Researchers in the Netherlands examined the relation between handedness and incidence of breast cancer in over 12,000 healthy, middle aged women born between 1932 and 1941.
Body measurements were taken and risk factors such as social and economic status, smoking habits, family history of breast cancer, and reproductive history were recorded.
They found that left handed women were more than twice as likely to develop premenopausal breast cancer as non-left handed women. Adjusting for risk factors hardly affected the overall association.
The origin of the association may lie in exposure to high levels of sex hormones before birth, which can induce left handedness as well as changes in breast tissue, say the authors.
"Although the underlying mechanisms remain elusive, our results support the hypothesis that left handedness is related to increased risk of breast cancer," they conclude.