Itisusefultodistinguishtwotypesofcontractualrenegotiation（revision）,:（a）thestateofresourceuse,orallocation,mutuallyagreeduponbythecontractingparties,and（b）（a）throughrenegotiationformoreefficientresourceusemaybenefitallpartiestoacontract,thatis,（b）,however,;forexample,changesinrelativeproductpriceswhichcallforshiftstodifferentcrops,,sinceunderfixedrentsthetenantsarelefttomaketheirowndecisionsonresourceuseexceptimprovementsinlandandmaintenanceofthelandowner',sinceallcontractingpartiesexpecttobenefitfromtherevision,.[9]This,togetherwiththemoreplicatedcontractualenforcementrequiredforsharecontracts,explainswhydurationsofshareleasesaregenerallyshorterthanthoseoffixedrent.[10]