U.S. adults under age 40 may be even more skeptical about the stock market than U.S. adults in their 40s are.

Researchers at the National Association for Variable Annuities, Reston, Va., have compiled figures supporting that conclusion in a summary of results from a September survey of 1,000 U.S. adults ages 18 and older.

Financial planners often estimate the percentage of a typical consumer’s portfolio that should be invested in stock and stock funds by subtracting the consumer’s age from 100. That would mean that a typical 20-year-old woman should consider investing about 80% of her assets in stock, and a 60-year-old man should consider keeping about 40% of his assets in stock.

But the NAVA researchers found that survey participants said they were willing to invest an average of just 21% of retirement assets in stock, even after survey staffers explained that investing in stock offers consumers the potential to earn higher returns.

Survey participants over age 40 showed a tendency to become more conservative as they neared retirement.

The average retirement portfolio percentage that participants were willing to put in stocks was about 30% for participants in their 40s, 21% for participants in their 50s and 15% for participants age 60 and over, researchers report.

For participants under age 40, who in theory should have suggested more aggressive portfolio allocations, the average suggested stock allocation was just 23%.

Income may explain much of the younger participants’ conservatism.

People under age 40 tend to have lower incomes than people in their 40s.

Survey participants with annual household incomes over $65,000 said they were willing to put about 32% of retirement assets in stock, but participants with household incomes between $35,000 and $65,000 said they were willing to put just 21% of retirement assets in stock.