Fully 81 percent have provided support to adult children and 39 percent to grandchildren over the past five years, the study found.
Because of the tough economy, today's multimillionaires are doling out so much to family that they are now acting as "the family bank," said the study, which looked at individuals with $5 million or more in investible assets.
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"They are the unsung financial heroes" for many families, said Ken Dychtwald, president and CEO of Age Wave, a research firm. "They're going far beyond what we might expect."
It also means that today's wealthy need to consider the needs of their broader family when planning their own retirement, the report said.
Despite the generosity, the money often comes with strings attached. "Family banks" also provide financial guidance and rules.
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Three quarters of the multimillionaires said they will stop giving financial help to a family member if they feel the money isn't being used wisely. More than a third will stop if they feel their money is not appreciated. And more than a third said they would reduce the inheritance planned for a family member if they feel the receiver is not financially responsible.