Breast cancer and handedness
I have always been disappointed neither of my children is left-handed (I’m a southpaw). I guess now I’m glad, at least about my daughter. A recent article in the BMJ shows a more than 2 fold increased risk of premenopausal breast cancer in left-handed women.
There is also a plausible mechanism in that increased exposure to sex hormones in utero increases the chances of a fetus being left-handed and might also increase the risk of breast cancer. The mechanism of sex hormones effect on handedness seems to be via disruption of the typical pattern of “lateralization” of the sides of the brain. Most people are right-handed and that seems to be the default pathway of brain development. High levels of sex hormones (both the authors of the article and I are being deliberately vague here) disrupt this default pathway and make handedness more a 50-50 proposition.
As far as breast cancer, exposure to hormones, particularly estrogens help breast tissue proliferate and play a crucial role in the dysregulated proliferation of many breast cancers (Tamoxifen acts by inhibiting estrogen induced proliferation). Presumably prenatal exposure can influence breast tissue to make it more likely to turn cancerous in later life.
On the other hand, the reported increased risk of breast cancer has broad confidence intervals. The risk for all breast cancers was not significantly higher in the left-handed although it almost made it (lower limit of relative risk was 0.99). For premenopausal cancers the lower bound of the relative risk was 1.15 (meaning at the lower bound left handed women had only a 15% increased risk)
One could also imagine that handedness might be a marker for an unmeasured environmental risk that also predisposed to breast cancer, such as exposure to tobacco smoke in utero, although this doesn't seem to be the case (it is always hard to rule out unmeasured confounders in retrospective studies).