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<p>You are failing to account for self-selection. Princeton has a disproportionate number of students from New Jersey, while Yale and Harvard have disproportionate numbers from Connecticut and Massachusetts, respectively. This is because people of those respective states tend to apply in disproportionate amounts and accept the offer (due to proximity) at also disproportionate percentages.</p>
<p>It’s just more noticeable with Stanford because you see “40%” and you think “wow! that’s almost half!”. And sure, 40% dwarfs the 12% of the US Population that California actually represents, but there are even more factors at work; California, for example, is more education oriented as a whole than some other states.</p>