Most of the world's population now lives in cities, which means fewer of us know our neighbors. Should we adapt to modern society by raising our guards and looking over our shoulder? Research says no—we are actually not trusting enough, and it could be costing us money.

A growing body of work has established that in laboratory studies, subjects who are less trusting of their peers make less money in investment and economic scenarios. For instance, participants in one 2009 study underestimated how many partners would return their money in an economics game, so they underinvested and wound up with lower earnings than they could have. A paper, published online in May in the Journal of Personality and Social Psychology, now finds some real-world evidence of the financial harm of cynicism.

Analyzing data from national surveys in the U.S. and Germany, psychologists Olga Stavrova and Daniel Ehlebracht of the University of Cologne found that people who reported cynical views of human nature had smaller incomes (by thousands of dollars) two and nine years later, compared with their more optimistic peers. The researchers ruled out several proposed explanations for the link between cynicism and income, including personality, health, education, age, gender and employment status.

Instead they propose that cynicism increases suspicion, which prevents cooperation. If that is true, cynicism should not be harmful in places where a high degree of suspicion is justified. Looking at 41 European countries, the researchers found that in nations with the highest murder rates and least volunteerism, cynicism did not correlate with lower income.

Giving others the benefit of the doubt, then, may not invite deception; it instead seems to pay off—literally, Stavrova says: “So in most places in the U.S. and western Europe, being less cynical might be a better way to go.”