Nature vs nurture: left vs. right confusion

There are two things I am bad at, that when I tell people they are either surprised, or confess to me that they are also not good at them. Today I'll tell you about the first one:

I cannot easily tell my left from my right.

Now the surest response to that confession from someone who thinks that is ridiculous --> "Just hold up your hands, your left one makes an L". Thanks.

It isn't that I need to make an L, it's just that I have to think about it. In my case, I sucked my fingers when I was a child, and I know that is my left hand, I actually make a fist with the two fingers I used to suck out to remember. Odd? Yeah but that's how much time has to pass. The only times this becomes a problem is when you are trying to give someone directions; especially when driving and you have to be quick about it. "Take a left, I mean the other left."

I've always wondered if there is any research on this, since multiple people have confessed to me they also can't quickly identify L/R. And I was delighted to find this blog post - everything she said was as if she was exactly in my mind!

Not to mention scientific data:

John R. Clarke, a professor of surgery at Drexel University in Philadelphia, estimates that 15 percent of the population faces some degree of left/right challenge. Eric Chudler, a neuroscientist at the University of Washington in Seattle, puts the figure higher, having found that more than 26 percent of college students and 19 percent of college professors acknowledge having difficulty telling left from right — occasionally, frequently or always.

Here is a test.
Basically there are some pictures of hands pointing up and down and you read them out loud. When you finish you get a time. Then you have more pictures of left and right. It took me more than twice as long to read the L/R ones as the U/D images.? What is that about?

poll by twiigs.com