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December holidays mean sleeping late, gobbling sugar cookies and watching ancient TV cartoon classics — unless you’re in 12th grade. Those high school seniors may instead be worrying whether they have taken enough Advanced Placement courses and exams to ensure that they’re ready for college — and that they can get into a good school.

A reader recently expressed a widespread view: If your classmates have taken 10 AP exams and you haven’t, you’re sunk. Fortunately, that’s wrong, a new study shows.

According to College Board and Princeton researchers looking at more than 400,000 entrants to about 100 colleges, the greatest gain comes not from taking 10 APs, but just one or two. The biggest boosts in first-year college grades and on-time graduation, they said, “are associated with students increasing their AP participation from zero to one AP exam and from one to two AP exams.”

“Taking and performing well on more than four to six AP exams does not markedly alter predicted first-year college grades and on-time bachelor’s degree attainment rates,” according to researchers Jonathan Beard, Julian Hsu, Maureen Ewing and Kelly E. Godfrey. The patterns hold true for students from wealthier families and students who come from families without substantial financial resources.

Beard, Hsu and Ewing work for the College Board. Godfrey works for Princeton University. Those who argue that their conclusions cannot be trusted because the College Board runs AP should note that they are encouraging less AP test-taking, which could translate into less revenue for their big nonprofit organization.

The study buttresses what is already known. And students have begun figuring this out — even those who care, more than anything, about getting into a certain celebrated college that’s older than our country. The smartest counselors are telling them that four to six AP courses and tests are enough to satisfy the ivyest Ivy.

Such colleges get many more applications from such students than they have room for, so final decisions depend not on college-level AP, International Baccalaureate or Cambridge tests but on the depth (not the number) of applicants’ extracurricular activities, the warmth of their recommendations, the charm of their essays and the relative scarcity of their family backgrounds at that college.

We Americans still find it hard to believe that big numbers don’t matter. A father in Fairfax County, Va., told me someone in authority at his daughter’s high school insisted she had to take nine or 10 APs to get into the University of Virginia. When I investigated, it turned out the issue for the student was her reasonable desire to take a regular U.S. history class rather than the AP version so she had time for more AP science.

Last week, I wrote about a student in Loudoun County, Va., whose counselor demanded he take four AP classes his junior year when he preferred to take just one, computer science, his senior year. He got the counselor to back off. He had time for his favorite courses and activities. This month, he was offered early admission to a highly competitive university, his top choice.

Having determined that the most productive AP experiences are the first two, the authors of the not-yet-published report, “Studying the Benefits of Taking APs: Relationships between the Number of APs, AP Performance, and College Outcomes,” recommend that college admission officers do more to discourage the AP arms race.

Students may “feel pressured to take many APs if they believe it will be a distinguishing factor in the college admission process,” they said. “Our research can help college admission officers further consider and possibly refine what they communicate to students, families, and school counselors.”

A small but growing number of high schools and nonprofit programs such as Advancement Via Individual Determination (AVID) have had success getting more students ready for the first or second AP course. To assist them, the College Board is adding pre-AP courses and AP study guides.

I think students should be allowed to take as many AP courses and tests as they want. Some teens tell me AP, IB and Cambridge are the least boring choices. But stacking tough courses high to impress colleges doesn’t work. If AP is new to you, better to take one course at a time and see what happens.