Based on the few studies that have been performed, being left-handed appears to be a marker of increased risk of breast cancer. The most recent study examining this question found that the risk of developing postmenopausal breast cancer among left-handed women was more than twice the risk of right-handers. The explanation that has been proposed for the link to handedness is that high exposure to estrogens in the womb influences the part of the brain that determines whether a person will be left-handed or right-handed.
In support of this theory, it has been noted that women exposed to diethylstilbestrol (DES) in the womb are more likely to be left-handed than the general population. High prenatal estrogen exposure is also known to influence breast development in such as way as to increase the likelihood of subsequent breast cancer. Therefore, to the extent that left-handedness is evidence of high estrogen exposure in the womb, it may also signal increased risk of breast cancer.
What is the significance of this information?
Information concerning a link between handedness and risk of breast cancer is not likely to be of use to women who have already been diagnosed with breast cancer. However, it might encourage those at high risk and those with left-handed daughters to be vigilant with respect to breast cancer screening and to take appropriate steps to mitigate breast cancer risk.