Largest twin registry

Sweden has the largest number of twins in its registry, yielding a total of 194,000 twins. Twin registries are becoming increasingly common and important for research purposes. They allow researchers to test behavioural and medical hypotheses using large samples and to identify twins with specific characteristics. Sweden has maintained such registries for many years, identifying all multiple birth individuals in its population. In this sense, Sweden's differs from other registries, such as the Australian Twin Registry (ATR), that rely on voluntary recruitment. Because the government is involved in the Swedish registries, the registry includes more twins than those in much larger nations. For example, Australia has a larger population, but there are fewer twins in the ATR. Some smaller twin registries are also valuable. The smallest one is a registry I maintain on virtual twins (same-age unrelated individuals reared together). Virtual twins are created when families adopt two near-in-age children or adopt a child that is close in age to one of their biological children. Critical requirements are that the individuals must both be in the same home by the age of one year, can differ by no more than nine months, must be enrolled in the same school grade and must be free of adverse birth events. The Fullerton Virtual Twin Study now includes 162 such pairs. Virtual twins are twin-like creatures that give us direct estimates of how much shared environments affect behavioural development.