Washington, D.C. ? This spring, some colleges have assembled waiting lists that rival the size of their incoming freshman classes, a measure of their uncertainty at a volatile time in higher education.

The swelling of wait lists in the past two years reflects the lingering economic downturn and an increasingly cautious approach by admissions offices. The recession has made it more difficult for admissions officers to discern which admitted students are likely to attend and has sapped endowments, leaving colleges less inclined to risk tuition dollars by failing to fill their freshman classes. Competitive colleges are processing record numbers of applications, further complicating the task of predicting who will enroll.

Colleges create the wait list as a sort of reserve fund, available for use if the school comes up short of students at the end of the regular admission cycle. No college wants to end up under-enrolled.

Nationwide, roughly one college in three employs a wait list. Its use is more common among the most selective colleges, according to a national survey by the National Association for College Admission Counseling. Roughly 30 percent of wait-listed students ultimately are admitted, although the percentage is much smaller at top colleges.

For example, the University of Virginia has offered admission to 6,900 students and wait-listed 3,750, a group large enough to fill the 3,240 spots for the Class of 2014. Most of the other top national universities in area, including Georgetown, Johns Hopkins, the University of Maryland and Virginia Tech, are tending deep wait lists of their own.

“Last year, wait lists at most places were much more active than normal, because people had no idea what was going to happen,” said Charles Deacon, dean of admissions at Georgetown. This year, colleges “are mostly not in any better shape than they were last year,” he said. “But they’ve had more time to prepare.”

Colleges wait-list many more students than they plan to enroll, knowing that a good share will tire of the wait and commit to another school. Still, academic officials say that some wait lists are needlessly long.

“Sometimes, frankly, it’s just hard to say no to so many great kids,” said Greg Roberts, dean of admission at U-Va.

“I’ll agree there’s no scenario where we’d exhaust the wait list and still not have the class we want,” said Henry Broaddus, dean of admission at William and Mary in Williamsburg, Va., where the is longer than last year’s by 142 students. “I think there’s an appropriate national conversation to have about ‘are these wait lists too big?’ “

May is wait-list month, when colleges tally deposits from students who have committed by the May 1 deadline and tabulate how many more students, if any, they will need to complete their freshman classes. By the start of June, most wait-listed students will have received a polite letter of rejection or, for a lucky few, a surprise telephone call offering admission.

Local deans say this year’s admission cycle, although tricky, was somewhat easier to predict than last year’s, a time of plummeting endowments and plunging stock prices. Colleges may empanel a wait list by a rough mathematical formula — say, one wait-listed student for every two admits. They don’t usually have a set number in mind.

Admission officers counsel the wait-listed to consider the long odds and mull options, even as they encourage them to submit additional grade reports and letters of recommendation, just in case.

“There’s no intent to peddle false hope here,” Broaddus said.

Making the phone call to a student admitted from the wait list is among the most pleasant duties assigned to admission officers, somewhat akin to contacting lottery winners.

“The last people in are the most grateful,” Deacon said. “They end up being the best alums.”