This is my understanding. Base on the history of how many have declined versus enrolled in the past, a school will know about how many to accept outright. For example, Stanford might accept 170 to fill a class of 86. If more than usual decline and they can't enroll 86 out of that initial group of 170, then they go to their wait list. The rumor is that Columbia only accepts the exact number to fill their class at first, and puts the rest on the wait list. As a result, they end up taking nearly half of their class off of the list. In other words, it can vary from one school to another, and from one year to another. If you ask Gower on medicalgold, he might be able to tell you more specifics about your school of interest. There are also some previously cited sites (say that three times fast)that tell how many people certain schools take after May (which is the biggest wait list movement month). Another example--in 2000, Hopkins took over 50 after May, but Stanford only 10 or 12 (These are from memory and may not be precise).