There might not be any direct link between smoking during pregnancy and children's anti-social behaviour, research at Cardiff University, Wales shows.
Actually there was a link between women who smoked when they were pregnant and anti-social behaviour in their children.
But the same link was not apparent in children who were born following IVF treatment with donated eggs or embryos - when there was no direct genetic link between mother and child.
If there was a direct link, both sets of children would have shown the same behaviour, it is argued.
The Cardiff University researchers, funded by the Wellcome Trust, were able to study IVF children, with differing degrees of genetic relation to their parents, to disentangle the effects of genetic influences and the prenatal environment.
The study is the first of its kind in the world to allow these effects to be separated, Madeleine Brindley reported for Western Mail.
Professor Anita Thapar, a clinical child psychiatrist at Cardiff University and the principal investigator on the study, said: "Smoking during pregnancy is clearly something that mothers need to stop because it has a detrimental affect on a child's health and their health.
"Only it doesn't look like that smoking affects the child's brain as had been believed earlier."
Prof Thapar added: "What we have been able to confirm is that cigarette smoke in pregnancy does lower birth weight regardless of whether the mother and child are genetically related or not, but the link with children's behaviour is different."