The hard data is piss poor. It's largely self-reported data collected by media agencies. Just google "Bank of Mom and Dad", there are lots of poorly done studies. This one is okay: http://paa2005.princeton.edu/papers/51443

The reason it's poor is because everyone has an incentive (bank, broker, parents, you) to under-report the parental funds. Parents send the gift money in advance (3 months to be exact), banks look the other way to meet equity requirements and the transaction takes place.