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<p>Physics and math are well-known at MIT for discouraging inbreeding.</p>
<p>Mollie’s right that generally the science departments discourage it more than the eng departments. Brain & cog sci is a notable exception to this. It preferentially takes its own.</p>
<p>This illustrates the danger of using acceptance rates to determine how hard it is to get into anything - aero/astro has an acceptance rate of 45% at the grad level, but that’s because it’s very self-selected (a lot of the grads were MIT undergrads). If you had a representative population of aero/astro students across the country applying, they wouldn’t get in at a rate of 45%.</p>