Final answer:
Regression is the ego defense mechanism where an individual reverts to a childlike behavior in response to stress.
Step-by-step explanation:
The ego defense mechanism described where an individual retreats to a more secure, earlier stage of life when faced with an unpleasant circumstance is known as regression. This is typically a subconscious process where the person attempts to escape the current stressors by exhibiting behaviors characteristic of a younger stage of psychological development. Regression is part of a broader spectrum of defense mechanisms that the ego uses to protect itself from anxiety. For example, a college student under immense stress during finals week might start sucking their thumb, a behavior they had not shown since early childhood.