The researchers compared non-disabled children with infants who had:
a developmental delay at age 9 months (in relation to hand-eye coordination, for example, or early communication gestures)
a longstanding limiting illness (such as type 1 diabetes or asthma)
special educational needs at age 7 (stemming from learning difficulties or impairments such as hearing loss).
They were able to analyse assessments of MCS children's behaviour at ages 3, 5 and 7 as parents had been asked about conduct problems, hyperactivity, emotional difficulties and whether their sons and daughters got on with children of the same age.
This enabled the researchers not only to record the emergence of any problems but establish whether the behaviour of disabled and non-disabled children had followed the same trajectory.
They also took into consideration family background factors known to be associated with child behaviour, such as income poverty, parental discipline and the closeness of the parent-child relationship.