After some thought on Larry Summers' hypothesis, there seems to be one major assumption that he's making that needn't be true - that the major culprit for high

After some thought on Larry Summers' hypothesis, there seems to be one major assumption that he's making that needn't be true - that the major culprit for high engineering and academic positions is aptitude. If this is the case, then he may well be correct, as it makes a lot of sense. But it is still a major assumption.