Women may not be fully aware of the benefits of screening
The Swiss research found that these women were far more pessimistic about their chances of developing the disease in the first place.
Cancer experts say it is important that women know about the true benefits of mammography before they give their consent.
However, they say that these benefits should not be overly downplayed - as this may discourage women from being screened.
The research team from the Institute of Social and Preventive Medicine at the University of Geneva quizzed 895 women aged between 40 and 80.
They were asked to estimate the quantity of deaths from breast cancer that regular mammography screening prevents in women over 50 years old.
More than half overestimated - some said the risk of death was reduced by as much as 75%.
Scare factor
The authors, writing in the Journal of Epidemiology and Community Health, said: "Our data suggest that fully-informed consent may reduce participation and hence the public health effectiveness of the programme.
"Screening programmes should be judged not only by the level of participation achieved, but also by the number of women who were able to reach a truly informed decision about mammography."
Professor Jack Cuzick, from the Imperial Cancer Research Fund, said that the charity was working on ways of educating women about the true reduction in breast cancer risks offered by screening.
He said: "Most people overly estimate their risk of disease - or the benefits they are going to get from screening.
"We are trying to develop a project on how to communicate risk to people.
"The benefits of breast screening in terms of over 50s are pretty clear now."