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Psychology advances knowledge by testing statistical hypotheses using empirical observations and data. The expectation is that most statistically significant findings can be replicated in new data and in new laboratories, but in practice many findings have replicated less often than expected, leading to claims of a replication crisis. We review recent methodological literature on questionable research practices, meta-analysis, and power analysis to explain the apparently high rates of failure to replicate. Psychologists can improve research practices to advance knowledge in ways that improve replicability. We recommend that researchers adopt open science conventions of preregi-stration and full disclosure and that replication efforts be based on multiple studies rather than on a single replication attempt. We call for more sophisticated power analyses, careful consideration of the various influences on effect sizes, and more complete disclosure of nonsignificant as well as statistically significant findings.

Previous studies have documented that smoking during pregnancy (SDP) is associated with offspring externalizing problems, even when measured covariates were used to control for possible confounds. However, the association may be due to non-measured environmental and genetic factors that increase risk for offspring externalizing problems. The current project used the National Longitudinal Survey of Youth and their children, ages 4-10 years, to explore the relations between SDP and offspring conduct problems (CP), oppositional-defiant problems (ODP), and attention deficit hyperactivity problems (ADHP) using methodological and statistical controls for confounds. When offspring were compared to their own siblings who differed in their exposure to prenatal nicotine, there was no effect of SDP on offspring CP and ODP. This suggests that SDP does not have a causal effect on offspring CP and ODP. There was a small association between SDP and ADHP, consistent with a causal effect of SDP, but the magnitude of the association was greatly reduced by methodological and statistical controls. Genetically informed analyses suggest that unmeasured environmental variables influencing both SDP and offspring externalizing behaviors account for the previously observed associations. That is, the current analyses imply that important unidentified environmental factors account for the association between SDP and offspring externalizing problems, not teratogenic effects of SDP. Keywordssmoking during pregnancy; conduct problems; externalizing problems; ADHD; children of twins; causation Smoking during pregnancy (SDP) has been consistently linked with externalizing problems in offspring, particularly in males (reviews in Cnattingius, 2004;Huizink & Mulder, 2005;Wakschlag & Hans, 2002;Wakschlag, Pickett, Cook, Benowitz, & Leventhal, 2002). SDP has been associated with parent-reported conduct problems (Ernst, 2001), arrest history from national crime registries (Brennan, Grekin, & Mednick, 1999;Rasanen et al., 1999), contact 2 All correspondence should be sent to: Brian D'Onofrio, PhD, Department of Psychological and Brain Sciences, Indiana University, 1101 E. 10th St., Bloomington, IN 47405, bmdonofr@indiana.edu. NIH Public Access Author ManuscriptDev Psychopathol. Author manuscript; available in PMC 2013 August 08. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscriptwith police obtained by city police records (Gibson, Piquero, & Tibbets, 2000), Oppositional Defiant Disorder (Wakschlag & Keenan, 2001), Conduct Disorder (Fergusson, Woodward, & Horwood, 1998;Wakschlag & Hans, 2002;Wakschlag & Keenan, 2001;Wakschlag et al., 1997;Weissman, Warner, Wickramaratne, & Kandel, 1999) and Attention-Deficit/ Hyperactivity Disorder (Mick, Biederman, Faraone, Sayer, & Kleinman, 2002;Rodriguez & Bohlin, 2005).Reviews of the literature note that the association is consistent with a causal connection because the association is specific to externalizing problems, has been found across diverse samples and measures, demonstr...

Hundreds of research articles have addressed the relationship between birth order and intelligence. Virtually all have used cross-sectional data, which are fundamentally flawed in the assessment of within-family (including birth order) processes. Although within-family models have been based on patterns in cross-sectional data, a number of equally plausible between-family explanations also exist. Within-family (preferably intact-family) data are prerequisite for separating within- and between-family causal processes. This observation reframes an old issue in a way that can be easily addressed by studying graphical patterns. Sibling data from the National Longitudinal Survey of Youth are evaluated, and the results are compared with those from other studies using within-family data. It appears that although low-IQ parents have been making large families, large families do not make low-IQ children in modern U.S. society. The apparent relation between birth order and intelligence has been a methodological illusion.