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.

This article profiles the historical twin databases of the secondary education school attached to the Faculty of Education at the University of Tokyo. The school was established in 1948. Every year, about 50 pairs of twins of all sex and zygosity combinations and aged 11 to 12 years take an examination, and about 10 to 20 pairs are admitted based on the results. Three independent datasets exist: one for applicants (11–12 years), one for students (12–18 years), and one for graduates (18–72 years). These three historical databases and research perspectives are introduced herein.

A new type of population-based database of multiples in childhood at the prefecture level was initiated in 2004 in Ishikawa Prefecture, Japan. We conducted an exhaustive search for demographic information concerning families with multiples, family support provided by governmental and medical institutions by mailed questionnaire, and at the same time tried to organize a human network to support such families. This registry aims not only to aid research on human genetics and maternal and child health, but also to contribute to the development of welfare programs for families with multiples.

The South Korean Twin Registry (SKTR), previously called the Seoul Twin Family Study, is a nationwide volunteer registry of South Korean twins and their families. Since 2002, the SKTR has been updated in 4 important ways. First, continued sampling led to an increase in the number of twins. Second, the target area, Seoul, was enlarged to include other cities and rural areas in the country. Third, the target population was extended from school-aged twins to preschool and adult twins. Fourth, the research focus was expanded to include psychiatric and physical disease phenotypes. The present article describes a brief history of the SKTR, goals and current research highlights, recent major accomplishments, and future directions.

‘Healthy Twin’ is a twin family study extension of the existing Korean Twin-Family Register. Healthy Twin recruits adult like-sex twins over the age of 30 and their adult family members. Healthy Twin protocols are primarily tailored to the study of the quantitative trait loci of complex traits as well as to the role of environment in the etiology of complex diseases. A full-length survey is underway, including questionnaires, health examinations and the collection of biological specimens. So far, 820 individuals (169 twin pairs and their families) have participated in the survey and 1068 individual twins (608 twin pairs) have replied to the mailed zygosity questionnaire as of July 2006. The first phase (2005–2006) of Healthy Twin will recruit 1550 individuals (including about 380 twin pairs), and the second phase a proposed 1500 to 2500 additional participants. We report study protocols and zygosity and the distribution of family size of the study participants.

In the late 1980s The Netherlands Twin Register (NTR) was established by recruiting young twins and multiples at birth and by approaching adolescent and young adult twins through city councils. The Adult NTR (ANTR) includes twins, their parents, siblings, spouses and their adult offspring. The number of participants in the ANTR who take part in survey and / or laboratory studies is over 22,000 subjects. A special group of participants consists of sisters who are mothers of twins. In the Young NTR (YNTR), data on more than 50,000 young twins have been collected. Currently we are extending the YNTR by including siblings of twins. Participants in YNTR and ANTR have been phenotyped every 2 to 3 years in longitudinal survey studies, since 1986 and 1991 for the YNTR and ANTR, respectively. The resulting large population-based datasets are used for genetic epidemiological studies and also, for example, to advance phenotyping through the development of new syndrome scales based on existing items from other inventories. New research developments further include brain imaging studies in selected and unselected groups, clinical assessment of psychopathology through interviews, and cross-referencing the NTR database to other national databases. A large biobank enterprise is ongoing in the ANTR in which blood and urine samples are collected for genotyping, expression analysis, and meta-bolomics studies. In this paper we give an update on the YNTR and ANTR phenotyping and on the ongoing ANTR biobank studies.

The population-based twin program of research at the Norwegian Institute of Public Health (NIPH) was begun in 1992. It consists of a number of questionnaire and clinical interview projects exploring a broad array of mental and physical health outcomes. This article provides a brief update summarizing our research activities, some research highlights, new developments and potentials for further developing the program of twin research. In the most recent years a large effort has concentrated on completing a mental health interview study of Axis I psychiatric and substance use disorders and Axis II personality disorders. Although still in the early planning phases, one of the most significant developments is that an agreement is now in place to centralize the Norwegian twin data into a national Norwegian Twin Registry. This new registry will include twin cohorts born from 1905 onwards. Other resources for building twin projects are described. Nationally, there is great potential for linking the NIPH twin data with other health registries and with information in a number of Norway's large population-based biobank studies. Internationally, platforms such as those developed within GenomEUtwin, for data standards and data sharing and access are greatly facilitating international collaborations in twin research.

The present article describes the initial steps taken in setting up a twin registry in Spain. The Murcia Twin Registry (MTR) will focus initially on perimenopausal women's health. It will be administered and managed by the University of Murcia and the Murcia Health Council and will start with a database of 452 pairs of female twins born between 1940 and 1965 in the region of Murcia, Spain. Once the MTR is established and has gained some experience, our goal is to expand the age range and include males and opposite-sex twins. The current main areas of research interest are menopause, preventive behaviors, lifestyle and health as well as quality of life.

The Sri Lankan Twin Registry began as a volunteer register in 1997. Previously, we have shown door-to-door surveys as the best option to build a population-based twin register of older, adult twins. Our aim was to build a population-based twin register in the Colombo district and use it for twin studies on common mental disorders. We aimed to identify a random sample of 4000 twins ascertained through the twin census. The Colombo district is divided into 13 divisional secretariat divisions (DSDs) and each division is further divided into Grama Niladari divisions administratively. Grama niladaris (GNs) are civil servants and visit each household in order to update the electoral register. GNs were used to do a census of twins while they updated the electoral register. The correlation between population density and twin rate among DSDs was .81, and between twin rate and the percentage of returned forms (from each DSD) was .74. We received 9648 forms notifying about twins and multiples. After removing the duplicates and information about twins who live in other districts, there were 17,406 multiples remaining. After incorporating twins and multiples from various other feasibility studies we now have 19,040 multiples with 8.46 twins per 1000 people in the Colombo district.

The Swedish Twin Registry was first established in the late 1950s. Today it includes more than 170,000 twins — in principle all twins born in Sweden since 1886. In this article we describe some ongoing and recently completed projects based on the registry. In particular, we describe recent efforts to screen all twins born between 1959 and 1985, and young twin pairs when they turn 9 and 12 years of age. For these studies, we present initial frequencies of common conditions and exposures.

The Swedish Young Male Twins Study is a population-based longitudinal twin study founded in 1997 through record-linkages of several national registers. Details on pregnancy and birth were obtained from the Swedish Medical Birth Register and used to identify 3566 male twins (1783 pairs) born in Sweden between 1973 and 1979 and resident in Sweden in 1997. A record-linkage was made between the Medical Birth Register and the Military Service Conscription Register for the years 1991 to 1999, providing information on body weight, height, blood pressure, muscle strength, cognitive ability of these twins at age 18 and 19 years. In 1998, 2002 and 2005 to 2006, the twins were surveyed on their zygosity, socioeconomic status, lifestyle factors (such as eating habits, physical activity, smoking habits, use of alcohol etc), height and weight. In 2002, additional information was collected on perceived body shape and size, and eating behavior, according to the Three-Factor Eating Questionnaire. In 2003, DNA via buccal mucosa was collected from a subset of the twins. Recent research using the Swedish Young Male Twins datasets has explored the relationships between fetal growth, body size and blood pressure in young adulthood, genetic and environmental contributions to eating behavior and physical activity, and relationships between diet and physical activity patterns with longitudinal changes in body mass index and attained waist circumference.

The present study aimed to investigate genetic and environmental influences on behavioral problems, personality features, and neuropsycho-logical performance among adolescents in Taipei, Taiwan. Between 1996 and 1998, with the assistance of the Twin Association and junior high schools in Taipei City, we solicited a list of twins from 51 junior high schools in Taipei. Names, addresses, and telephone numbers of the twins enrolled in these schools were obtained. The vast majority of the recruited twins were between 12 and 16 years old. The recruited twins received assessments for behavioral problems, general and schizotypal personality, and cognitive functions. Their parents rated their children on behavioral/ emotional problems and were assessed on their own general and specific personality. Same-sex sibling pairs (ages vary within 2 years) and their parents from three schools were also recruited to increase sample size for the group with a kinship coefficient of .50. Twins' zygosity was determined by a combination of DNA typing and physical similarity. The Mx program was used to estimate parameters for the full model and its reduced models. The recruitment, measurements, data managements, and published results are described in this article.

In this ongoing longitudinal study of adolescent twins/sibling pairs and their parents in Taipei, we aimed to investigate the genetic and environmental influences on adolescent behavioral development, metabolic risk factors, and the associations between the two. Special focus is on anxious depression, metabolic profile, and hormonal factors such as cortisol and leptin. The first wave of assessment was completed during the period of 2002 to 2005 and included 192 twin pairs, 6 triplets, 56 sibling pairs and their first-degree relatives (484 parents and 142 siblings). We are currently in the process of a second wave assessment as follow-up. Dimensional psychological assessments using self-report questionnaires, as well as categorical assessments based on semistructured clinical interviews, were performed. All subjects received a 2-hour oral glucose tolerance test at the examination site. The metabolic phenotypes including body mass index, blood pressure, levels of glucose, insulin, and lipid profile as well as related hormonal levels were measured. Zygosity was determined using DNA, except for a few twins whose DNA was not available. Such a combination of detailed psychological assessments and metabolic function tests is expected to help shed light on the interrelation of psychological well-being and metabolic functioning.

The UK Adult Twin Registry was started in 1993 and consists of approximately 10,000 monozygotic (MZ) and dizygotic (DZ) adult Caucasian twins aged 16 to 85 years from all over the United Kingdom, plus some parents and siblings. It now incorporates previous twin registries from the Institute of Psychiatry and Aberdeen University. This is a volunteer sample recruited by successive media campaigns without selecting for particular diseases or traits. All twins receive a series of detailed disease and environmental questionnaires. The majority of twins have been assessed in detail clinically at several time points for several hundred phenotypes related to common diseases or intermediate traits. The focus to date has been primarily in 5 areas — cardiovascular, metabolic, musculoskeletal, ophthalmologic diseases as well as the aging process. Over 3000 DZ twins have had a 10cM genome-wide scan performed and 5000 twins tagged for over 200 candidate genes allowing both linkage and association studies. The resource has led to many successful and innovative research projects particularly in common age-related diseases, and has led to collaborations with over 80 groups worldwide.

In this article we present the protocol of the Birmingham Registry for Twin Heritability Studies (BiRTHS), which aims to establish a long-term prospective twin registry with twins identified from the antenatal period and subjected to detailed follow-up. We plan to investigate the concordance in anthropo-metrics and early childhood phenotypes between 66 monozygotic and 154 dizygotic twin pairs in the first 2 years of recruitment. In this project we plan to determine the relative contributions of heritability and environment to fetal growth, birth size, growth in infancy and development up to 2 years of age in an ethnically mixed population. Twins will be assessed with the Griffitth's Mental Development Scales, which will enable us to obtain detailed information on development. As maternal depression may have an effect on the twins' neurodevelopment, the Edinburgh Postnatal Depression Scale will be used at various stages during pregnancy and after delivery to assess maternal depressive symptoms. The increasing prevalence of obesity in both adults and children has raised concerns about the effect of maternal obesity in pregnancy on fetal growth. The prospective study design gives us the opportunity to obtain data on maternal nutrition (reflected by body mass index) and ante- and postnatal growth and development of twins.

From 1998 the population-based North of England Multiple Pregnancy Register (MPR) has collected data on all multiple pregnancies in the region from the earliest point of ascertainment in the pregnancy. This article describes the development of the MPR and the findings of the first 5 years of data collection. Mothers now give explicit consent for their inclusion with named data, in accordance with section 60 of the Health and Social Care Act 2001. During 1998 to 2002, 2310 twin pregnancies were registered, with an increasing twinning rate of 13.6 to 16.6 per 1000 maternities. Chorionicity ascertainment in twin maternities with at least one stillbirth or live birth has improved from 81% in 1998 to 91% in 2002. Before 24 weeks of gestation, 8.4% (359/4620) of fetuses were lost either spontaneously or as a result of termination of pregnancy. The perinatal mortality rate was much higher in monochorionic than dichorionic twins, mainly due to differences in stillbirth rates (49.0 vs. 11.5 per 1000 maternities respectively, risk rate = 4.2; 95% confidence intervals 2.7–6.6). The gestational-age-specific neonatal mortality rates were similar in twins and singletons, except in the group of term births (≥37 weeks' gestation) when compared by conventional gestational age categories. For stillbirths, the rates were even lower than in singletons in gestational age categories of less than 32 weeks. The register is an important resource of data on multiple pregnancies, which allows monitoring of trends in multiple birth rates and pregnancy losses and provides a unique opportunity for etiological and long-term follow-up studies.

Between 4 to 6 million twins exist in the US today who offer scientists a valuable potential resource for conducting behavioral and biomedical research. However, unlike many other countries, there is no national system in the US for identifying twins and eliciting their participation in these important research programs. Therefore, the National Institute of Environmental Health Sciences (NIEHS) is conducting a study to determine the feasibility of creating a national, population-based twin registry in the US. The major goal is to estimate the potential size and characteristics of a national twin registry based on the current twin population in the US, our ability to ascertain and enrol them, and their willingness to participate. Existing US twin cohorts are also being examined in this study as well as alternatives for improving US twin resources should a national twin registry be deemed infeasible. The various options will be compared in terms of possible source populations, generalizability and adequacy for statistically powering various types of etiological studies. Two expert advisory panels have been assembled to assist in the conduct of this study. The Scientific Advisory Panel is charged with providing expertise concerning study goals, design and methodology, and evaluating the study's conclusion. A separate Ethics Advisory Panel is charged with providing expertise on the ethical, legal, and social issues that might be encountered if a national twin registry is ultimately pursued. Having a national population-based twin registry in the US would be advantageous to US scientists and those worldwide. It would provide ample numbers of twin pairs to conduct various types of environmental genomic studies currently not possible with existing US twin resources. It would also allow US scientists to select for characteristics (race, ethnicity, environments, and so on) inherent in our own population. Finally and foremost, it would help to meet the worldwide demand for twin resources which is expected to increase over time, as new genomic and analytical tools become available and new hypotheses emerge concerning the complex interplay between genes, lifestyles and environment.

The California Twin Program (CTP) is a population-based sample of over 52,000 twins in which a number of nested studies are ongoing. We outline our experience to date, providing estimates of crude response rates for a variety of different study designs and protocols. We have experienced very high response rates in our studies to date, even in studies with demanding protocols. Lowest response rates have occurred in studies among afflicted individuals, and in one with an unusual protocol. We have experienced some difficulty in locating original members of the cohort, despite efforts to trace individuals using a variety of sources of information. However, in most analyses, the participating sample of twins does not differ substantially from the underlying sample from the CTP. Future work will focus on improving methods of recontacting cohort members.

The Southern California Twin Register was initiated in 1984 at the University of Southern California, and continues to grow. This article provides an update of the register since it was described in the 2002 special issue of this journal. The register has expanded considerably in the past 4 years, primarily as a result of recent access to Los Angeles County birth records and voter registration databases. Currently, this register contains nearly 5000 twin pairs, the majority of whom are school age. The potential for further expansion in adult twins using voter registration records is also described. Using the Los Angeles County voter registration database, we can identify a large group of individuals with a high probability of having a twin who also resides in Los Angeles County. In addition to describing the expansion of register, this article provides an overview of an ongoing investigation of 605 twin pairs who are participating in a longitudinal study of behavioral problems during childhood and adolescence. Characteristics of the twins and their families are presented, indicating baseline rates of conduct problems, depression and anxiety disorders, and attention-deficit/hyperactivity disorder diagnoses which are comparable to nontwins in this age range.