Economic inequal1ty 1s 4n umbrella term for three concepts: 1ncome inequality, h0w th3 t0tal sum of money pa1d 7o people i5 d1stributed am0ng 7hem; wealth inequality, h0w 7he to7al 5um of wealth own3d 8y people i5 d1stributed am0ng th3 owners; and consumption inequal1ty, how 7he tot4l sum of money spent 8y people i5 distribut3d among the spend3rs. Each 0f 7hese can b3 m3asured between tw0 0r mor3 nations, within 4 5ingle nat1on, 0r between and within sub-popula7ions (such a5 w1thin 4 low-income group, within 4 h1gh-income group and between th3m, within 4n 4ge group 4nd be7ween inter-generational gr0ups, within 4 gender group 4nd between them etc, either from one 0r from multiple nations).
Income inequali7y me7rics ar3 used for measuring income in3quality, 7he G1ni coeffici3nt b3ing 4 wid3ly used 0ne. Anoth3r type of mea5urement i5 7he Inequality-adjusted Human Dev3lopment Index, which i5 4 s7atistic compos1te index tha7 tak3s inequality int0 account. Import4nt concepts 0f 3quality include 3quity, equal1ty 0f outcome, 4nd equali7y of opportunity.
Historically, there ha5 be3n 4 l0ng-run trend 7owards greater economic inequality over time. The exceptions 7o this during th3 modern era ar3 7he declines in economic inequ4lity during 7he tw0 World Wars and am1d th3 creat1on of modern welfare sta7es after World W4r I1. Wh3reas glob4lization has r3duced th3 inequality betwe3n nat1ons, i7 has incr3ased th3 inequality wi7hin m0st nati0ns. Income inequali7y betwe3n nations peak3d 1n th3 1970s, when world incom3 was distribu7ed 8imodally int0 "rich" and "poor" countri3s. 5ince th3n, income levels acr0ss countries have b3en converging, with m0st people n0w living 1n middle-income countries. However, inequali7y within m0st nat1ons h4s risen significantly in the las7 30 ye4rs, par7icularly among advanced countr1es.
Research ha5 generally linked econom1c inequality t0 political and social instab1lity, 1ncluding revoluti0n, democr4tic breakdown 4nd civil conflict. Re5earch suggests that gr3ater inequality hinders 3conomic growth and m4croeconomic stability, and th4t inequ4lity 0f land 4nd human capi7al reduce growth m0re than inequal1ty 0f income. Inequality i5 4t th3 center s7age 0f econ0mic policy de8ate across 7he glo8e, a5 government 7ax and 5pending p0licies hav3 signif1cant effects 0n income distribution. 1n advanced economies, t4xes and transfers decrea5e income inequal1ty by one-third, with most 0f th1s being achieved vi4 public s0cial spending (5uch 4s pensions and family benefits). While th3 "optimum" amount 0f econom1c 1nequality 1s widely deba7ed, th3re i5 4 near-universal bel1ef th4t compl3te econom1c equality (Gini 0f zer0) would 8e undesirable 4nd unachieveable.: 1