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<p>Well, can’t be by junior year, since most of them will have grades too low to have a realistic chance at medical school. But perhaps it may be due to the fact the biology is the least math-heavy of the sciences, so students who like science but are not good at math end up in biology by default.</p>
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<p>Could be as costly in terms of labs, but cheaper in terms of faculty and staff, because of the poor other employment prospects for biology PhDs compared to engineering PhDs.</p>