The problem with overoptimism was illustrated by the investment-bank collapses of 2008.

ECONOMIST: Sometimes it helps if investors are gloomy

In an editorial accompanying the study, Dan L. Longo, an editor at the journal, suggested the varied genetic makeup of tumors described in the study stands in contrast to "overoptimism" among proponents of personalized medicine.

WSJ: 'Personalized Medicine' Hits a Bump