This is going to be confounded surely... parents who use cannabis during pregnancy are not 'the same as everyone else except they use cannabis'. They probably,

This is going to be confounded surely... parents who use cannabis during pregnancy are not 'the same as everyone else except they use cannabis'. They probably, or might I say, almost certainly, have higher rates of psychiatric disorders, the propensity for which they pass onto their children. So the size of the effect will be less than what is claimed, but in any case don't use cannabis during pregnancy.