<p>I haven’t seen any assertion that “picking two majors will boost grades”. Can you explain where you have seen this assertion?</p>
<p>There is clearly a self-selection going on for double majors, but your conclusion that this invalidates comparisons of the two groups has not been supported. For example, you postulate that the double major group is more “gifted” than the single-major group, yet if you would bother to examine the evidence provided you will see that the SAT/GRE scores of the two groups are not significantly different.</p>
<p>The one thing that is so far completely unsupported in this discussion is the notion that a double major inherently reduces GPA.</p>