The difficulty with using evidence to prove an economic system good or bad is that "good" and "bad" are value terms. Some people would call an economic system good which increases overall GDP but more recent theorists are beginning to factor in other such issues as health, happiness, and income equality (or inequality). That the US system has worked successfully for the 20th century to increase GDP, and generally the condition of its citizens, is generally self-evident. Recent rises in inequality combined with declining performance in health and educational outcomes, though, suggests that it may not be the "best" system for the 21st century. Chinese state capitalism has been an especially significant challenger to the US model in terms of GDP and per capita income growth over the past decade.