Most do not believe that their main job is to maximise investment returns for their charity, according to the study by financial consulting firm Watson Wyatt.
This is in spite of Charity Commission rules stipulating that trustees must always seek "the maximum return consistent with commercial prudence".
The research found other gaps between trustee behaviour and Charity Commission rules. Guidelines say that trustees should consider the need for diversification in investment assets, but nearly one-third of trustees with financial qualifications said their investment portfolios should consist of UK assets only.
At the same time, 80 per cent of respondents believed diversification was essential to good investment practice.
The average equity stake of charities in the survey was 65 per cent, which Watson Wyatt describes as "a relatively high exposure to the stock market". But most trustees said the optimal allocation was between 25 per cent and 50 per cent.
Mirko Cardinale, senior consultant at Watson Wyatt said: "This research sheds light on some potentially serious problems related to long-term planning of investment and spending by UK charities, as well as on their governance structures."
The survey, of 129 trustees in 106 organisations, found that only 40 per cent of charity board members see an explicit connection between spending decisions and investment performance.
The research also revealed the desire for external advice when it comes to investment issues. Most investment decisions are currently taken by trustees, but most think that the majority of them should be handed over to outside experts.
Tessa Akpeki, head of the trustee and governance team at the NCVO, agreed.
"Trustees think it is best left to experts and I think this is rightly so," she said. "What is needed is a greater awareness of the importance of the area, and the need for boards of trustees to ask the experts the right types of questions."
The study also asked whether some trustees have too many commitments.
Figures show 28 per cent sit on more than five charity boards.
"This raises questions about whether they have enough time for all of them and whether they can be sufficiently independent," said Cardinale.