The study for the Copenhagen Consensus, a project to analyse the costs and benefits of different solutions to world problems, argues that peacekeeping is a good investment, despite the experience of the United States and its allies in Iraq.

"This much criticised area actually seems to be a very effective form of development assistance. (But) it needs to be longer term -- there's a tendency to pull troops out too soon," report co-author Paul Collier, professor at Oxford University's Centre for the Study of African Economies, told Reuters.

The report says most civil wars break out in small, low-income, low-growth nations -- unlike Iraq's, which was triggered by an international invasion.

In a typical country, it says, a $100 million a year peacekeeping initiative reduces the risk of renewed conflict over a decade from 38 percent to 16.5 percent.

It puts the cost of civil war to the country, its neighbours and the international community at up to $250 billion, so each percentage point of risk reduction is worth $2.5 billion.

If the peacekeeping investment rises to $850 million a year, the cost over a decade hits $8.5 billion but the conflict risk falls more than 30 percentage points to 7.3 percent, representing a gain of $75 billion.

The report, which claims to be the first cost-benefit analysis of U.N. peacekeeping initiatives, is based on a study of civil conflicts around the world over the past four decades.

SECURITY GUARANTEES

It argues that peacekeeping is even more cost-effective when provided in the form of an "over the horizon" security guarantee -- a commitment to send in foreign troops if needed.

France had implicitly provided such a guarantee for decades to its former colonies in Africa, reducing the civil war risk in those countries by three-quarters, the study said, adding that Britain now operates a similar arrangement in Sierra Leone.

It suggested such a guarantee could be supplied by the United Nations or the African Union to protect governments which had come to power through certified democratic elections.

"There should be U.N. guarantees," Collier said in a telephone interview. "It would be much better if it were just regularised as U.N. guarantees instead of these ad hoc, hit-and-miss former colonial (arrangements)."

Providing a credible international security force would cost about $2 billion a year but the gains, in terms of lower conflict risk and higher economic growth, would be between 11 and 39 times higher, the report said.

It said a global package combining peacekeeping, over-the-horizon guarantees, aid and limits to military spending by post-conflict countries would cost $10.8 billion a year, but the benefits to the world would be at least five times higher.

By comparison, the total budget for U.N. peacekeeping operations for the year to June 30 is $6.7 billion.

The study said the risk of new civil wars breaking out had increased, partly as a result of commodity booms which fan conflict over resources like oil.

Ironically, the spread of democracy in low-income states was another risk factor because democracies find it harder than repressive governments to put down revolts or coups.