A British study by Andrew Oswald and Nattavudh Powdthavee found that the more daughters you have, the more likely you are to vote for left-of-center parties. The reverse happens with sons.
Specifically: " For each daughter, holding family size constant, a parent is approximately 2 percentage points more likely to vote left."
Nathan Silver at fivethirtyeight.com has the link to the full study.
(I thank the mother of our two daughters and one son, all good Democrats, for finding this.)