Final answer:
Marsha's little sister is exhibiting the foot-in-the-door technique by agreeing to a small favor (getting water) and being more likely to agree to a larger one (getting a snack) due to the desire for consistency in one's actions.
Step-by-step explanation:
Marsha's little sister's willingness to comply with her sister's request is known as the foot-in-the-door technique. This persuasive strategy involves acquiring a person's agreement to a small request, which increases the likelihood that they will agree to a subsequent larger request. The strategy is grounded in the principle of consistency, where our past actions influence our future ones, and we prefer to act consistently with our previous behavior.
In the case of Marsha and her sister, by complying with the small task of getting a glass of water, the sister has shown her willingness to help. When Marsha asks for a snack next, her sister is more likely to comply because she has already committed to the initial behavior of assisting Marsha. This persuasion technique demonstrates how small initial commitments can lead to larger ones, a concept useful in various aspects of life, from sibling dynamics to marketing strategies and social conformity.