Eric S. Maskin
From the Nobel Foundation:
While in England, he got caught up in a problem inspired by the work of another new friend, Leo Hurwicz (Ken [Kenneth Arrow] had introduced Maskin to him at Stanford): under what circumstance is it possible to design a mechanism (that is, a procedure or game) that implements a given social goal (formally, a social choice rule)? After struggling with that question for most of the year, he finally realized that monotonicity (now sometimes called "Maskin monotonicity") was the key: if a social choice rule doesn't satisfy monotonicity, then it is not implementable; and if it does satisfy this property it is implementable provided no veto power, a weak requirement, also holds. The proof of the latter finding was constructive, that is, he showed how one can explicitly design an implementing mechanism. But the mechanism was fairly cumbersome, and so he was most grateful to Karl Vind, his discussant at that summer's Econometric Society meeting in Paris, who suggested a nice simplification. Maskin wrote up the full details of his results in the paper "Nash Equilibrium and Welfare Optimality" during his first term as an assistant professor at MIT that fall. But he didn't actually publish the paper for another twenty years: there seemed little point, since it was already well known in its working paper form.
Nobel Laureate, The Sveriges Riksbank Prize in Economic Sciences in Memory of Alfred Nobel, 2007