149 7 Major Depressive Disorder Shauna L. Clen, Douglas S. Mennin, and David M. Fresco Major depressive disorder (MDD) is one of the most prevalent psy- chiatric disorders (Kessler, Chiu, Demler, Merikangas, & Walters, 2005) and foremost public health threats of the 21st century (Judd & Kuno- vac, 1997). Emotionality is central to the diagnosis of MDD (American Psychiatric Association, 2000), as the disorder is characterized by both high levels of negative emotions and low levels of positive emotions (see Morris, Bylsma, & Rottenberg, 2009, for a discussion). Despite the cen- tral role of emotionality in MDD and other forms of psychopathology, emotional functioning has historically been a neglected research area in psychology (Greenberg, 2002; Samoilov & Goldfried, 2000). However, in the past 10 years, emotion-based psychological research is growing as researchers turn toward emotion regulation research and affective neuroscience to gain a greater understanding of psychological disorders, including MDD (e.g., Davidson, Pizzagalli, Nitschke, & Putnam, 2002; Rottenberg, Gross, & Gotlib, 2005; Teasdale, 1999). Emotions are integral to human functioning as they promote behav- iors that ensure survival. Emotional responding is the result of the activa- tion of motivational systems that have been triggered in response to pun- ishment, reward, or the need for current action (i.e., fight-or-flight system; Carver & Scheier, 1998; Gray & McNaughton, 2000; Higgins, 1997). Emotional responses include physiological responses (e.g., heart rate), behavioral responses (e.g., facial expressions; moving to escape danger), From Distress Tolerance: Theory, Research, and Clinical Applications. Edited by Michael J. Zvolensky, Amit Bernstein, and Anka A. Vujanovic. Copyright 2011 by The Guilford Press. All rights reserved.