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LSADT Details
The LSADT Study Population
The study is based on the Danish Twin Registry, which was established in 1954 as the first nationwide twin registry in the world. The registry includes all twin pairs born in Denmark between 1870 and 1910 and all same-sex pairs born between 1911 and 1930. The birth registers from all 2,200 parishes of the relevant calendar years were manually scrutinized to identify all twin births. Through regional population registers (in operation since 1924) and other public sources, a search was made for the twins, or whenever needed, their closest relatives. As soon as a twin was traced, a questionnaire was sent to him or her. If neither of the partners were alive, a questionnaire was sent to the closest relative. Specific questions about the degree of similarity between the partners of a pair were included in the questionnaire to assess zygosity in same-sex twins. For twins dying or emigrating at an early age it was impossible to obtain reliable data to be used in zygosity classification. Consequently, pairs were not followed up if one or both partners died or emigrated before age 6 years.
Zygosity
The validity of zygosity classification based on answers to mailed questionnaires has been evaluated by comparison with the results of later blood group determinants and the misclassification rate has been found to be less than 5%.
The Survey
The residence of all twins in the study population was identified in the Central Person Registry in January 1995 through the personal identification number for each twin. During two weeks in the beginning of February 1995 the twins received a letter explaining the study and asking permission for an interviewer to come to their residence in order to conduct a health related 1-hour face-to-face interview. A total of 100 Interviewers from The Danish National Institute of Social Research were used. In 1958, this Institute was set up by law in order to conduct and disseminate research into social policy, working life, social aspect of housing, health, and living conditions among the Danish population. The interviewers are not medically or paramedically trained. The interviewers from The Danish National Institute of Social Research have substantial experience interviewing the elderly, however.
All 100 interviewers received detailed training during January and February 1995, and were closely monitored during the three month period during which interviews were completed (February-April 1995). When a twin was not able to participate due to physical or mental handicaps, a proxy respondent was sought (closest relative or professional care-provider). Only when at least three unsuccessful attempts to contact a twin at his or her residence at different times had been made, the twin was considered to be impossible to contact.
Classic twin analysis is based on correlations within pairs, and in order to avoid inflated estimates of twin similarity due to potential interviewer bias, the two twins in a pair were never interviewed by the same interviewer, not even in cases where the co-twins lived together. A pilot-study performed in November 1994 comprising 100 twins from the study population was included in the analysis, because it was made just a few months before the main survey, used the same procedure, and the pilot questionnaire differed only minimally from the one used in the main survey.
The questionnaire includes questions on self-rated health, diseases, medicine, Activities of Daily Living (ADLs), cognitive abilities, depression, and life circumstances and events. From 1999, physical tests were included: grip strength, agility, speed, and spirometri. DNA samples were obtained through blood spots or cheek swabs. (For details see: Christensen K. Biological Material in Household Surveys. The Interface between Epidemiology and Genetics. In National Research Council "Cells and Surveys. Should Biological Measures Be Included in Social Science Research?" Committee on Population; Finch CE, Vaupel JW and Kinsella K (eds.). Commission on Behavioral and Social Sciences and Education. Washington DC, National Academy Press 2000: 42-63. [purchase PDF])
English version of the 1999 questionnaire (PDF), 1997 questionnaire (PDF), and 1995 questionnaire (PDF).
Selected Publications from LSADT
Christensen K, Frederiksen H, Vaupel JW, McGue M. Age trajectories of genetic variance in physical functioning: A longitudinal study of Danish twins aged 70 years and older. Behavior Genetics 2003;33(2):125-135.
McGue M, Christensen K. The heritability of depression symptoms in elderly Danish twins: Occasion-specific versus general effects. Behavior Genetics 2003;33(2):83-93.
Frederiksen H, Christensen K. The influence of genetic factors on physical functioning and exercise in second half of life. Scand J Med Sci Sports 2003;13(1):9-18.
Frederiksen H, Gaist D, Bathum L, Andersen K, McGue M, Vaupel JW, Christensen K. Angiotensin I - Converting Enzyme (ACE) gene polymorphism in relation to physical performance, cognition and survival. A follow-up study of elderly Danish Twins. Annals of Epidemiology 2003;13(1):57-65.
Christensen K, Kohler HP, Basso O, Olsen J, Vaupel JW, Rodgers JL. The correlation of fecundability among twins: Evidence of a genetic effect on fertility? Epidemiology 2003;14(1):60-64.
Bak S, Gaist D, Sindrup SH, Skytthe A, Christensen K. Genetic liability in stroke. A long-term follow-up study of Danish twins. Stroke 2002;33:769-774.
Christensen K, Gaist D, Vaupel JW, McGue M. Genetic contribution to rate-of-change in functional abilities among Danish twins aged 75 years or more. American Journal of Epidemiology 2002 Jan 15;155(2):132-139.
Petersen HC, Jeune B, Vaupel JW, Christensen K. Reproduction Life History and Hip Fractures. Annals of Epidemiology 2002;12:257-263.
Frederiksen H, McGue M, Jeune B, Gaist D, Nybo H, Skytthe A, Vaupel JW, Christensen K. Do Children of long-lived Parents age more successfully? Epidemiology 2002;13:334-339.
Wienke A, Christensen K, Skytthe A, Yashin AI. Genetic analysis of cause of death in a mixture model of bivariate lifetime data. Statistical Modeling 2002;2:1-14.
Tomassini C, Rosina A, Billari FC, Skytthe A, Christensen K. The Effect of Losing the Twin and Losing the Partner on Mortality. Twin Research 2002,5(3):210-217.
Iachina M, Jørgensen B, Christensen K, Iachine I. Analysis of functional abilities for elderly Danish twins using GEE models. Twin Research 2002,5(4):289-293.
McGue M, Christensen K. The heritability of level and rate-of-change in cognitive functioning in Danish twins aged 70 years and older. Experimental Aging Research, 2002;28(4):435-51.
Skytthe A, Kyvik K, Holm NV, Vaupel JW, Christensen K. The Danish twin registry: 127 cohorts of twins. Twin Research, 2002;5(5):352-357.
Kjeldsen MJ, Kyvik KO, Friis ML, Christensen K. Genetic and environmental factors in febrile seizures: A Danish population-based twin study. Epilepsy Research 2002, 51:167-177.
Frederiksen H, Gaist D, Petersen HC, Hjelmborg J, McGue M, Vaupel JW, Christensen K. Hand Grip Strength - a phenotype suitable for identifying genetic variants affecting mid- and late- life physical functioning. Genetic Epidemiology 2002, 23:110-122.
Johnson W, McGue M, Gaist D, Vaupel JW, Christensen K. Frequency and heritability of depression symptomatology in the second half of life: evidence from Danish twins over 45. Psychological Medicine, 2002; 32(7):1175-85.
Bathum L, Pedersen HC, Rosholm JU, Petersen PH, Vaupel JW, Christensen K. Evidence for a substantial genetic influence on biochemical liver function test: results from a population based Danish twin study. Clinical Chemistry 2001;47:81-87.
Brix TH, Kyvik KO, Christensen K, Hegedüs L. Evidence for a major role of heredity in Graves' disease - a population based study of two Danish twin cohorts. Journal of Clinical Endocrinology and Metabolism 2001;86(2):930-934.
Rodgers JL, Kohler H-P, Kyvik KO, Christensen K. Behavior genetic modeling of human fertility: Findings from a contemporary Danish twin study. Demography 2001;38(1):29-42.
Bathum L, Christiansen L, Nybo H, Andersen-Ranberg K, Gaist D, Jeune B, Petersen NE, Vaupel JW, Christensen K. Association of mutations in the hemochromatosis gene with shorter life expectancy. Archives of Internal Medicine 2001;16:2441-2444.
Christensen K, Frederiksen H, Hoffman HJ. Genetic and Environmental Influences on Self-Reported Reduced Hearing among the Elderly and Oldest-Old. Journal of the American Geriatric Society 2001;49(11):1512-1517.
Christensen K, Wienke A, Skytthe A, Holm NV, Vaupel JW, Yashin AI. Cardiovascular Mortality in Twins and the Fetal Origins Hypothesis. Twin Research 2001; 4: 344-349.
Christensen K, Ørstavik K-H, Vaupel JW. "The X Chromosome and the Female Survival Advantage: An Example of the Intersection between Genetics, Epidemiology and Demography". I: Population Health and Aging: Strengthening the Dialogue Between Epidemiology and Demography. Annals of the New York Academy of Sciences, 2001;954:175-183.
McGue M, Christensen K. The heritability of cognitive functioning in the very old: Evidence from Danish twins aged 75 and older. Psychology and Aging 2001;16:272-280.
Tomassini C, Billari FC, Rosina A, Christensen K. Born together - die together. Live together - die together. The role of the partner and of the co-twin on longevity at very old ages. Genus, 2001; Vol. LVII (3-4): 63-82