The findings of this study do not support the assumption that adulthood ADHD is necessarily a continuation of childhood ADHD. Rather, they suggest the existence of 2 syndromes that have distinct developmental trajectories.

Background A novel approach is explored for improving causal inference in observational studies by comparing cohorts from high-income with low- or middle-income countries (LMIC), where confounding structures differ. This is applied to assessing causal effects of breastfeeding on child blood pressure (BP), body mass index (BMI) and intelligence quotient (IQ).Methods Standardized approaches for assessing the confounding structure of breastfeeding by socio-economic position were applied to the British Avon Longitudinal Study of Parents and Children (ALSPAC) (N ≃ 5000) and Brazilian Pelotas 1993 cohorts (N ≃ 1000). This was used to improve causal inference regarding associations of breastfeeding with child BP, BMI and IQ. Analyses were extended to include results from a meta-analysis of five LMICs (N ≃ 10 000) and compared with a randomized trial of breastfeeding promotion.Findings Although higher socio-economic position was strongly associated with breastfeeding in ALSPAC, there was little such patterning in Pelotas. In ALSPAC, breastfeeding was associated with lower BP, lower BMI and higher IQ, adjusted for confounders, but in the directions expected if due to socioeconomic patterning. In contrast, in Pelotas, breastfeeding was not strongly associated with BP or BMI but was associated with higher IQ. Differences in associations observed between ALSPAC and the LMIC meta-analysis were in line with those observed between ALSPAC and Pelotas, but with robust evidence of heterogeneity detected between ALSPAC and the LMIC meta-analysis associations. Trial data supported the conclusions inferred by the cross-cohort comparisons, which provided evidence for causal effects on IQ but not for BP or BMI.Conclusion While reported associations of breastfeeding with child BP and BMI are likely to reflect residual confounding, breastfeeding may have causal effects on IQ. Comparing associations between populations with differing confounding structures can be used to improve causal inference in observational studies.

Objective To examine the effects of early social, anthropometric, and behavioural variables on physical activity in adolescence.

WHAT'S KNOWN ON THIS SUBJECT:Maternal prenatal smoking is strongly associated with various child psychological problems. It is not clear if this association reflects biological intrauterine effects of smoking or confounding by broader social, environmental, and genetic factors. WHAT THIS STUDY ADDS:Using multiple approaches for exploring causality we found evidence that whilst other psychological problems are unlikely to be causally related to maternal smoking, there is evidence that maternal smoking is causally related via intrauterine effects to child conduct/externalizing problems. abstract + OBJECTIVE: To explore associations of maternal prenatal smoking and child psychological problems and determine the role of causal intrauterine mechanisms. PATIENTS AND METHODS: Maternal smoking and child psychological problems were explored in 2 birth cohorts in Pelotas, Brazil (n ϭ 509, random subsample), and the Avon Longitudinal Study of Parents and Children (ALSPAC) in Britain (n ϭ 6735). Four approaches for exploring causal mechanisms were applied: (1) crosspopulation comparisons between a high-income and a middleincome country; (2) multiple adjustment for socioeconomic and parental psychological factors; (3) maternal-paternal comparisons as a test of putative intrauterine effects; and (4) searching for specific effects on different behavioral subscales. RESULTS: Socioeconomic patterning of maternal prenatal smoking was stronger in the ALSPAC compared with the Pelotas cohort. Despite this difference in a key confounder, consistency in observed associations was found between these cohorts. In both cohorts, unadjusted maternal smoking was associated with greater offspring hyperactivity, conduct/externalizing problems, and peer problems but not with emotional/internalizing problems. After adjusting for confounders and paternal prenatal smoking, only the association with conduct/externalizing problems persisted in both cohorts (conduct problems in the ALSPAC cohort, odds ratio [OR]: 1.24 [95% confidence interval (CI): 1.07-1.46], P ϭ .005; externalizing problems in the Pelotas cohort, OR: 1.82 [95% CI: 1.19 -2.78], P ϭ .005; ORs reflect ordinal odds ratios of maternal smokers having offspring with higher scores). Maternal smoking associations were stronger than paternal smoking associations, although statistical evidence that these associations differed was weak in 1 cohort. CONCLUSION: Evidence from 4 approaches suggests a possible intrauterine effect of maternal smoking on offspring conduct/ externalizing problems. Pediatrics 2010;126:e57-e65 AUTHORS: