Hundreds of twins helped researchers in northern Serbia to explore the influence of genetic and social factors on intelligence.
"We learned things about ourselves," admit Ana and Maja Markovic, twin sisters in their 30s who who were among around 530 people taking part in the scientific study at the University of Novi Sad.
"When we were younger, under the influence of our environment and our parents, we used to be a lot more similar," says Maja.
"And now with the influence of other experiences that we have been through individually, we are very different from each other."
Her twin Ana described their involvement in the research as a "really fascinating experience," adding: "We got to know more about our own selves, while they were taking information from us."
The study showed that cognitive capacity is "largely determined by certain genetic factors, which are hereditary, while success in school is mostly determined not only by genetic factors but also by social and environmental factors," said psychologist Snezana Smederavac.
"It is not enough to have a high IQ if you don't have an enviroment to stimulate intelligence it will not help you a lot in life," said Smederavac, who coordinated the study.
Serbia, a country of 7.1 million people, has more than 1,000 twins, official figures show.
In September, the study will be carried out on twin children.