Research by Stephen Haber and Victor Menaldo found that increases in natural resource reliance do not induce authoritarianism, but may instead promotes democratization. The authors say that their method rectifies the methodological biases of earlier studies which revolve around random effects: "Numerous sources of bias may be driving the results earlier studies on the resource curse, the most serious of which is omitted variable bias induced by unobserved country-specific and time-invariant heterogeneity." In other words, this means that countries might have specific, enduring traits that gets left out of the model, which could increase the explanation power of the argument. The authors claim that the chances of this happening is larger when assuming random effects, an assumption that does not allow for what the authors call "unobserved country-specific heterogeneity". These criticisms have themselves been subject to criticism. One study re-examined the Haber-Menaldo analysis, using Haber and Menaldo's own data and statistical models. It reports that their conclusions are only valid for the period before the 1970s, but since about 1980, there has been a pronounced resource curse. Authors Andersen and Ross suggest that oil wealth only became a hindrance to democratic transitions after the transformative events of the 1970s, which enabled developing country governments to capture the oil rents that were previously siphoned off by foreign-owned firms.