A range of potential confounding factors (selected due to being likely risk factors for the outcome in addition to being correlates of the exposure) that were measured in ALSPAC were included in the analyses. Based on our earlier work [18], the following prenatal maternal and socio-demographic variables obtained during the pregnancy were categorised for analysis: maternal age (≤20, 21–34, or ≥35 years); parity (none or ≥1); highest level of maternal education (based on the UK examination system- <‘O’ levels, ‘O’ levels, ‘A’ levels, degree); the number of times per day the mother smoked during the second trimester (response categories were 0, 1–4, 5–9, 10–14, 15–19, 20–24, 25–29, and 30+); and use of cannabis and other illicit drugs during the pregnancy, home ownership and whether currently married (all dichotomized to yes/no). Maternal mental health was measured at 18 weeks gestation and when the child was 11 years old using the Edinburgh Postnatal Depression Scale (EPDS)—this measure has been validated outside the postnatal period [24]. As scores of >12 are highly associated with a diagnosis of a depressive disorder, this cut off
