Up until I reached drinking age (three years after I actually quit drinking), I hated sharing my birthday. I figured my twin sister and I got half the cash in our birthday cards as our two older sisters. But now I’m relieved to have someone with whom I can commiserate about getting older. So the first thing I do on my birthday (today), is call Trish and wish her a good one.

I also call her on the afternoons I doubt the biological basis of my depression–after chats with morons who insist my disease is make-believe. It only takes a few minutes of conversation before I’m nodding my head again. Yep! It’s genetic.

Now Trish has managed her moods much better than I have. She’s never ordered off a psych ward menu, or obsessed about all the different ways she could kill herself (at least I hope not). I don’t even think she has a head doctor. However, (and I mean this in the most complimentary way), she’s nuts too. In fact, our whole family is nuts. We’d make a great holiday basket, like every other American family.

Psychiatrists today know depression and bipolar disorder are genetic partly because of the consistent results of twin studies and adoption studies. In twin studies, researchers find one twin with major depression or bipolar disorder and then get permission to study the other twin. According to J. Raymond DePaulo, Jr., M.D., author of “Understanding Depression,” the studies have found that “genetically identical twins are more than twice as likely to share these diseases (unipolar or bipolar) than are genetically fraternal or nonidentical twins.” However, a sibling or child of a person with major depression or manic depression still has a higher chance of developing the disorder compared to the general population.

Especially interesting to me are the new genetic findings of obsessive-compulsive disorder, since my twin sister and I both have suffered from that. For example, close relatives of people who develop early-onset OCD (which I did–disguised in religious scrupulosity) are up to nine times more likely to develop it themselves.

Eight months ago, two papers that were published simultaneously in the medical journal “Archives of General Psychiatry” pinpointed a specific gene (SLC1A1) that appears to play a key role in the development of OCD.

So as I blow out a few dozen candles (and then blow them out again just to make sure), I wish my twin the same thing that I wish for myself: peace and serenity, and more twin studies leading to better treatment for all who are anxious and depressed.