More than half of those surveyed in a Ponemon study backed by Internet security firm PC Tools indicated they would reveal mobile phone numbers, email addresses or other information when told they might get something for nothing.
"Even in scenarios where people realize it is too good to be true, they are falling for it," said PC Tools senior manager of online strategy Eric Klein.
"I don't know why people keep falling for it, really," he added.
Cyber crooks have long exploited human nature with scams relying on "social engineering" to get people to reveal secrets such as passwords or unwittingly install computer viruses.
Manipulations can range from telling people they will be entered in prize drawings after filling out detailed surveys or getting them to open booby-trapped files said to contain sexy or graphic imagery.
"The results found a clear difference between how aware consumers think they are of scams and how likely they are to be taken in by the given scenarios," Ponemon Institute researchers concluded.
"It is clear from the findings that the threat posed by scams is still being underestimated."