If you're a women with a twin brother, your chances of conceiving may be reduced by 25%. According to researchers, this is due to the testosterone female twins are exposed to in the womb.

In addition to infertility, there are other interesting side effects -- most notably, that women with male twins seem to be "masculinized." The study found that these women are more likely to exhibit traits, attitudes and behaviors typically associated with men -- which, researchers believe, accounts for the fact that they are 15% less likely to marry than other women.

Of course, it works both ways, so the male twin is also exposed to estrogen -- and previous research has shown that certain physical characteristics, including facial features, can be altered by exposure to hormones of the opposite sex. However, unlike testosterone, both male and female fetuses have similar amounts of estrogen to begin with, so men are less likely to be affected.