Some time ago, some 50-60 years ago (in my country at least) if you were a lefty the teachers at school would tie your left arm to your desk and force you to write with your right hand. They saw the whole being a lefty thing as a defect or some kind of negative aspect. Even in my school days there wasn’t much awareness or care for lefties, there was barely one lefty chair which I had to fight with some other kid everyday. Regardless, I like being a lefty. We even have our own day (August 13th)! :D
They’re saying that a higher proportion of the Mensa population is left-handed than the general population. I’ll try and give some math to back up what they’re saying. Assume that 5% of the general population is qualified to be a part of Mensa. Then their 20% left-handed means that 1% of the general population would be left-handed and qualified to part of Mensa while 4% would be right handed and qualified to be part of Mensa. So when you pit these people against their handedness populations, 1% out of the 15% left-handed population is 6.67%. This is percentage of left-handed people qualified to be in Mensa. Doing the same math for the right-handed population, you get the 4% out of 85% or 4.7% of the right-handed population qualified to be in Mensa. Thus, if you’re left-handed, you get about a 40% higher chance of being in Mensa than if you were right-handed. This is all napkin math, and it assumes that 5% of the general population is qualified to be in Mensa, when in fact it is 2% (98th percentile of IQ scores). I chose 5% to simplify the math. Yes, I’m left-handed.