Which fallacy is this? Assume that all people are either right-handed or left-handed, and likewise either right-footed or left-footed. 90% of people are right-handed. 90% of right-handed people are right-footed, but only 50% of left-handed people are left-footed as well. Which is more common: left-handedness or left-footedness?
STOP here, and have a go at answering that question first before continuing.
And then read on:
The answer is easy enough to get if you calculate it. Yet it feels counter-intuitive. Two people I tested this on (another used maths and got it right) assumed that because right-footedness is so dominant among right-handers and common among left-handers, that right-footedness should be even more common than right-handedness. There's some sort of fallacy at work here: any idea of what it is?

You could argue that what you're assuming about handedness is a false dichotomy. Since some people are ambidextrous. 
There's nothing wrong with the statement above as it is written as a statement about folks in general. You may want to rule out ambidextrous people as outliers (they're very rare). 
You're really just saying that people who are left handed aren't left footed as frequently. So if you looked at left and right footed folks and compared that to handedness, you'd see "rightfootedness" outnumbers that of the left.
