Exposure to tobacco smoke in the womb can lead to behaviour problems by the age of ten. Researchers from Munich studied 5,991 children and their parents. They found that children who had been exposed to tobacco smoke in the womb were 1.9x more likely to develop behaviour problems. Children exposed to tobacco smoke after birth were at 1.3x greater risk. Children who were exposed to tobacco smoke both while in the womb and later had double the risk of behaviour problems. The kind of behavioural problems the children were at an increased risk of included hyperactivity, attention-deficits and problems in their relationship with their peers.