Final answer:
The therapist is likely using the facial-feedback hypothesis, which suggests that facial expressions can affect emotional experiences, and smiling specifically can enhance happiness.
Step-by-step explanation:
The therapist's advice for the client to smile may be based on the facial-feedback hypothesis. According to this hypothesis, facial expressions can influence emotional experiences. Research has supported this idea, showing that facial expressions can affect both emotional and physiological reactions. For instance, smiling can enhance the feeling of happiness, while frowning can lead to fewer positive emotions.
In the context of different theories, the therapist's suggestion does not align with the James-Lange theory of emotion, which posits emotions arise from physiological arousal. It also does not align with the cognitive-mediational approach of Lazarus or Schachter's two-factor theory of emotion, which incorporate cognitive processes or labels in the experience of emotions. Rather, the facial-feedback hypothesis purely focuses on the physical act of smiling influencing emotional states.