Last year, I spoke to a group of parents of high school students in Connecticut about college admissions and a parent asked me afterward why it’s so hard to get accepted to college these days. I said it’s only difficult to get into some colleges.

“Yes, selective colleges,” she corrected herself, but then added, “what makes a college selective?”

It’s a good question that most students anxious to go to one probably can’t answer. As I mentioned in last week’s column, selectivity is largely defined by Barron’s in its publication, “Profiles of American Colleges.”

But our perception of selectivity has also changed drastically in the last three decades.

Recently, I uncovered a copy of the U.S. News & World Report college guide from 1988, the first year the magazine published annual rankings of colleges (before that it had published them every other year). The guide is a window into a much different era in college admissions, one that many of today’s parents of high school students experienced as applicants. What is perhaps most revealing is a list of acceptance rates for top-ranked colleges.

Many of the numbers seem like typos when compared to today’s rates. Take Johns Hopkins University. In 1988, it accepted 54 percent of applicants; last year, its acceptance rate was 11 percent. Johns Hopkins is not alone in seeing its rate plummet over the last three decades. The University of Pennsylvania was 35 percent in 1988; last year, 9 percent. Washington University in St. Louis, 62 percent; last year, 16 percent.

Not all colleges have seen their selectivity drop so dramatically. Only about 10 percent of colleges are substantially more selective now compared to 50 years ago, according to research by Caroline M. Hoxby, an economics professor at Stanford University.

So why have acceptance rates fallen precipitously at dozens of colleges?

Simply put, they are getting more applicants than ever before. Even though more students are applying, these top-ranked schools haven’t substantially increased the size of their incoming classes. The top 20 national universities in the U.S. News & World Report rankings, for instance, enroll only about 100,000 students out of 17 million undergraduates nationwide. So the denominator is rising as the numerator stays the same in an equation that is more like the odds of playing the lottery for most students and parents.

For these top-ranked schools, the admissions business has evolved from a local and regional industry to a national one in which colleges can attract applicants from a wider geographic reach. That has led to what Hoxby calls a “re-sorting” of college students where “their choices are driven far less by distance and far more by a college’s resources and student body.”

These colleges have also encouraged more applicants with outreach to high school students and mailings. Colleges buy more than 80 million names of test takers from the College Board annually. In some cases, schools simply encouraged more students to apply as a way to improve their standings in the rankings: More applicants help drive down acceptance rates, a key metric in the methodology of many rankings.

Technology has also played a role as applying to college has become ever easier. Colleges now receive, on average, 94 percent of their applications online. More than 800,000 students used the Common Application last year to submit some 3.5 million applications to more than 700 colleges. Plenty of students today apply to colleges they have hardly any intention of attending.

Proposals to improve the efficiency and fairness of the admissions system proliferate. One common refrain promotes a matching system, like the National Resident Matching Program, that uses a computerized algorithm to pair the preferences of medical school graduates and residency programs as closely as possible, based on rank-order lists each side submits. Establishing such a system for more than 3 million high school seniors is surely a daunting, if not impossible, task.

Even so, some colleges are planning the building blocks for such a tool. Nearly 60 schools experimented last year with a new shared application that aims to compete with the popular Common Application. One feature of the tool, designed by the Coalition for Access, Affordability and Success, is a virtual “locker.” It allows students as early as ninth grade to upload their written work, videos, photos and other materials that show off their potential beyond a transcript. At first, the locker is accessed only by students, but over time they can open it to their parents, counselors and, most important, to colleges through the admissions process.

Such a tool imagines a future where instead of waiting for applications to arrive each year, colleges could conduct searches of data that students and parents choose to make available.

The current admissions system was designed for a different era when fewer students were going to college and it was more difficult to apply. Given the ease of applying to college these days and the incentives offered by the rankings, some colleges have been able to game the system through a combination of strategic marketing and public relations to look more selective.

Admissions has long been the lever college leaders pulled to improve prestige. In higher education, it’s difficult, if not impossible, to stray far from the pack and think differently about how to recruit classes. While most colleges accept a majority of a students who apply, the schools that get outsized attention are selective colleges, which have built an admissions system that is becoming increasingly impossible to navigate for a generation of anxious families.