Final answer:
The marshmallow study conducted by Walter Mischel measured the self-regulation or delay of gratification in children, which correlated with later success in life including higher SAT scores and more stable relationships.
Step-by-step explanation:
The quality measured in the study using marshmallows that was so telling is self-regulation or delay of gratification. Psychologist Walter Mischel designed a study to assess this quality in young children by offering them a choice between one immediate marshmallow or two marshmallows if they waited until the researcher returned. The follow-up studies on these children revealed that those who exhibited greater self-control by waiting for the larger reward were generally more successful later in life, as evidenced by higher SAT scores, better peer relationships, less likely to have substance abuse issues, and more stable marriages.