Students from wealthy families are more likely than those from poor families to go to college, and those whose parents pay their way are more likely to graduate. But according to More Is More or More Is Less? Parent Financial Investments During College, a study by Laura Hamilton, a sociology professor at the University of California, Merced, greater parental contributions were linked with lower grades across all kinds of four-year institutions.

“It’s a modest effect, not big enough to make the kid flunk out of college,” said Dr. Hamilton, whose study was published in this month’s American Sociological Review. “But it was surprising because everybody has always assumed that the more you give, the better your child does.”

The negative impact on grades was less at elite institutions than at other private, expensive, out-of-state colleges. The higher graduation rate of students whose parents paid their way is not surprising, she said, since many students leave college for financial reasons.