What you know determines where you go, according to a new book that sets out to determine why the smartest low-income students forgo the most selective colleges.

It’s not that poor kids aren’t as smart as rich ones, researcher Alexandra Walton Radford finds. Nor do top schools turn them down. In fact, she reports, low-income prospects have a big advantage in the admissions process at the most selective colleges.

The problem is that few of them apply, thanks to high school counselors and peers who know little about the admissions process, and parents who often know even less.

“Colleges must go beyond trying to ease the path of less-affluent students,” writes Walton, who, with Princeton sociologist Thomas Espenshade, previously wrote a high-profile analysis that found that whites and Asians needed higher grades and scores to get into top colleges than blacks and Hispanics. “They have to tackle their low application rates directly.” Poor students are not applying to elite colleges, “and institutions need to do a better job of finding them and encouraging them to submit an application.”

Only 3 percent of students at the nation’s 146 most selective colleges and universities are from the bottom 25 percent of the income scale. And Walton says it’s not necessarily because they’re less academically well prepared than students at the top.

She surveyed 900 valedictorians at all socioeconomic levels, and found that those from low-income families had parents who were not as involved in helping them apply to college as their high-income counterparts. Low-income parents also emphasized cost more, and were more likely to encourage their children to stay close to home.

“’You’re just going to go where we can afford to send you,’” one low-income valedictorian said her parents told her. “I … had to get a scholarship if I wanted to go somewhere else.”

The public high schools most of these students attended were described as equally unhelpful, with guidance counselors characterized as uninformed and, by the students themselves, as “pretty lousy” and “incompetent.”

At best, the counselors are overworked. The ratio of counselors to students at public high schools is 1 to 285, and of college counselors to students, 1 to 338. Only one in four public high schools even has at least one full-time college counselor, compared to three out of four private ones.

Nor did less-affluent valedictorians find easy access to information about financial aid. Only 28 percent of parents of students in the lowest income brackets had a good understanding of financial aid, compared to nearly 60 percent of high-income parents, and low-income students also were unlikely to receive accurate or comprehensive information about it from their high schools.

“All I knew is that I asked my dad about it and he’s like, ‘No, we don’t want to take any financial aid,’” one student said. “I guess it’s like a pride thing for him. So I didn’t really look into it.”

The student-loan provider Sallie Mae reported in 2011 that 13 percent of families with annual incomes under $35,000 did not apply for financial aid.

So while more than half of top SAT and ACT scorers from the upper-income category enroll in top institutions, only a third of low-income standouts do. Three-fourths of high-income valedictorians end up enrolling at the nation’s most-selective colleges, compared to 43 percent of low-income ones.

This lopsided ratio comes despite the fact that, when they do apply, low-income students (also described as those with low socioeconomic status, or SES) are more likely to be admitted than high-income applicants. Sixty-three percent are accepted to the most selective institutions, compared to 50 percent of their middle-class peers and 54 percent of wealthy students.

“Given the greater socioeconomic hurdles less-affluent students face, admissions officers may be more impressed when low-SES students reach the top of their class than when middle- or high-SES students manage to do so,” Walton writes.

The book, Top Student, Top School: How Social Class Shapes Where Valedictorians Go to College, is published by the University of Chicago Press.

In it, Walton urges more publicity about need-based aid, better explanations of net price versus sticker price, and better high school counseling.

“Many of our best and brightest students, who deserve to attend our nation’s best colleges and would thrive at them, are prevented from doing so because of their social class origins,” she says.

The information universities and colleges are now providing, Walton writes, is not particularly helpful.

Glossy promotional mailings, for example, don’t sway top students. Many said they never even read them.

“I mean, I haven’t heard of anybody really saying, ‘Oh, I got a letter in the mail from College X and that made me go there,’” one student said.