The longer CEOs stay in power—and a new study suggests most of them do, exceeding the optimal tenure length by about three years—the more likely chief executives are to limit outside sources of market and customer information, ultimately hurting firm performance.

Research titled How does CEO tenure matter? The mediating role of firm-employee and firm-customer relationships examines why a longer CEO tenure may not always produce positive results for firm performance.

According to the study, the average CEO holds office for 7.6 years, but the optimal tenure length is 4.8 years.