The Norwegian Institute of Public Health in Oslo has an ongoing program of twin research using population-based cohorts of twins. The current database includes information on twins identified through the Medical Birth Registry of Norway and born from 1967–1979, altogether 15,370 twins. This is a longitudinal study with a cohort sequential design whereby new cohorts are recruited into the study at 5–6 year intervals. Sub-samples of these twins have participated in questionnaire studies and clinical assessment sub-projects. These projects include national and international collaborations. Our primary areas of interest include mental health and psychological well-being, obesity, asthma and allergies, health behaviors and health perceptions, comorbidity, and perinatal influences on health outcomes. This paper provides a brief overview of the data, sample, and the various research projects associated with this twin program of research.

Sri Lanka is an island with genetic diversity between the five main population groups. Our twin registry is the first in the developing world. Initially, we established a volunteer cohort of 4600 twin pairs through a competition advertised in the media. In addition, we have volunteer cohorts, birth registration-based cohorts through hospitals, and community-based cohorts. There is also a nationwide population-based younger twin cohort (1992–1997) traced through the Department of Birth and Death Registration. Additionally, we have adapted a Zygosity determination questionnaire and validated it. Establishing ethical guidelines for twin research was a priority because the field of bio-ethics is at an early stage of development in Sri Lanka. These guidelines were from a developing world perspective. A sister organization, the Multiple Birth Foundation, was formed to cater to twins and their special needs and to represent their interests, and several branches have been formed. We intend to build capacity by establishing a genetic lab and through crosscultural collaboration. Our vision is to establish a multidisciplinary research foundation. Based on our research findings, we plan to build services to cater to needs of twins by working with professionals, statutory services and government policy makers.

Since the Swedish Twin Registry was first established in the late 1950s to study the importance of smoking and alcohol consumption on cancer and cardiovascular diseases, it has been expanded and updated on several occasions. The focus has similarly broadened to most common complex diseases. The content of the database is described, ongoing projects based on the registry are summarized, and we review some of the principal findings on aging, cancer and cardiovascular disease that have come from the registry. Ongoing efforts and future plans for the STR are discussed. Among others, we plan blood collection and genotyping to study the genetic bases of complex diseases, a first contact ever with the cohorts born after 1958, and in-depth studies of selected diseases, such as Parkinson’s disease and chronic fatigue syndrome.

A new Swedish twin register was created in 1998. All male twins born in Sweden 1973–1979 were identified in the Swedish Medical Birth Register. 3566 twins (1783 pairs) who were alive and residents of Sweden in 1997 were included in the new register. Data on birthweight, birthlength, gestational age, and so forth, were obtained from the Medical Birth Register. Record linkage was effected with the Military Service Conscription Register for the years 1991–1999. Anthropometrical data and information on blood pressure were obtained from this latter source. In 1998 a questionnaire was mailed to all twins in the register. Questions were posed about zygosity, socioeconomic conditions, lifestyle factors (physical activity, smoking habits, etc.), body shape and body size. Complete responses were obtained from 2654 twins (1327 pairs). In 2002 the cohort is being followed-up by a more extensive mailed questionnaire. Data will be collected about socioeconomic conditions, lifestyle, eating behaviour (TFEQ), body size and body shape. In the next few years genomic DNA will also be collected from buccal mucosa (using brush techniques).

A regional population-based Multiple Pregnancy Register was established in 1998, with the aim of collecting detailed information on multiple pregnancies to enable research into mortality and morbidity in multiples. Multiple pregnancies are notified to the Register as soon as they are detected, irrespective of whether they resulted in a spontaneous abortion, termination of pregnancy or registered birth. Nine hundred and twenty-six twin pregnancies were recorded during 1998–99, giving a twinning rate of 14.8 per 1000 maternities (rate at birth 13.0 per 1000 maternities). Sixty one per cent of twin pregnancies were detected before 13 weeks of gestation. Chorionicity was determined in 82.6% of 849 twin maternities with at least one stillbirth or livebirth. The fetal loss rate before 24 weeks of gestation was 10.5% (194/1852). The perinatal and infant mortality rates were 40.6 per 1000 births and 32.6 per 1000 livebirths respectively. A prospective Multiple Pregnancy Register not only allows monitoring of trends in multiple birth rates and mortality, but also etiological research and long-term follow-up studies.

The Registry consists of nearly 10,000 monozygous and dizygous adult caucasian twins aged 18–80 from all over the UK and was started in 1993. This is a volunteer sample recruited by successive media campaigns without selecting for particular diseases or traits. All twins receive a series of disease questionnaires. In addition over half the twins have been assessed in detail clinically for several hundred phenotypes related to common diseases or intermediate traits. The focus has been primarily on cardiovascular, metabolic, musculoskeletal, dermatological, and opthalmological diseases. Over 3000 DZ twins have had a genome wide scan performed as well as many candidate genes allowing both linkage and association studies. The registry has led to many successful innovative research projects, particularly in common diseases previously thought to be predominantly environmental and helped positionally clone some novel genes for common diseases.

The Twins Early Development Study (TEDS) focuses on the early development of the three most common psychological problems in childhood: communication disorders, mild mental impairment and behavior problems. The TEDS twins were assessed longitudinally at 2, 3, 4 and 7 years of age in order to investigate genetic and environmental contributions to change and continuity in language and cognitive development; it is multivariate in order to examine the origins of comorbidity; and it uses a large sample in order to study abnormal development in the context of normal development. The twins were identified from birth records of twins born in the UK in 1994–96. More than 15,000 pairs of twins have been enrolled in TEDS and the participating families are representative of the UK. The measures at 2, 3 and 4 years are administered by parents. At 7 years, children are assessed for language and cognitive development using telephone testing, parents and children are interviewed about behavior problems, and teachers also assess behavior problems as well as academic achievement. One set of findings is that the same genes largely contribute to both language and cognitive problems and the same genes affect normal and abnormal development, a result that suggests that general impairment may be a better target for genetic research than specific language impairment independent of nonverbal cognitive problems. DNA has been obtained so far for more than 4000 pairs and is being used initially in molecular genetic studies of language problems and hyperactivity.

The Mid-Atlantic Twin Registry (MATR) is a population-based registry of twin pairs ascertained from birth records and school system records of Virginia, North Carolina, and South Carolina. The MATR was formed in 1997 with the merging of the Virginia and North Carolina Twin Registries, and it expanded to include South Carolina when access to twin birth records in that state was granted in 1998. Registered twins (“participants”) number more than 51,000, with approximately 46,000 of these individuals representing complete pairs. Roughly two-thirds of MATR participants are over age 18, with a mean age of approximately 35 years. These participants have primarily been drawn from the more than 170,000 identical and fraternal twin pairs born in the three states between 1913 and 2000. Twins and their family members have participated in numerous research projects, ranging from general health surveys to studies on specific health topics such as cardiovascular disease; depression and anxiety; seizures; behavioral development; pregnancy complications; conduct disorder; drug use, abuse, and dependence; cleft lip/palate; obesity; and chronic fatigue syndrome. The MATR has established a privacy policy and strict standard operating procedures to protect the confidentiality of participant data. The MATR considers a limited number of qualified requests per year from investigators interested in recruiting MATR participants into their research studies.

The Southern California Twin Register is the result of an effort to recruit twins of all ages in the city of Los Angeles and surrounding areas. The register currently includes an ethnically diverse sample of more than 2600 twin pairs. The most recently recruited pairs have been drawn primarily from computerized records of enrollments in local public school districts, and are comparable in sex and ethnic distributions to the general public school population. An ongoing twin study of social and moral development in children, based on 600 pairs of 9-year-old twins from this register, is also described briefly in this paper.

We describe the prevalence of chronic diseases and conditions in a large cohort of twins, which has been developed to facilitate studies of the role of genetics and environment in the development of disease. The California Twin Program (CTP) comprises twins born in California between 1908 and 1982. Birth records from all multiple births (256,616 in total) were linked (multiple times between 1990 and 2001) with the California Department of Motor Vehicles (DMV) roster of licensees to obtain address information. The linkages have revealed 161,109 matches and, because of less complete DMV records in some years, were less successful in older females than in all others. To date over 51,000 of these twins have completed a detailed 16-page mailed risk factor questionnaire. Based on estimates of numbers of individuals receiving a questionnaire, our crude response rates are as high as 63.6% (among females currently in their 50s), with an overall crude response rate of 37.9%. Similar to our previous report regarding the first 42,000 twins, the current group who have completed the questionnaire are representative of the population from which they were drawn (in terms of age, sex, race and residential distribution). Self-reported disease frequencies are provided, along with current estimates of future cancer incidence and mortality rates likely to be observed in the group. We outline our plans for cohort expansion, additional studies using the cohort, and future plans for inviting collaboration.

The Southern Illinois Twins registry consists of a small group of twins from the southern Illinois area and some areas in Missouri and Kentucky, as well as siblings of twins and single-tons who have been involved in the study. The current project has as its primary focus the identification of variables that predict aggressive and prosocial behaviors in preschoolers, especially to explore the etiology of individual differences in aggressive and prosocial behaviors from a behavior genetic perspective. Twins are tested yearly within months of their birthdays, from ages 1 year to 5 years. Siblings are tested at 5 years of age. At ages 1–4 years, children are tested on a cognitive battery and mother–child interactions are videotaped. At 5 years, children engage in a peer play situation in the lab. Results thus far suggest that aggression during play in the lab shows genetic influence, corroborating earlier parent rating studies.

“Project Grow-2-Gether” is a child nutrition study of same-sex, 3- to 7-year-old monozygotic and dizygotic twin pairs. The study attempts to bridge two bodies of literature that have rarely interfaced with respect to obesity and ingestive behavior: the first being behavioral genetic approaches to obesity-related traits, and the second being developmental approaches focusing on parent–child relationships. The overarching aim of Project Grow-2-Gether is to disentangle genetic from potential home-environmental influences on child eating behavior and body fat. This paper reviews the rationale for Project Grow-2-Gether, its procedures, and core phenotypic measurement battery. A focus of the study is acquisition of controlled food intake measurements obtained in the laboratory, measurement of specific home environmental variables, and multi-method evaluation of parent–child feeding relations. Future directions may involve longitudinal assessment of child growth and molecular analyses for specific genes that influence child eating behavior.

The Vietnam Era Twin (VET) Registry is composed of 7369 middle-aged male-male twin pairs both of whom served in the military during the time of the Vietnam conflict (1965–1975). The Registry is a United States Department of Veterans Affairs resource that was originally constructed from military records; the Registry has been in existence for more than 15 years. It is one of the largest national twin registries in the US and currently has subjects living in all 50 states. Initially formed to address questions about the long-term health effects of service in Vietnam the Registry has evolved into a resource for genetic epidemiologic studies of mental and physical health conditions. The management and administration of the VET Registry is described with particular attention given to the processes involved with database maintenance and study coordination. Several waves of mail and telephone surveys have collected a wealth of health-related information on Registry twins. More recent data collection efforts have focused on specific sets of twin pairs and conducted detailed clinical or laboratory testing. New Registry initiatives for the future include the construction of a web site and the development of a DNA repository.

The Minnesota Twin Family Study is a longitudinal study of 11-year-old and 17-year-old twins and their parents designed to examine factors related to the etiology of substance abuse and related problems. At study intake, the twins and their parents participate in a day-long assessment in our laboratory that includes measures of endophenotypes (e.g., event-related potentials, EEG, autonomic nervous system reactivity, startle eye-blink), psychopathology, personality, cognitive ability, anthropometry, and environmental risk/protective factors. DNA derived from blood is also collected. A parallel longitudinal study of adolescent adoptive siblings, biologically related siblings, and their parents is also underway. Over 1500 twin families and 350 adoptive and biological sibling families have already entered the longitudinal phase of the study. This article provides an overview of study methods, highlights published findings, and describes procedures in place to foster collaboration with other investigators.

The Minnesota Twin Registry is a birth-record-based twin registry. Begun in 1983, it includes data for 4307 surviving intact pairs born in Minnesota between 1936 and 1955. In addition, the Registry includes 901 twin pairs born in Minnesota from 1904 to 1934, as well as 391 male pairs born in Minnesota from 1961 to 1964. The research focus is primarily on human individual differences assessed by self-report. Questionnaires completed by the participants include measures of personality, occupational interests, demographics, and leisure-time activities. We outline major contributions that have resulted from Registry research, as well as current and future research directions.

The NAS-NRC Twin Registry is one of the oldest, national population based-twin registries in the United States. It consists of 15,924 white male twin pairs born in the years 1917–1927 (inclusive) both of whom served in the armed forces. The registry, which has been in operation more than 30 years, has collected data from a variety of sources. Records-based, computerized data have come largely from the Department of Veterans Affairs, and there have been three major epidemiologic questionnaires, undertaken roughly every 15 years. Classic twin studies on a variety of medical conditions were the early focus of the registry, which now has a strong focus on chronic disease epidemiology. Work on a DNA specimen bank has been proceeding slowly, but is now a top priority, due to the increasing force of mortality in this twin cohort.