Twins may be double trouble in the beginning, but by the time they are 5, they exhibit higher levels of cooperative behavior with each other and their peers than do single children.

"By the time they are 5, twins show minimal conflict," said Deborah Vandell, professor of psychology and human development at the University of Texas at Dallas. "We see lovely cooperative play with twins that we don't see with singletons."Vandell studied the social behavior of 32 pairs of twins in the Dallas area at regular intervals over five years. The study group included both identical and fraternal twins, but all pairs were same sex pairs, half of them girls and half boys, she said.

She spoke about her research last week at Utah State University during a conference on infancy and early childhood.

Much previous twin research has focused on teasing out the roles of environment versus genetics in explaining characteristics, or on the identity development in twins, she said. Vandell was interested in their development of social relationships, the quality of twins' relationships with each other, their mothers and other children.

"Mothers of twins are very busy," Vandell said. "They are trying to balance multiple things at one time, and they face dilemmas balancing the needs of two children. One crying twin settles down and then the other cries. We observe more crying in twin babies than in single children."

Studies have shown that the overall amount of language and affection given by a mother of twins is less than that of a mother of a single child, she said.

"Mother fatigue may say something about that," she said.

Young twins ignore each other or fight over toys and their mother, she said. However, by age 5, they show low levels of conflict, with interactions that are almost exclusively positive.

"Observed on the playground, they interacted positively with their classmates and with each other. They are doing well and seem to have a special appreciation of their peers."

Vandell has also studied the mothers' styles of interaction with their twin children, she said, and has observed developmental changes over time.

She is still analyzing data from the twin study, she said.