Various investigators have proposed that people may feel simultaneous positive and negative affect. However, experimental evidence from tests of a recent theory about the intensity of emotion (J. W. Brehm, 1999) suggests that even when they are invited by the experimental design, positive and negative emotions do not occur at the same time. When people have been instigated to feel a particular emotion, such as happiness, and then are given a reason (e.g., sad news) for not feeling happy, they report continued happiness but no increase in sadness unless the reason for feeling sad is very great, in which case sadness replaces happiness. The present paper briefly reviews the underlying theory and evidence, and discusses implications.