Call it viral, buzz or word-of-mouth advertising: Getting customers to spread the word about a new product through their social or professional networks is a hot strategy in the marketing world. Its proponents insist that the technique — whether online or face-to-face — is sure to boost a company’s return on investment (ROI).

But how can companies find the right individuals to deliver the message? Marketers may wonder if they are finding the best “seeding points” — that is, well-connected people at the hub of social networks who will latch on to a product and promote it widely among the people they know.

New research led by Wharton marketing professors Raghuram Iyengar and Christophe Van den Bulte , working with University of Southern California preventive medicine professor Thomas W. Valente, has found that traditional targets may not be as influential as previously thought.…