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ASK just about any high school senior or junior — or their parents — and they’ll tell you that getting into a selective college is harder than it used to be. They’re right about that. But the reasons for the newfound difficulty are not well understood.

Population growth plays a role, but the number of teenagers is not too much higher than it was 30 years ago, when the youngest baby boomers were still applying to college. And while many more Americans attend college than in the past, most of the growth has occurred at colleges with relatively few resources and high dropout rates, which bear little resemblance to the elites.

So what else is going on? One overlooked factor is that top colleges are admitting fewer American students than they did a generation ago. Colleges have globalized over that time, deliberately increasing the share of their student bodies that come from overseas and leaving fewer slots for applicants from the United States.

For American teenagers, it really is harder to get into Harvard — or Yale, Stanford, Brown, Boston College or many other elite colleges — than it was when today’s 40-year-olds or 50-year-olds were applying. The number of spots filled by American students at Harvard, after adjusting for the size of the teenage population nationwide, has dropped 27 percent since 1994. At Yale and Dartmouth, the decline has been 24 percent. At Carleton, it’s 22 percent. At Notre Dame and Princeton, it is 14 percent.

The frenzy over admissions at top colleges can seem nonstop: the last-minute flurry as accepted students decide by May 1 where to attend, the Supreme Court battles over affirmative action, the applications that some high school juniors have already begun writing. Yet the globalization of these colleges has been largely missing from the discussion.

This globalization obviously brings some big benefits. It has exposed American students to perspectives that our proudly parochial country often does not provide in childhood. “It would be a lesser education for them if they didn’t get a chance to interact with some international students,” asWilliam Fitzsimmons, the dean of admissions at Harvard since 1986, told me. The trend also fits with the long American tradition of luring some of the world’s most talented people here. Many international students who come for college never leave. Some of them found companies or make other contributions to society.

Yet the way in which American colleges have globalized comes with costs, too. For one thing, the rise in foreign students has complicated the colleges’ stated efforts to make their classes more economically diverse. Foreign students often receive scant financial aid and tend to be from well-off families. For another thing, the country’s most selective colleges have effectively shrunk as far as American students are concerned, during the same span that many students and their parents are spending more time obsessing over getting into one.

Many numbers for individual colleges here come from Noodle, a company that provides advice on education decisions. I combined the numbers with census data on the number of 18- to 21-year-olds in the United States to examine what share of college-age Americans in four different years — 1984, 1994, 2004 and 2012 — were attending various elite colleges.

The share for any individual college is minuscule, of course. In 2012, about 33 out of every 100,000 American 18- to 21-year-olds were attending Harvard, down from 45 per 100,000 in 1994. These changes in the share tell you how much harder, or easier, admission has become for American teenagers on average. Between 1984 and 1994, it became easier at many colleges. The college-age population in this country fell during that time to 14.1 million in 1994 from 16.5 million in 1984, and the number of foreign students was relatively stable.

I attended college in the early 1990s, and these numbers made me realize how easy the application process was for me and my peers, relative to almost any other time over the past half century. By the 2000s, the so-called echo boom in births had increased the number of college-age Americans. It reached 17.9 million in 2012. The number of foreign students was growing at the same time. They now constitute close to 10 percent of the student body at many selective colleges, nearly double the level of the early 1990s.

The result is those big declines in the number of available seats for any given American teenager. Only colleges that have rapidly expanded their student bodies, like Columbia and the University of Chicago, have avoided the pattern.

Obviously, the averages do not apply equally across the board. For students from the Northeast applying to elite colleges in the region, college admissions have probably become even more difficult in recent decades than these statistics suggest. Not only have colleges globalized, they have also become less regional, admitting more students from states like North Carolina, Texas and Washington.

To many individual students, the newfound difficulty probably doesn’t cause much harm (even if it does cause angst). Over the last 20 years, several large colleges, like N.Y.U. and the University of Southern California, have improved markedly, effectively increasing the number of seats on elite campuses, Noodle has noted.

And there is still scant evidence that the selectivity of the college one attends matters much. Students with similar SAT scores who attended colleges of different selectivity — say, Penn and Penn State — had statistically identical incomes in later years, according to research by the economists Stacy Dale and Alan Krueger.

THERE was one exception, though: low-income students, who did seem to benefit from attending an elite college. Maybe they benefited more from the social contacts they made there or were more likely to drop out if they did not attend a top college.

Either way, the research underscores a problem with the way colleges have globalized. With only a handful of exceptions (including Harvard, Amherst, M.I.T. and Yale), colleges have not tried hard to recruit an economically diverse group of foreign students. The students instead have become a revenue source.

Sarah Turner and Kelli Bird, University of Virginia economists, have found that the enrollment of undergraduate foreign students fluctuates with the economic growth and exchange rates of those students’ countries of origin. The pattern is much stronger among undergraduates than doctoral students — a sign that the undergraduates’ families are paying their way.

In recent years, college administrators have repeatedly claimed that enrolling a more economically diverse group of students is a top priority. But their actions don’t always match their words. While some have made progress, the students at many remain overwhelmingly affluent. On average,about 15 percent of students at elite colleges receive Pell grants, which as a rule of thumb go to students in the bottom half of the income distribution.

Foreign students — typically well-off ones — have become another group that college admissions offices have decided should be well represented in every freshman class, along with “legacy” applicants (the children of alumni), varsity athletes and underrepresented minorities. A large fraction of these groups comes from high-income families. And all of them, along now with students from around the world, are a higher priority for colleges than poor students.

Low-income applicants are left to compete for the remaining slots with applicants who have the highest test scores, most impressive extracurricular activities and most eloquent essays.

The globalization of elite colleges, then, is a fitting case study of how higher education has transformed itself in the last half century. After decades of being dominated by male students coming from a narrow network of prep schools, these schools have become a patchwork of diversity — gender, race, religion and now geography. Underneath the surface, though, that patchwork still has some common threads.