Research shows that oil wealth lowers levels of democracy and strengthens autocratic rule. According to Michael Ross, "only one type of resource has been consistently correlated with less democracy and worse institutions: petroleum, which is the key variable in the vast majority of the studies that identify some type of curse." A 2014 meta-analysis confirms the negative impact of oil wealth on democratization. A 2016 study challenges the conventional academic wisdom on the relationship between oil and authoritarianism. Other forms of resource wealth have also been found to strengthen autocratic rule. A 2016 study finds that resource windfalls have no political impact on democracies and deeply entrenched authoritarian regimes, but significantly exacerbate the autocratic nature of moderately authoritarian regimes. A third 2016 study finds that while it is accurate that resource richness has an adverse impact on the prospects of democracy, this relationship has only held since the 1970s. A 2017 study found that the presence of multinational oil companies increases the likelihood of state repression. Another 2017 study found that the presence of oil reduced the likelihood that a democracy would be established after the breakdown of an authoritarian regime. A 2018 study found that the relationship between oil and authoritarianism primarily holds after the end of the Cold War; the study argues that without American or Soviet support, resource-poor authoritarian regimes had to democratize while resource-rich authoritarian regimes were able to resist domestic pressures to democratize. Prior to the 1970s, oil-producing countries did not have democratization levels that differed from other countries.