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For the same reason that schools like Wabash College, Hampden-Sydney College, and Morehouse College don’t admit women. </p>
<p>It was historically quite normal for private colleges and universities to be single-sex; this included top schools like Princeton, Dartmouth, and Yale (which were male only). Most male-only schools went coed in the 1960s or 1970s, and so did many female-only schools (like Vassar, Skidmore, or Connecticut College). </p>
<p>But some schools have remained single-sex. The female-only schools are the best known, but there are still some male-only schools as well.</p>
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<p>Incidentally, private schools can also discriminate on the basis of religion. Some private colleges and universities are associated with particular religious denominations, and won’t accept applicants who don’t share their particular religious beliefs.</p>