The study followed around 400 children who were born before the 28th week, or about 7 months of gestation.
"These are children who are more socially reserved, more anxious, and have significant difficulties when it comes to attention," said Johanna Månsson, a researcher behind the study in an interview with Swedish Radio News.
Over one third of the children studied had cognitive functions that were under the expected level. Those could include memory, information processing, ability to recognize patterns and ability to discern similarities and differences. The study also showed that those children born early had more trouble with language and motor skills.