Are firms with cheery workers more likely to make investors smile? According to Alex Edmans of Wharton, a business school, choosing to invest in a portfolio comprising companies that have happy employees (as measured using Fortune‘s “100 best companies to work for,” most recently published in January this year) is likely to bring rewards. Mr. Edmans suggests that such investors enjoy gains beyond what can be explained by the risks they take. From 1999 to 2008 the Fortune portfolio has provided a 4.1 percent annual return above that of the broader CRSP index (which includes all shares traded on Nasdaq, the New York Stock Exchange and American Stock Exchange). But advocates of corporate social responsibility should be cautious about inferring that employee satisfaction brings high returns. Other variables, such as good management, may be at work.