I found an interesting article on msnbc.com. The link I sent to my twin brother said, "Having a male twin can reduce woman's fertility. Women who have a male twin are less likely to marry and have children, perhaps because of being exposed to their brother's testosterone for nine months in the womb, researchers reported."
And then came my twin's reply via email: "And males twins are more prone to drink to excess and mumble to themselves, due to having been subjected to female criticism prior to birth..."
Smarty pants. :)