This paper presents a theoretical economic model assessing the effects of the level of
mandatory genetically modified (GM) / non-GM coexistence regulations on market and welfare
outcomes. We assume vertical differentiation of GM and non-GM goods on the consumer
side. Producers are heterogeneous in their production cost for GM crops. Producers of non-
GM crops face a probability of having their harvest downgraded if gene flow from GM fields
raises its content in GMOs (genetically modified organisms) above the labeling threshold. The
government may impose on GMO producers mandatory ex ante isolation distances from non-
GM fields in order to decrease the probability of non-GM harvest downgrading. It may also
introduce an ex post compensation to non-GMO farmers for profit losses due to harvest downgrading,
imposing GMO farmers’ participation to a compensation fund via a tax on GM seeds.
Assuming endogenous crop choices and prices, we study the effects of ex ante regulation and
ex post liability of GMO producers on market equilibrium, on the achievement of coexistence,
and on both global and interest group welfare
