The study's authors — three management scholars and a statistician — say planners are systematically biased toward excessive optimism, which dam promoters exploit with deception or blatant corruption. The study finds that actual dam expenses on average were nearly double pre-building estimates, and several times greater than overruns of other kinds of infrastructure construction, including roads, railroads, bridges and tunnels. On average, dam construction took 8.6 years, 44 percent longer than predicted — so much time, the authors say, that large dams are “ineffective in resolving urgent energy crises.”