The role of mental outlook in cancer is controversial
Women who have a positive outlook may decrease their chances of developing breast cancer, say Israeli researchers.
The small study, published in the BioMed Central journal, also found that getting divorced, or being bereaved could increase the risk.
But the researchers admitted that women were questioned after their diagnosis, which might significantly change their outlook on life.
UK experts said it was hard to compare different women's emotional stresses.
The role of mental outlook on cancer remains controversial, with some studies suggesting that it might play a role.
Meanwhile, others have found no significant effect, either on the likelihood of developing the illness in the first place, or on your chances of surviving it.
The latest study looked at 255 women with breast cancer and compared their answers in a questionnaire on mental outlook and life events with 367 healthy control subjects.
They found that a generally positive outlook appeared to reduce the chance of breast cancer by a quarter.
In addition, exposure to one or more of the traumatic "life events" such as loss of a parent or a spouse increased the risk by more than 60%.
Lead researcher Dr Ronit Peled, from Ben-Gurion University, said that women who had been exposed to a number of negative events should be considered an "at-risk" group for breast cancer.
"We can carefully say that experiencing more than one severe and/or moderate life event is a risk factor for breast cancer among young women.
"On the other hand, a general feeling of happiness and optimism can play a protective role."
But Dr Sarah Cant, from Breakthrough Breast Cancer, maintained that there was no clear evidence that positive or negative experiences could affect breast cancer risk.
"Emotional stress is highly subjective and is difficult to measure accurately.
"Women in this study were interviewed after breast cancer was diagnosed when they may be more likely to recall feeling depression and anxiety.
"The researchers also didn't account for other factors known to affect breast cancer risk such as family history or weight.
"Breast cancer is a complex disease and there is unlikely to be one single cause."