argument that female orgasm is a nonadaptive byproduct of orgasm in men. We hold this view because we disagree with the criteria Lloyd uses to evaluate evolutionary hypotheses, and because we believe Lloyd defines female orgasm too narrowly, ignoring critical information about its affective aspects.

New legislation giving parents a voice in deciding their twins' school placement situation (same or separate classrooms) was passed in Minnesota in spring 2005. Since then, parents in several other states have started petitions and lobbying efforts to persuade their senators and representatives to follow suit. A review and update of these exciting developments is presented, followed by a report from the recently established Asian Society for Twin Studies. Reviews of two new twin studies — bases of political attitudes and age effects on dizygotic twinning — are presented. The uniquely informative lives of several twin pairs are also documented.

Attention-deficit hyperactivity disorder (ADHD) is a common childhood disorder which occurs more often in twins than singletons. This article focuses on the psychosocial consequences of having a co-twin with ADHD. Specifically, the level of anxiety (generalized and separation) in non-ADHD children who have a co-twin with ADHD is examined using data from the Australian Twin ADHD Project (ATAP). Parental report data on 501 dizygotic (DZ) twin pairs aged 6 to 15 and their siblings were used to examine (i) anxiety symptoms in twin pairs discordant for ADHD, (ii) how the effects of an ADHD twin on their co-twin and siblings are related to the type of ADHD, and (iii) whether the effects are greater for the nonaffected twin than nontwin siblings. Results show that anxiety was high in co-twins of children with the combined subtype of ADHD, with increased symptoms of both generalized and separation anxiety. Inattentive ADHD had smaller effects, which were confined to generalized anxiety and were specific to the co-twin rather than other siblings. These results have clinical implications in managing the entire multiple birth family where one twin has ADHD, and also has implications for genetic analysis in modeling the relationship of ADHD to internalizing disorders.

The Osaka University Aged Twin Registry (OUATR) is the largest adult twin registry in Japan. Since its establishment in 1974, the OUATR has conducted a number of studies with particular focus on the environmental contribution to physical–cognitive–mental aging, longevity and aging-dependent diseases in later adulthood. The registry consists of 12,000 pairs of Japanese twins born between 1900 and 1935. Two hundred and fifty pairs of twins have undergone comprehensive medical examination to date. Follow-up questionnaires have been mailed out on a regular basis, for the purpose of checking current vital statuses, health conditions, and so forth. The main objective of this longitudinal twin study is to contribute to the prevention of lifestyle-related diseases and the promotion of successful aging.

Twin Research and Human Genetics, 9(1).

This study examined the influence of twin status (monozygotic [MZ], dizygotic same-sex [DZss] and dizygotic opposite-sex [DZos]) on friendship patterns. It examined the friendships of 60 children from 30 twin pairs, mean age 8 years, 4 months. The study sought to establish how number of friendships and degree to which these are shared vary according to twin status. Additionally, it sought to assess the children's meaning of shared friendship and to examine whether there were group differences according to twin status. Results indicated that, while number of friends did not vary, the degree to which friendships were shared was significantly associated with twin status. MZ children shared approximately 50% of their friends, DZss 25% and DZos 5%. Group patterns emerged from interviews with the children concerning shared friendships. MZ twins were characteristically positive or accepting about shared friends; DZos pairs shared fewer friends and were, therefore, less challenged by the twin situation in negotiating friendships; and DZss pairs were more diverse in their reaction with some expressing ambivalent or negative views about the twin situation and sharing of friendship. The results are presented in the light of their implications for twin children's developmental and scholastic progress.

Since established in 1998, the Keio Twin Project (KTP) has been dedicated to investigating genetic and environmental sources contributing to human psychological traits in adolescence and young adulthood. A population-based twin registry was constructed by the KTP through the use of official residential records in the Tokyo area, and to date requests to participate in our research have generated 1040 pairs of twins and triplets of age 14 to 30, forming one of the largest twin registries in Asia. Our comprehensive datasets, obtained through questionnaires, performance tests, and physical measurements, cover a wide range of human traits: personality, psychiatry, mental health, sociality, cognition, and physical index. Demographic variables and environment of upbringing are also sought by twins and by some parents. This extensive information allows us to clarify the genetic and environmental overlap across multiple traits as well as specificities unique to single traits. Adding an evolutionary psychology perspective to the behavior genetics framework is currently being attempted in order to develop a grand theory of human genetics.

The Tokyo Twin Cohort Project (ToTCoP) is a large-scale longitudinal study of 5 years based on 1619 pairs of infant twins reared together. The purpose of the study is to construct a population-based twin registry in Japan and to investigate human growth and development and twin themselves. It covers behavioral, neurological, physical and environmental variables measured by questionnaire, home visiting and brain imaging technology. The full registry contains over 47,000 multiple births collected from the Basic Resident Register, and the targeted population is 3070 probable twins of 0 to 2 years old. Preliminary analysis of the entry questionnaire data showed no serious sampling biases. Descriptive statistics of parental characteristics (parental age, gestation age, parity and placentation, maternal weight, parenting stress) and children's characteristics (body size at birth, 4 and 10 months of age, milk consumption, and sleeping and social behavior) and their correlations, genetic and environmental contributions and correlations are reported.

The definition and nature of twin language has been a focus of recent studies concerned with the phenomenon. There has been a call for a tighter definition and understanding of the meaning of twin language (Thorpe et al., 2001). This article sought to identify social factors associated with the parent report of twin language and thus provide further understanding of the phenomenon. Data from 583 mothers of twins aged 25 to 59 months were analyzed using multiple logistic regression. Factors included in the modeling of parent-report twin language included social experience factors such as presence of siblings, attendance at preschool education and reports of nonverbal play. It was found that twin pairs who didn't have an older sibling, who showed frequent nonverbal play and who didn't attend preschool were more likely to have a twin language. Moreover, in the group not having an older sibling, the influence of whether twins attended preschool or not was strong and the odds ratio was 0.589 (95% confidence intervals 0.360–0.963). The findings suggest that social experience factors are important predictors of the parent reporting of twin language.