Left with thoughts about Lefty

I don't get it. This morning the Today Show was talking about twins, and some of the recent discoveries in prenatal behavior that have been made by observing multiples in the womb. Meredith Viera threw out this idea that there is a theory that left-handed people are actually the surviving single from a set of twins, where one embryo was lost/absorbed into the body early on in the pregnancy.

Now, I've heard of the lost twin theory - there was an article about it in the New Yorker years ago, and it fascinated me. The idea that I could have had a possible womb-mate, someone who disappeared without anyone even knowing of her existence... I romanticized the idea, identifying myself with the author of the article, who had always felt that he was "incomplete" without having any idea why.

But what I don't understand is this. For the left-handed theory to be true, wouldn't every set of living twins have to contain a lefty and a righty? I don' t think that is true, is it? Or is the theory based on something about the residual grief of losing your embryo-buddy that causes a chemical/biological reaction that makes you left-handed?

Ah, Meredith, you puzzle me.