Children with a twin of the opposite sex may do better than their classmates when it comes to emotional and social development, claim researchers.
A boy and girl twin
A survey carried out in Finland found that boy and girl twins were more developed than both same-sex sets and singleton children.
The researchers involved, from the University of Jyvaeskylae, suggest that the best qualities of each assist the progress of the other.
Traditionally, twins are thought to take longer to develop certain social, emotional and language skills.
These twins tend to shape and modify each others' behaviour
Lea Pulkkinen, Jyvaeskylae University
Twins are often smaller at birth than singleton babies, and this is thought to contribute to marginally delayed development.
In addition, a delay in picking up language skills is often blamed on very young twins choosing to babble at each other rather than attempt communication with their parents.
However, the Finnish study, which involved 4,000 sets of twins, comparing them with 22,000 of their classmates, found no evidence of any social disadvantage.
Pupils in classes with twins were asked to evaluate their classmates, rating their "friendliness" or how easily they became irritated.
The same-sex twins fared no worse than their singleton schoolmates, and the mixed-sex pairs actually did better.
Lea Pulkkinen, the psychologist who carried out the study, said: "We did not find differences between twins and non-twins, except from the cases where the twins are of opposite sex - these scored higher in positive characteristics."
Many parents have noticed that girl babies are less active than boy babies, while boys have poorer social skills.
Pulkkinen has the theory that contact with a girl twin develops the social side of boys, while girls also benefit from contact with their twin brothers.
She said: "These twins tend to shape and modify each others' behaviour, so the girls become more active, and the boys more socially skilful."
The study was published in the journal Twin Research.