[3] The Swedish Twin Registry was first developed in the late 1950s to study the effects of environmental factors, like alcohol and tobacco, on chronic disease risk while controlling for potential genetic confounding factors.
When the registry was first started, researchers contacted every parish in Sweden to obtain records of multiple births between 1886 and 1925.
In 1960 and 1961, a questionnaire asking about demographic and health-related information was sent to all living same-sex twins that had been identified in Sweden at the time.
[6] More recently, the STR has obtained records of twin births in Sweden from the country's National Board of Health and Welfare.
[4] More recently, STR researchers have contacted and screened twins born from 1959 to 1985.