Through the early 1990s and early 2000s, average scores on the SAT college entrance exam moved steadily upward. Now, for the last five years, they've been drifting back down.

The reason? Unlike on the multiple-choice sections of the test itself, there's no one right answer. But a big factor is the larger, more diverse group of students taking the tests, combined with a widening scoring gap between the best-performing groups and those that whose numbers are growing fastest.

Results released Tuesday show the high school class of 2009 earned a combined score of 1509 on the three sections of the exam, down two points from last year. The average reading and writing scores dropped one point each, while math scores held steady.

Experts caution against reading too much into the national average SAT score, given the test-taking pool changes over time and can vary widely among states. Still, the average score is now down nine points since 2006, when the writing section was first included and the test moved to a combined 2400-point scale.

Math scores are higher over the last decade, but reading scores are four points below their 1999 level.

The College Board, which administers the exam, emphasized the growing diversity of SAT-takers. Minorities made up 40 percent of last year's group, and more than a quarter of the 1.5 million test-takers reported English was not their first language at home.

That's good news in that more students aspire to college, but it also weighs down the overall scores because, on average, students from most minority groups score lower.

The exception is Asian-Americans, whose average combined score surged 13 points to a combined 1623, while scores for whites fell 2 points to 1581. For black students, average scores dropped 4 points to 1276. Average scores for two of the three categories the College Board uses for identifying Hispanics also declined, and overall ranged from 1345 to 1364.

Men also widened their advantage over women by 3 points; men scored 1523 on average compared to 1496 for women. The difference comes mostly from math scores.

Students reporting their families earned over $200,000 scored 1702, up 26 points from a year ago. That group is comparatively small, but the sharp increase could fuel further criticism the exam favors students who can afford expensive test-prep tutoring.

The SAT remains the most common college entrance exam, though the rival ACT has nearly caught up in popularity. Most colleges accept either, and a growing minority no longer require either one.

Still, fewer than half of high school graduates take the three-hour, 45-minute SAT, and the group is tilted toward higher-achieving, college-bound students.

"I just don't think it's a good gauge of what's going on nationally," said Tom Loveless, senior fellow at the Brookings Institution, who said the SAT remains a useful tool, when combined with high school GPA, for evaluating whether individual students are prepared for college.

Experts generally pay closer attention to the National Assessment of Educational Progress, because, unlike college entrance exams, it represents the entire population of students.

On those exams, K-12 black and Hispanic students have made bigger gains than whites since the 1970s. Since 2004, they've made improvements in reading and math at every level or age tested, but the achievement gap between minority and white students has remained wide because whites have also done better.

College Board officials don't attribute the widening SAT scoring gap directly to race but to factors that correlate with race, such as the likelihood of exposure to a rigorous high school curriculum. Students taking a core curriculum — including four years of English and three each of science, social science and history — scored 44-46 points higher on each section of the SAT.

"Our data suggest the gap is widening as academic preparation widens," said Wayne Camara, the College Board's vice president of research and development.

White kids are more likely to have access to advanced college-prep subjects than blacks and Hispanics, and the success of Asian-Americans on the SAT is also probably due in part to their push to enroll in such courses.

Whatever Asian-Americans are doing, educators want to bottle it.

"For students who are planning to attend college, there's this one group that's outperforming everybody," said Seppy Basili, senior vice president at Kaplan Test Prep. "So what is it about this group? Can we do something to study it?"