Neuroticism’s Importance in Understanding the Daily Life Correlates of Heart Rate Variability Scott Ode and Clayton J. Hilmert North Dakota State University Desiree J. Zielke Indiana University–Purdue University Indianapolis Michael D. Robinson North Dakota State University Individual differences in high-frequency heart rate variability (HRV) have been conceptualized in terms of a greater capacity to self-regulate problematic outcomes, but have also been conceptualized in terms of greater moment-to-moment flexibility. From a self-regulation perspective, higher HRV should be inversely correlated with trait neuroticism and problematic daily outcomes. From a flexibility perspec- tive, high HRV should result in more state-like functioning—that is, functioning that is more contextual and less trait-like in nature. In the latter case, HRV and trait neuroticism may interact to predict problematic outcomes such that neuroticism should be a less consequential predictor at higher levels of HRV. The flexibility perspective was systematically supported in a daily experience-sampling protocol. Implications focus on theories of neuroticism and HRV. Keywords: neuroticism, heart rate variability, stress, negative affect, self-regulation The distinction between flexible responding and self-regulated responding is a subtle but important one in relation to constructs such as perseveration, inhibition, and switching. A useful way of disentangling flexibility and self-regulation processes involves ex- amining their relation to the broadly dysfunctional trait of neurot- icism in combination with problematic daily outcomes known to be correlated with neuroticism. To the extent that self-regulation processes are involved, the measure should be inversely related to neuroticism or the daily outcomes. On the other hand, to the extent that flexibility processes are involved, neuroticism and the mea- sure should interact to predict the outcomes. In particular, flexi- bility should reduce or eliminate neuroticism– outcome relation- ships (Robinson & Compton, 2007; Robinson, Goetz, Wilkowski, & Hoffman, 2006; Robinson, Wilkowski, & Meier, 2006). We use this assessment strategy to differentiate two perspectives on heart rate variability (HRV). HRV: Self-Regulation or Flexibility? Changes in heart rate have long been viewed as important in understanding reactions to the environment (Obrist, 1981; Selye, 1956; Vila et al., 2007). However, heart rate reflects a heteroge- neous mix of sympathetic and parasympathetic nervous system activity, and it is important to disentangle their separate influences (Berntson & Cacioppo, 2007; Berntson, Norman, Hawkley, & Cacioppo, 2008). Whereas activity in the sympathetic branch increases heart rate and does so relatively slowly, activity in the parasympathetic branch decreases heart rate and does so quickly (Shepherd & Vatner, 1996). The latter fast-acting parasympathetic influence can be isolated by assessing individual differences in high frequency (0.15– 0.40 Hz) HRV (Friedman, 2007; Porges, 2007). There is now a reasonably sized literature showing that individ- ual differences in HRV have predictive value in understanding outcomes important to the personality, social, clinical, and emotion literatures (Appelhans & Luecken, 2006). Yet, there appears to be a subtle, often implicit, disagreement as to how best to understand the findings reported to date. On the one hand, higher levels of HRV have been conceptualized in terms of individual differences in self-regulation abilities or capacities (e.g., Thayer & Lane, 2000). From this perspective, higher levels of HRV should almost uniformly be beneficial in mitigating problematic processing rou- tines or outcomes. On the other hand, higher levels of HRV have been characterized in terms of individual differences in flexible responding (e.g., Beauchaine, Gatzke-Kopp, & Mead, 2007). From this perspective, higher levels of HRV should not be viewed in terms of self-regulation abilities or capacities, but rather in terms of a greater range of responding to situational input. We review the HRV literature according to this distinction among perspectives. Self-regulation processes are those involved in the controlled override of problematic mental tendencies, broadly conceptualized (Rothbart, Ellis, & Posner, 2004). It is thus informative that Hansen, Johnsen, and Thayer (2003) reported some evidence for the idea that individuals high in HRV perform better in a working memory task, a classic measure of controlled processing abilities (Engle, 2002). This result may be somewhat ambiguous, however, because cognitive flexibility is a major contributor to performance Scott Ode, Clayton J. Hilmert, and Michael D. Robinson, Department of Psychology, North Dakota State University; and Desiree J. Zielke, Depart- ment of Psychology, Indiana University–Purdue University Indianapolis. Scott Ode acknowledges support from a North Dakota State University Graduate Research Fellowship. Correspondence concerning this article should be addressed to Scott Ode, Psychology, NDSU Department 2765, PO Box 6050, Fargo, ND 58108-6050. E-mail: Scott.Ode@ndsu.edu Emotion © 2010 American Psychological Association 2010, Vol. 10, No. 4, 536 –543 1528-3542/10/$12.00 DOI: 10.1037/a0018698 536