Sibling differences in low birth weight, dopaminergic polymorphisms, and ADHD symptomatology: Evidence of GxE Dylan B. Jackson a,n , Kevin M. Beaver b,c a University of Texas at San Antonio, USA b Florida State University, USA c King Abdulaziz University, Saudi Arabia article info Article history: Received 25 September 2014 Received in revised form 28 January 2015 Accepted 30 January 2015 Keywords: Attention deficits Hyperactivity Perinatal risk DAT1 DRD2 DRD4 Moderating effects abstract Low birth weight has been found to increase the risk of attention-deficit hyperactivity disorder (ADHD) in children. However, few studies adequately control for shared environmental influences (e.g., concentrated disadvantage, family structure) or examine whether interactions between birth weight and genetic factors predict ADHD. The present study addresses these limitations in prior research by examining a) whether sibling differences in low birth weight status are significantly predictive of sibling differences in behaviors symptomatic of ADHD and b) whether sibling differences in dopaminergic genes interact with sibling differences in low birth weight status to predict sibling differences in ADHD symptomatology. The results suggest that low birth weight siblings are at significantly greater risk of exhibiting symptoms of ADHD during childhood relative to their normal birth weight siblings. Moreover, possessing greater genetic risk on three dopaminergic genes (DAT1, DRD2, and DRD4) relative to a sibling appears to exacerbate the link between sibling differences in birth weight and sibling differences in ADHD symptomatology. Limitations and directions for future research are discussed. Published by Elsevier Ireland Ltd. 1. Introduction A growing body of research indicates that low birth weight children are at-risk for a variety of maladaptive outcomes as they develop. Physical, neurological, psychological and behavioral dimen- sions of development all appear to be influenced by a child's weight at the time of their birth (Gray et al., 2004; Indredavik et al., 2005; Hack, 2006; Marshall et al., 1999; Tibbetts and Piquero, 1999). To illustrate, low birth weight individuals have a higher risk of serious physical ailments, including chronic lung disease (Marshall et al., 1999) and high blood pressure (Hack, 2006), as well as serious psychological ailments, such as anxiety disorders (Indredavik et al., 2005). Research has even suggested that being born low birth weight can increase the likelihood behavioral problems during childhood (Gray et al., 2004) as well as an early-onset of offending (Tibbetts and Piquero, 1999). A particularly important behavioral correlate of low birth weight is attention-deficit hyperactivity disorder (ADHD) (see Bhutta et al., 2002). A large number of studies have revealed that individuals who are born low birth weight are at an increased risk of exhibiting attention deficits (Botting et al., 1997; Hack et al., 2009) and hyperactivity problems (Lahti et al., 2006; Mick et al., 2002) by middle to late childhood. Furthermore, birth weight has been found to significantly influence whether children meet the criteria for an ADHD diagnosis, above and beyond increasing their severity of ADHD symptoms (Hack et al., 2009; Shum et al., 2008). A handful of studies have also examined low birth weight differences in sibling pairs and whether such differences lead to distinct developmental outcomes. To date, these studies have found significant learning and behavioral deficits in the low birth weight sibling relative to the normal birth weight sibling (Fletcher, 2011; Hultman et al., 2007). For instance, a study by Hultman and colleagues (2007) suggested that Swedish twins who are divergent on low birth weight status tend to exhibit differing levels of ADHD symptomatology. Other studies intimate the potential for sibling birth weight differences to predict correlates of ADHD, such as learning difficulties (Fletcher, 2011). A number of scholars have also explored the conditions under which the influence of low birth weight on ADHD symptomatol- ogy might be the strongest (Laucht et al., 2001; Tully et al., 2004; Shah et al., 2013). To date, several potentially important modera- tors of the effect of low birth weight on ADHD-related traits and behaviors have been examined, including the biological sex of the child (Martel et al., 2007), socioeconomic status (Kelly et al., 2001), maternal responsivity (Laucht et al., 2001), and parental warmth or negativity (Tully et al., 2004; Shah et al., 2013). Most of these studies have suggested that the effect of low birth weight on ADHD is conditional in nature (see Kelly et al., 2001; Laucht et al., Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/psychres Psychiatry Research http://dx.doi.org/10.1016/j.psychres.2015.01.025 0165-1781/Published by Elsevier Ireland Ltd. n Corresponding author. Tel.: þ1 626 825 4856. E-mail address: DylanBakerJackson@gmail.com (D.B. Jackson). Please cite this article as: Jackson, D.B., Beaver, K.M., Sibling differences in low birth weight, dopaminergic polymorphisms, and ADHD symptomatology: Evidence of GxE. 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