n cnmcs, th Jvns prdx (; smtms Jvns ffct) ccrs whn tchnlgcl dvncmnts mk rsrc mr ffcnt t s (thrby rdcng th mnt ndd fr sngl pplctn); hwvr, s th cst f sng th rsrc drps, f th prc s hghly lstc, ths rslts n vrll dmnd ncrss csng ttl rsrc cnsmptn t rs. Gvrnmnts hv typclly xpctd ffcncy gns t lwr rsrc cnsmptn, rthr thn ntcptng pssbl ncrss d t th Jvns prdx.
n 1865, th nglsh cnmst Wllm Stnly Jvns bsrvd tht tchnlgcl mprvmnts tht ncrsd th ffcncy f cl s ld t th ncrsd cnsmptn f cl n wd rng f ndstrs. H rgd tht, cntrry t cmmn nttn, tchnlgcl prgrss cld nt b rld pn t rdc fl cnsmptn.
Th ss hs bn r-xmnd by mdrn cnmsts stdyng cnsmptn rbnd ffcts frm mprvd nrgy ffcncy. n ddtn t rdcng th mnt ndd fr gvn s, mprvd ffcncy ls lwrs th rltv cst f sng rsrc, whch ncrss th qntty dmndd. Ths my cntrct (t sm xtnt) th rdctn n s frm mprvd ffcncy. ddtnlly, mprvd ffcncy ncrss rl ncms nd cclrts cnmc grwth, frthr ncrsng th dmnd fr rsrcs. Th Jvns prdx ccrs whn th ffct frm ncrsd dmnd prdmnts, nd th mprvd ffcncy rslts n fstr rt f rsrc tlztn.
Cnsdrbl dbt xsts bt th sz f th rbnd n nrgy ffcncy nd th rlvnc f th Jvns prdx t nrgy cnsrvtn. Sm dsmss th ffct, whl thrs wrry tht t my b slf-dftng t prs sstnblty by ncrsng nrgy ffcncy. Sm nvrnmntl cnmsts hv prpsd tht ffcncy gns b cpld wth cnsrvtn plcs tht kp th cst f s th sm (r hghr) t vd th Jvns prdx. Cnsrvtn plcs tht ncrs cst f s (sch s cp nd trd r grn txs) cn b sd t cntrl th rbnd ffct.