Let's work it out: 1 in 10 are lefty, 1 in 1000 have Downs. Supposedly 50% of people with Downs are lefty, therefore: P(A) = 0.001 P(B|A) = 0.5 P(B) = 0.1 P

Let's work it out: 1 in 10 are lefty, 1 in 1000 have Downs. Supposedly 50% of people with Downs are lefty, therefore: P(A) = 0.001 P(B|A) = 0.5 P(B) = 0.1 P(A|B) = P(B|A)xP(A)/P(B) = 0.5x0.001/0.1 = 0.005 = 0.50% So the chances of having Downs given being a lefty is 1 in 200, or which is 5 times higher than the base rate, but still quite rare. It's probably more a case of "having developmental abnormalities interferes with handedness" rather than "being a lefty predisposes you to having sundry developmental abnormalities".