Question

Seven-year-old marvin and his four-year-old brother are sitting at the table as their mother pours milk into their glasses. both glasses are the same size and shape. however, marvin's mother notices a scratch/nick on the lip of his glass and decides that it is safer if she pours that milk into another glass so marvin doesn't cut his lip. the new glass is shorter and much wider, and when she pours the milk into in, marvin's younger brother laughs and says that now he has more milk than marvin. however, marvin is not upset because he knows that he still gets the same amount of milk. marvin is exhibiting

Answer 1

Based on the description, marvin is exhibiting :Decentering

Decentering is a psychological distress that happen to someone after being replaced from the center of attention.
Before Marvin's brother is born, Marvin received the full attention from his parents. But now after his brother is born, most of the attention is given to his brother, which is the most likely reason why Marvin is upset