Wait For It! A Twin Study of Inhibitory Control in Early ChildhoodAbstractIntroductionMethodParticipantsProcedureObserver ratings of inhibitory controlParent ratings of inhibitory controlDescriptive statistics and phenotypic correlationsTwin correlationsUnivariate model-fitting analysesMultivariate model-fitting analysesResultsDescriptive statisticsGenetic and environmental influences on individual differences in inhibitory controlTo what extent do genetic influences on parent-rated IC overlap with genetic influences on observer-rated inhibitory control?DiscussionAcknowledgmentsReferencesORIGINAL RESEARCHWait For It! A Twin Study of Inhibitory Control in EarlyChildhoodJeffrey R. Gagne•Kimberly J. SaudinoReceived: 3 June 2009 / Accepted: 7 November 2009 / Published online: 21 November 2009Ó Springer Science+Business Media, LLC 2009Abstract Inhibitory control (IC) is a dimension of childtemperament that emerges in toddlerhood and involves theability to regulate behavior in response to instructions orexpectations. In general, children with low levels of IChave more cognitive and social difficulties, and higherlevels of problem behaviors. Unfortunately, there is apaucity of research on the heritability of this importantbehavioral dimension. The present study used a twin designto examine the extent to which genetic and environmentalfactors contribute to individual differences in IC. Labora-tory and parent assessments of IC were conducted on 294same-sex twin pairs (133 MZ, 161 DZ) at 24 months ofage. Model-fitting analyses showed that genetic factorsaccounted for 38 and 58% of the variance in laboratory-and parent-rated IC, respectively. Multivariate geneticanalyses also revealed that the covariance betweenobserved and parent-assessed IC could be predominantlyexplained by common genetic influences.Keywords Temperament  Inhibitory control Childhood  Twins  Laboratory ratingsIntroductionInhibitory control (IC) is a dimension of temperament thatdevelops over early childhood and involves the regulationof behavior, typically in response to instructions orexpectations. Individuals who develop sufficient IC areable to inhibit behavior when it is necessary. Deficient orunderdeveloped IC often results in impulsive behavior. Ingeneral, children with typically developing IC have fewercognitive difficulties, more stable temperaments, and lessbehavioral maladjustment (Eisenberg et al. 2001, 2004;Hughes et al. 2000; Schachar et al. 1995; Wolfe and Bell2003).Child temperament is defined as individual differencesin both reactivity and self-regulation, is moderately stable,and collectively forms the basis of later personality (Der-ryberry and Rothbart 1997; Goldsmith et al. 1987; Rothbartand Ahadi 1994; Rothbart and Bates 2006). In the Rothbartmodel of temperament, IC is the latest developing com-ponent of the broad effortful control (EC) factor. ECemerges in infancy and the second year, is considered self-regulatory, and involves the efficiency of executive atten-tion, including the ability to inhibit a dominant response(IC) and the ability to activate responses (Derryberry andRothbart 1997; Rothbart 1989a, b; Rothbart and Ahadi1994; Rothbart and Bates 2006). IC becomes distinct as adimension of temperament around the age of 2 years, andcontinues to develop in the toddler and preschool years(Kochanska et al. 1996; Rothbart 1989a). It is important tonote that although IC and EC are intended as overlappingyet somewhat distinct constructs (Rothbart and Bates 2006)the terms are sometimes used interchangeably to describethe same behaviors.IC is positively associated with executive functioningskills such as working memory, planning and attentionalflexibility (Chiappe et al. 2000; Conway et al. 2000;Pallodino et al. 2003; Passolunghi and Siegel 2001; Wolfeand Bell 2003), as well as theory of mind (Frye et al. 1995;Perner et al. 1987; Perner et al.1999; Russell et al. 1991).Edited by Deborah Finkel.J. R. Gagne (&)Department of Psychology, University of Wisconsin-Madison,1202 West Johnson Street, Madison, WI 53706-1611, USAe-mail: [email protected]. J. SaudinoBoston University, Boston, MA, USA123Behav Genet (2010) 40:327–337DOI 10.1007/s10519-009-9316-6Children who develop appropriate levels of IC also haveincreased levels of social competence in middle childhood(Lengua 2003; Nigg et al. 1999; Olson et al. 1999).Although these social and cognitive outcomes are consis-tently linked to IC, the importance of investigating theetiology of IC is largely attributed to relations with childbehavior problems and psychopathology, and the potentialrole of IC as an endophenotype for relevant childhooddisorders.The behavioral disorder that is most strongly associatedwith IC in childhood is Attention Deficit HyperactivityDisorder (ADHD; Pliszka et al. 1997; Schachar and Logan1990; Schachar et al. 1995). ADHD-diagnosed childrentypically have poorer IC as compared to children withdiagnoses of anxiety disorders and oppositional defiantdisorder (ODD), who in contrast to ADHD children do nottypically show deficits in IC (Oosterlaan et al. 1998;Oosterlaan and Sergeant 1996; Schachar et al. 2000).However, lower levels of IC may lead to an increased riskfor other forms of psychopathology, including disordersco-morbid with an ADHD diagnosis in childhood. In fact,in at least one meta-analysis, children with conduct disor-der (CD) display similar levels of IC deficits as those withADHD, or those co-morbid for ADHD and CD (Oosterlaanet al. 1998). In addition, similar decrements in IC wereobserved in children with co-morbid ADHD and ODD inone other study (Scheres et al. 2001).Although a large body of research supports the presenceof an inhibition deficit in ADHD, several more recentinvestigations suggest that inhibitory deficits may not behomogenous. A meta-analysis of inhibitory ability andADHD indicates that the Stroop Color-Word Task does notsupport an ‘‘interference control’’ deficit in 6–27 year-oldswith ADHD (Van Mourik et al. 2005). The authors positthat inhibition deficits in ADHD are largely the result ofinappropriate methods used to calculate interference scoreswith the Stroop task. Another review supports the theorythat impaired executive functioning in ADHD is nothomogenous, but rather, involves multiple cognitive pro-files that include poor IC as indexed by both standardresponse inhibition and delay aversion tasks (Castellanoset al. 2006). Performance on these two different tasks inADHD children