WASHINGTON (Reuters) - Many Americans fear they can do nothing to protect themselves from cancer, and they may be creating a self-fulfilling prophecy, U.S. researchers said on Thursday.

Their survey of more than 6,000 people found that nearly half -- 47 percent -- agreed that "nearly everything causes cancer" and that more than a quarter felt there was little they could do about it.

Yet an estimated two thirds of cancer cases can be prevented with common-sense measures, such as not smoking, eating plenty of fruits and vegetables and avoiding too much sun, according to the American Association of Cancer Research and the National Cancer Institute.

"They say, 'Well, there is nothing much you can do about it,' and, as our survey shows, they indeed do nothing about it," said Jeff Niederdeppe of the University of Wisconsin, who helped lead the study.

"Yet we know that approximately 30 percent of cancer deaths are attributed to smoking, while 15 to 20 percent are linked to overweight and obesity. Clearly, there are very specific things we can do to reduce our personal risk for getting cancer."

Niederdeppe and colleagues analyzed a National Cancer Institute survey that asked people whether they agreed with three statements about cancer. About 47 percent of those surveyed agreed that "it seems like almost everything causes cancer," while 27 percent agreed that "there's not much people can do to lower their chances of getting cancer."

Most of all, Americans are confused, with 71.5 percent agreeing that "there are so many recommendations about preventing cancer, it's hard to know which ones to follow," researchers wrote in their report, published in the journal Cancer Epidemiology, Biomarkers & Prevention.

People who agreed with at least one of the three statements were less likely than others to exercise weekly and eat five daily servings of fruits and vegetables, said Niederdeppe and colleagues.

Those who said there were too many recommendations were the most likely to smoke, and overall, less-educated people were more likely to hold one or more of the three beliefs.

Part of the problem could be the way medical research is reported, with one discovery at a time dribbling out and some findings contradicting others.

Cancer is the second-leading killer of people in the United States and most developed countries, after heart disease. Half of all men and one third of all women will develop some type of cancer in their lifetime.