Smoking and externalizing problems also co-occur. Of considerable interest, the comorbidity between smoking, ADHD, conduct problems and other dimensions of delinquency has been partly attributed to maternal smoking during pregnancy (SDP). Multiple studies show increased rates of smoking and externalizing psychopathology in offspring of mothers with a history of SDP, however the extent to which the effects of SDP on offspring outcomes are genetic or environmental remain inconclusive. However, as SDP is a good indicator of maternal nicotine dependence and smoking persistence, it is often difficult to disentangle the independent effects of SDP on offspring smoking from familial transmission of vulnerability68. Insight into this confound may be offered by a recent study comparing related mother-offspring pairs with offspring conceived by assisted reproductive technologies, such as oocyte donations (i.e. where the mother is genetically unrelated to the offspring). This study reported that the link between ADHD and SDP was more noticeable in related pairs, thus arguing against an environmental/causal role of SDP69
