By Dr. Marciene Mattleman

PHILADELPHIA (CBS) - Everyone knows incentives are motivating – gold stars and attendance certificates. Cash always sounds best.

But new research on student achievement published by the National Bureau of Economic Research conducted by a team from several universities involving 43,000 students, sheds light on what incentives work best.

Some students were promised no rewards; others, a trophy, $10 or $20 in cash given immediately or a month afterward. While older students preferred cash, younger students were wooed by a trophy as easily as by money.

Prior research has shown that people value rewards less the longer they have to wait and value something they have already and are at risk of losing.

Rewards worked better if they were given before a test. Participants signed a form acknowledging receipt and wrote what they would do with it.

The article suggests that it’s better to reward behavior kids can control, not just telling them to get better grades.

Read more about timing rewards in Education Week.