Mothers in the study reported on whether their kindergarten child had at least one friend and on the quality of their child's friendships.

Researchers then compared the progress of children with no friends, low-quality friendships, average-quality friendships, and high-quality friendships. Teachers provided feedback on children's behaviour problems in kindergarten and first and third grades.

"As we expected, high-quality kindergarten friendships that featured cooperation and sharing, taking turns, low levels of hostility, and little destructive conflict, gave children-especially boys-practice in positive interaction, which they demonstrated in grades 1 and 3," Engle, the leas author of the paper, said.

Engle noted that friendship quality was important for both boys and girls in kindergarten. Kindergarten kids with high-quality friendships tended to have fewer behaviour problems and better social skills than those whose friendships were of low or moderate quality.

In contrast, kids who had low-quality kindergarten friendships had more behaviour problems during kindergarten.

She also said that the differences in friendship quality for boys versus girls didn't show up until the children were older.

"Boys who had no friends in kindergarten had more behaviour problems, but not until they had reached first and third grades," she added.

The study has been published in Infant and Child Development.