Final answer:
Joan's behavior is best described by the psychological concept of Cognitive dissonance.
Step-by-step explanation:
Joan's behavior can be described by Cognitive dissonance, which is the psychological concept that best fits her actions. Cognitive dissonance occurs when there is a conflict between beliefs, attitudes, or behaviors, causing discomfort. In this case, Joan found out she was pregnant and had to stop drinking caffeine, despite experiencing tiredness and headaches. This creates a conflict between her desire for caffeine and her belief that it is harmful to her pregnancy.