Left-handed womb twin survivors

In the case of identical twins, who develop from a single zygote,  like this, for example:

MONOZYGOTIC MONOCHORIONIC TWINS

A lot depends on exactly how and when the original zygote splits into two.
Under particular circumstances,  the twins that result are mirror images of each other.  That means that one is right-handed and one is left-handed.

Where one of these mirror twins dies,  the survivor may be left-handed or right-handed, depending.

 Now Charles Boklage, who can always be relied upon to cast a fresh light on all things about twinning, has clarified the left-handedness of twins: click this link for more:  Biology of Mirror-Image Twins.

He says: 

Twinning and nonrighthandedness are somehow genetically related. The exact mechanisms involved are not clear. But it is a good example of something very important that most people don't understand about human developmental genetics. Many of the most important genes do not show up every time they are present.
Most twin conceptions result in no births at all, and for every twin pair born, about 10-12 people who grew from twin embryos are born single. It is actually possible that every lefthander in the world is a twin. I'm not saying I believe that, but it is not impossible and there is no evidence against it.

Now an interesting fact is that left-handed twins have a particular gift, which may have contributed to their survival. 

The study: Led by Nick Cherbuin, the research team from the Australian National University recruited 80 right-handed volunteers and 20 lefties. They measured the transfer time between the two sides of the brain by measuring reaction times to white dots flashed to the left and right of a fixed cross, notes the BBC. This was then compared with how well participants were able to spot matching letters in the left and right visual fields, a task that required them to use both sides of their brain at the same time.
The results:

• In the right-handed volunteers, there was a strong correlation between how quickly information was transferred across the left and right hemispheres and how quickly people spotted matching letters, reports the BBC.
• No matter the speed, the strongly left-handed volunteers were far better than their right-handed counterparts at processing information across the two sides of the brain. The pattern wasn't as clear-cut for those who were only mildly left-handed.

The study findings were published in the medical journal Neuropsychology.

So if you are a leftie, your brain works both ways - lucky you!