Psychological Science XX(X) 1 –6 © The Author(s) 2012 Reprints and permission:

A core concern in psychology has been to identify the source of stable individual differences in personality and behavior (Penke, Denissen, & Miller, 2007). Impulsivity is a trait that has been shown to be influenced by both environmental and genetic factors (Bezdjian, Baker, & Tuvblad, 2011). One of the more robust findings in behavior genetics research is the asso-ciation of a 48-base-pair variable number of tandem repeats (VNTR) in exon III of the dopamine D4 receptor (DRD4) gene with attention-deficit/hyperactivity disorder (ADHD; Gizer, Ficks, & Waldman, 2009; Tharpar, Langley, Owen, & O’Donovan, 2007). The DRD4 polymorphism has also been associated with continuous measures of impulsivity-related traits in some studies but not in others (Bidwell et al., 2011; Munafò, Yalcin, Willis-Owen, & Flint, 2008). Part of this inconsistency may be explained by variation in the devel