<p>Georgetown & Princeton require pictures.</p>
<p>Partially, it's about economics: the wealthy kids can afford the dental surgery and the plastic surgery. They can also afford the nice clothes.</p>
<p>There's somewhat of a campus environment issue: the more athletic schools (like Williams) place a lower emphasis on outward signs of wealth, of which beauty is one. I imagine that, generally, more Greek life would equate to more pressure to look good.</p>