Dr. prentice, principal of an elementary school, instructs her teachers to employ a new aggression-reduction program with the second graders and to keep using it on that group of students for the next three years. she finds that, relative to the start of the program, aggression in that group of students is down 35%. this example illustrates the possibility of as a plausible rival hypothesis.

The answer is maturity.

Explanation:

In the example, the age difference might be the cause for the changes in attitude, as it was performed over the course of three years (and thus it's difficult to test the validity of the program itself).

In fact, some studies have shown that less psychologically mature teenagers tend to present more aggresive behaviours. However, not much further research has been done.

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