And another time-proven rule of investing — that people should put a substantial amount of their money in the stock market — might have its detractors, no matter what their I.Q. scores. That is especially possible given the volatility in the financial markets in recent years.

Yet only about half of all American adults have money in the stock market, directly or indirectly. So maybe something else is going on. If people can’t figure out the financial markets on their own, they can entrust their money to professionals or heed professional advice. The real problem may not be that many people lack investing savvy or smarts. Perhaps what they lack is trust, or confidence in whom to trust.

Three economists, Luigi Guiso of the Einaudi Institute for Economics and Finance, Paola Sapienza of Northwestern and Luigi Zingales of the University of Chicago, argued in a paper published in 2008 that many households avoid investing directly in stocks out of vague fears that they might be deliberately misled or cheated. Using results from a survey of households, this time in the Netherlands, the economists showed that those who indicated a high level of trust were 50 percent more likely to invest in the stock market. They were also more likely to have diversified their stock holdings. The paper, titled “Trusting the Stock Market,” was published in The Journal of Finance.

Knowing whom to trust, and relying on those who are trustworthy, is itself an aspect of intelligence. Mr. Guiso and his co-authors cited research that suggested that investment decisions relied significantly on a part of the brain called the Brodmann area 10. This region of the frontal cortex is believed to be associated with our ability to make inferences about others’ preferences and beliefs based on their actions. Such social intelligence seems to reward some people more than others with an ability to put standard investment advice into practice.

Successful investing requires that we judge other people, and it relies on an ability to develop a good model of others’ minds. It requires that we put into perspective recent angry rhetoric against Wall Street and understand that, while some criticisms are surely justified, others are just as surely exaggerated.

Anyone, regardless of background or education, may worry about being misled. The professionals tell us that the stock market is the best place to invest, but such assurances don’t help us when the market swoons. Many pros assured us that housing prices would never decline, either.