I'm seeing the year 1968 at http://thecomputerboys.com/?tag=black-art#fn-156-1
Is there a 1966 reference out there somewhere? Or perhaps the actual study was done in '66 and didn't make it into press until '68? I didn't pull the original paper.
(The fact checkers at the New Yorker are typically very sharp.)
edit: I pulled the original paper (10.1145/362851.362858). The studies were done in 1966!
Interesting to see the genesis of the myth of the 10x programmer.