American Express Co.'s net income grew 9 percent in the second quarter, as spending by cardholders increased and the credit card issuer set aside less money to cover potential credit losses. American Express cardholders tend to be more affluent than other credit card users, which is one reason the company has done well as the nation's economy has gradually improved since the recession. [...] consumer spending at retail stores picked up an average of 0.5 percent in the April-June quarter after a severe winter weighed on sales earlier this year. Increased retail spending can help drive profits for credit card issuers like American Express.