Why Am I Left-Handed?

> Left-handedness does not seem to be purely genetic. Two left-handed parents have a left-handed child only 25% to 30% of the time. If one identical twin is left-handed, there’s only a 20% to 30% chance the other is too. This suggests a genetic component alongside some developmental randomness.

It seems that 20-30% is double or triple the likelihood of the being lefthanded normally, implying that actually there is a stronger correlation towards genes being a factor.

If at least one of the twins was left-handed, we would expect odds of 19% among sets of twins compared to 10% among babies generally, but then expect only 1% odds of twins, both twins would be left-handed if there was no correlation. So, the expected odds of exactly one being lefthanded is then 1:19, or just over 5%. Having actual odds of this being 20-30% shows that whatever is causing this is likely to affect both babies the same.

Having said that, I don't necessarily think we can just jump to genes - it could also be due to something environmental during pregnancy, after all both babies are growing in the same environment.