In which of the following conditions is flat affect most commonly observed?

Flat affect is most commonly associated with depressive disorders because it reflects a significant reduction in emotional expressiveness. Individuals with depression often experience a lack of emotional responsiveness; they may seem emotionally blunted or unable to convey feelings through facial expressions, voice tone, or gestures. This condition can be a result of the pervasive sadness and emotional drain that characterize depressive states, making it challenging for individuals to display typical emotional responses.

In contrast, manic episodes are usually marked by heightened emotional expressiveness, including euphoria, increased energy, and a range of intense emotions. Anxiety disorders often involve heightened emotions such as fear or worry, rather than a lack of emotion. Similarly, adjustment disorders tend to present with emotional responses related to stressful life events, which typically do not include the flat affect seen in depression. Therefore, among the given conditions, depressive disorders are the most strongly associated with flat affect.