The Danish study of 11,000 youngsters shows that twins are catching up. "We interpret our findings as indicating that improvements in obstetric and pediatric practices over the past 50 years have largely ameliorated the cognitive disadvantage identified in twins by earlier research, despite more twins of extremely low birth weight now surviving the first year of life," said lead researcher Professor Kaare Christensen.
Researchers identified 3,411 twins and 7,796 singletons who were born in Denmark between 1986 and 1988. This data was obtained from the Danish national birth register. The researchers tabulated information like birth weight, gestational age at birth, and parents' age and education.
"It is possible that our findings differ from those of earlier research because of country specific factors or differences in measurement. Our research is based on measures of academic achievement rather than IQ. These differences are not likely to be a major factor in accounting for the differences in findings," Prof Christensen said.
The study is published online by the British Medical Journal.