More than a third of Americans age 24 to 28—or 35%—have debts that exceed their assets, according to research from Dartmouth College assistant professor Jason Houle. That’s roughly double the proportion of their peers in the late 1980s and mid-1970s. Mr. Houle, in an article published in the academic journal Social Problems, compares Americans between ages 24 and 28 across three generations: those from today, which he calls Generation Y; early baby boomers, who were that age in the mid-1970s; and late baby boomers from the late 1980s. He finds the share of young Americans with debt—if not the overall dollar amount—has actually fallen from prior generations. Today, 75% of young Americans have debt, compared with 76.5% of late baby boomers at the same age and 78.2% of early baby boomers. But big shifts in the types of debt held by the groups have led to far different experiences. Younger Americans today are taking on far less mortgage debt and far more student and credit-card debt than the early and late boomers did at the same age, according to Mr. Houle’s research, which relied on Labor Department data. Only 19.8% of today’s young Americans have home-related debt, down from 29.9% of their peers in the late 1980s and 43.1% of those in the mid-1970s. Conversely, 22.4% of young Americans today have education debt, compared with 5.1% among late baby boomers and none among early boomers. Mr. Houle attributes the shifts to several forces. He links cuts in public funding for colleges to rising tuitions that have led students to take on more education-related debt. Also, more Americans are attending school—and delaying getting married and having kids—than prior generations. “As the transition to adulthood has protracted, and the costs of education have risen, young adults have shifted their credit use away from home mortgage debt and towards student loan and consumer debt,” Mr. Houle writes. The new research comes amid a debate among economists about whether rising student debt and falling homeownership rates among young Americans are linked. Researchers have yet to reach a consensus, since it’s hard to determine whether other factors—such as tightened lending standards and stagnant wages—are the main force.