Leonid Hurwicz, the economist who last year became the oldest person ever to win a Nobel prize, helped transform economic thinking in the second half of the 20th century.

For years economists had been passionately debating the rival merits of state planning versus free markets. In both systems, people had incentives to lie to bureaucratic planners or to employers about their interests, their skills or their circumstances. "Leo", who has died at the age of 90, founded the field of "mechanism design", a new way of thinking which focused on giving people incentives to tell the truth and to do so in a way that would benefit society as a whole.