Mr. Scudder’s most recent stint as a consultant, on the Nam Theun 2 Dam in Laos, delivered his final disappointment. He and two fellow advisers supported the project because it required the dam’s funders to carry out programs that would leave people displaced by the dam in better shape than before the project started. But the dam was finished in 2010, and the programs’ goals remain unmet. Meanwhile, the dam’s three owners are considering turning over all responsibilities to the Laotian government — “too soon,” Mr. Scudder said in an interview. “The government wants to build 60 dams over the next 20 or 30 years, and at the moment it doesn’t have the capacity to deal with environmental and social impacts for any single one of them.

“Nam Theun 2 confirmed my longstanding suspicion that the task of building a large dam is just too complex and too damaging to priceless natural resources,” he said. He now thinks his most significant accomplishment was not improving a dam, but stopping one: He led a 1992 study that helped prevent construction of a dam that would have harmed Botswana’s Okavango Delta, one of the world’s last great wetlands.

Part of what moved Mr. Scudder to go public with his revised assessment was the corroboration he found in a stunning Oxford University study published in March in Energy Policy. The study, by Atif Ansar, Bent Flyvbjerg, Alexander Budzier and Daniel Lunn, draws upon cost statistics for 245 large dams built between 1934 and 2007. Without even taking into account social and environmental impacts, which are almost invariably negative and frequently vast, the study finds that “the actual construction costs of large dams are too high to yield a positive return.”

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.”

DAMS typically consume large chunks of developing countries’ financial resources, as dam planners underestimate the impact of inflation and currency depreciation. Many of the funds that support large dams arrive as loans to the host countries, and must eventually be paid off in hard currency. But most dam revenue comes from electricity sales in local currencies. When local currencies fall against the dollar, as often happens, the burden of those loans grows.