Eli5:what’s the cause of someone being right or left handed? And why the vast majority is right handed?

Question

Eli5:what’s the cause of someone being right or left handed? And why the vast majority is right handed?

In: Biology

Answer 1

I’m a lefty (from a family of five where every single person was lefty); my wife and her mom are righty, but supposedly my FIL was born lefty but the nuns beat it out of him in parochial school…who knows.

Anyway.

We have fraternal twin boys.

Twin #1:
– 100% righty for writing
– 100% lefty for kicking
– 50/50 for throwing

Twin #2
– 100% lefty for writing
– 100% righty for kicking
– 50/50 for throwing

Odd twist:
– they both have simian creases on both left and right hands (and so do I). I only recently discovered this phenomenon. Apparently it’s only in 1.5% of the population…FOR ONE HAND.

Me, and my twin sons all have it both hands. I’d love to know the probability for that.