In Tribal Leadership: Leveraging Natural Groups to Build a Thriving Organization, authors David C. Logan and John King draw from several decades of consulting experience to examine the corporate cultures at a number of leading companies, including Amgen, Intel, American Express, and Prudential. What makes these companies so successful? Tribes - the groups that naturally form within the company - are the secret to lasting success, the authors say. Birds flock, fish school, and people "tribe." The authors learned that what separates average tribes from those that excel is culture, and they contend that tribal culture exists in stages, on a scale of 1 to 5, going from undermining to history-making.