Andrew Papworth, a marketing consultant and author, reached this conclusion after analysing 23 charity legacy appeals in magazines and newspapers in 2007.
But the conclusion, published in a report called Will It Work?, were challenged by two legacy experts, Richard Radcliffe and Paul Farthing, who argued that specific appeals do work.
The report says that many of the appeals offered information or advice about writing a will in return for a bequest. It concludes that few organisations encouraged people at the same time to become donors and to start relationships with charities.
"They just run ads saying 'please leave us some lovely money' and maybe put a telephone number and website address on the bottom," Papworth said. "It's the 'cast your bread on the water and hope' school of advertising. They might as well feed £50 notes into the shredder."
The report argues that people often make wills because of major life changes and that appeals for legacies are unlikely to be seen by people at the right time.
Radcliffe, legacy consultant with charitable legacy agency Smee & Ford, disagreed with the report.
"Typically, 60 per cent of legacies are from people who have distant relationships with charities," he told Third Sector. "Maybe they are relatives or friends of people who have benefited from charity services in the past. If you don't advertise specifically to them, you will never get their legacies."
Farthing, director of high-value relationships at Cancer Research UK, said: "Legacy communications are there to try to get the message across about the impact of legacies and explain how their money is spent. We can't do that through a tick box."
For a copy of the report, contact email@example.com.