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Yoked pairs of subjects solved puzzles such that one member of each pair was given choice about what puzzles to work on and how much time to allot to each, while the yoked subject was assigned the same puzzles and time allotments as those chosen by the first subject. It was predicted and found that subjects who chose the activities and time allotments -in other words, who had additional self-determination--would be more intrinsically motivated than subjects doing the same activity without choice.
Zuckerman et al. (Sat,) studied this question.
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